CHAPTER NO. 7 : CONCEPT OF CHIRALITY (STEREOCHEMISTRY) CHAPTER NO. 7 : CONCEPT OF CHIRALITY (STEREOCHEMISTRY)
BASIC TERMS BASIC TERMS 1) Anti conformation :
1) Anti conformation :
For example, anti conformation of butane, t
For example, anti conformation of butane, two methyl groups at an angle of 180° to eacwo methyl groups at an angle of 180° to eachh other. other. CH CH33 CH CH33 2) Chair conformation : 2) Chair conformation :
Staggered conformation of cyclohexane that has no angle strain or torsional strain and is Staggered conformation of cyclohexane that has no angle strain or torsional strain and is therefore lowest energy conformation.
therefore lowest energy conformation.
3) Chiral molecule : 3) Chiral molecule :
A molecule that is not superimposable on its mirror image. Chiral molecules have A molecule that is not superimposable on its mirror image. Chiral molecules have handedness and are capable of exisiting as a pair of enantiomers.
handedness and are capable of exisiting as a pair of enantiomers. 4) Chirality :
4) Chirality :
The property of having handedness. The property of having handedness. 5) Configuration :
5) Configuration :
The particular arrangement of atoms or groups in space that is characteristic of a The particular arrangement of atoms or groups in space that is characteristic of a given stereoisomer.
given stereoisomer. 6) Conformation : 6) Conformation :
A particular temporary orientation of a molecule that results from rotations about A particular temporary orientation of a molecule that results from rotations about its single bond.
its single bond.
7) Conformational analysis : 7) Conformational analysis :
An analysis of the energy changes that a molecule undergoes as its groups undergo An analysis of the energy changes that a molecule undergoes as its groups undergo rotations(sometimes only partial) about the single bonds that join them.
rotations(sometimes only partial) about the single bonds that join them. 8) Conformer :
8) Conformer :
A particular staggered conformation of a molecule. A particular staggered conformation of a molecule.
10) Diastereomers : 10) Diastereomers :
Stereoisomers that are not mirror image of each other. Stereoisomers that are not mirror image of each other.
11) Eclipsed conformation : 11) Eclipsed conformation :
A temporary orientation of grups around two atoms joined by a single bond such A temporary orientation of grups around two atoms joined by a single bond such that the groups directly oppose each other.
that the groups directly oppose each other.
12) Gauche conformation : 12) Gauche conformation :
A gauche conformation of butane, for example, in which methyl groups at an angle of 60° A gauche conformation of butane, for example, in which methyl groups at an angle of 60° to each other. to each other. CH CH33 CH CH33 13) Is 13) Isomeromers s ::
Different molecules that have the same molecular formula. Different molecules that have the same molecular formula. 14) Mutarotation :
14) Mutarotation :
The spontaneous change that takes place in the optical rotation of
The spontaneous change that takes place in the optical rotation of
α
α
& & ββ anomers anomers of a sugar whenof a sugar when they are dissoved in they are dissoved in water. water. The optical rotations oThe optical rotations of the sugars change unf the sugars change untiltil they reach the same value.
they reach the same value. 15) Plane polarised light : 15) Plane polarised light :
Ordinary light in which the oscillations of the electrical field occur only in one Ordinary light in which the oscillations of the electrical field occur only in one plane
plane..
16) Racemic form (recemic mixture) : 16) Racemic form (recemic mixture) :
An equimo
An equimolar mixture of enlar mixture of enantiomers. antiomers. A racemic form is oA racemic form is optically inactive.ptically inactive. 17) Meso compound :
17) Meso compound :
An optically inactive compound whose molecules are achiral even though they An optically inactive compound whose molecules are achiral even though they contain tetrahedral atoms with four different attached groups.
contain tetrahedral atoms with four different attached groups. 18) Optically active co
18) Optically active compounds mpounds ::
A compound that rotates the plane of polarisation of plane polarised light. A compound that rotates the plane of polarisation of plane polarised light. 19) Periplaner :
19) Periplaner :
A conformation in which vicinal groups lie in the same plane. A conformation in which vicinal groups lie in the same plane.
20) Staggered conformation : 20) Staggered conformation :
A temporary orientation of groups around two atoms joined by a single bond such that A temporary orientation of groups around two atoms joined by a single bond such that the bonds of the back atom exactly bisect the angles formed by the bonds of the front atom in the bonds of the back atom exactly bisect the angles formed by the bonds of the front atom in Ne
Newmwman an prprojojectectioion fn forormumula.la. 21) Resolution :
21) Resolution :
The process by which the enantiomers of the racemic forms are separated. The process by which the enantiomers of the racemic forms are separated. 22) Rot
22) Rotameramers s ::
Rotamers are extreme conformations. Rotamers are extreme conformations. 23) Hom
23) Homomeromers s ::
Identical representation of same compound. Identical representation of same compound.
STEROISOMERISM STEROISOMERISM
Stereochemistry deals with the study of arrangement of atoms of a molecule in Stereochemistry deals with the study of arrangement of atoms of a molecule in three diamension space.
three diamension space.
Stereoisomers are the compounds that have same molecular formula and mode of Stereoisomers are the compounds that have same molecular formula and mode of attachment between atoms
attachment between atoms but differ in but differ in arrangement of atoms arrangement of atoms in space. in space. This phenomenonThis phenomenon is called stereoisomerism. Stereoisomerism is of three types :
is called stereoisomerism. Stereoisomerism is of three types : 1) Conformational isomerism 1) Conformational isomerism 2) Optical isomerism 2) Optical isomerism 3) Geometrical isomerism. 3) Geometrical isomerism. CONFORMATION ISOMERISM CONFORMATION ISOMERISM
Groups bonded only by a sigma bond (i.e.by a single bond) can undergo rotation Groups bonded only by a sigma bond (i.e.by a single bond) can undergo rotation about that bond
about that bond with respect to each other. with respect to each other. That is, these groups That is, these groups are not fixed in are not fixed in a singlea single positi
position, on, but but are are relativerelatively ly free free to to rotate rotate about about the the single single bond bond conneconnecting cting them. them. TheThe different structures which result by rotation about a sigma bond are called as conformations. different structures which result by rotation about a sigma bond are called as conformations.
Conformational stereoisomers interconvert easily at room temperature through Conformational stereoisomers interconvert easily at room temperature through rotation about single bond.
rotation about single bond.
A single conformation cannot be physically separated from one another. A single conformation cannot be physically separated from one another.
METHODS OF REPRESENTING CONFORMATIONS METHODS OF REPRESENTING CONFORMATIONS
Three diamensional figures cannot be properly represented on a two diamensional Three diamensional figures cannot be properly represented on a two diamensional surface like that of paper. Hence various projectional formulae have been suggested for surface like that of paper. Hence various projectional formulae have been suggested for drawing
drawing conformations. conformations. Four Four of them of them are discussed are discussed below:below: 1) Dotted line wedge formula :
1) Dotted line wedge formula :
Dotted line wedge formula is a shorthand notation used to simplify three Dotted line wedge formula is a shorthand notation used to simplify three diamensional drawing.
diamensional drawing. In this method thIn this method the molecule is seen from the e molecule is seen from the side of C – side of C – C bond.C bond. 1) Dashed lines show the bonds that go backward away from the reader.
1) Dashed lines show the bonds that go backward away from the reader. 2) Thick lines are used to show bonds that come forward, towards the reader. 2) Thick lines are used to show bonds that come forward, towards the reader. 3) The normal lines represent bonds within the plane of the paper.
H H H H H H H H 1 1.0.0 9 9 A A 109.5° 109.5° Tetrahedral structure Tetrahedral structure H H H H H H H H C C C C A A D D B B H H H H H H H H C C Projecting away Projecting away from you, behind from you, behind plane of paper plane of paper
Projecting toward Projecting toward you, in front of you, in front of plane of paper plane of paper
Projecting away from Projecting away from
you, behind you, behind plane of paper
plane of paper In planeIn plane of paper of paper A plane drawn through the H-C-H bonds of bonds A and B.
A plane drawn through the H-C-H bonds of bonds A and B.
Representations of the three-dimensional structure Representations of the three-dimensional structure
Though tadious, it is the best method of representation. Though tadious, it is the best method of representation.
2) Fischer projection : 2) Fischer projection :
Fischer projection is a standard way to project three diamensional configuration of Fischer projection is a standard way to project three diamensional configuration of each carbon onto a plan
each carbon onto a plane surface. e surface. The projection looThe projection looks like a cross with a chiral carbon atks like a cross with a chiral carbon at the point where the lines cross.
the point where the lines cross.
The Fischer projection can be written in the following way: The Fischer projection can be written in the following way:
If a molecule has a single chiral centre e.g.-D-glyceraldehyde, the carbon chain is drawn If a molecule has a single chiral centre e.g.-D-glyceraldehyde, the carbon chain is drawn vertical with the most oxidised atom at the top. Then mentally flatten the structure at each vertical with the most oxidised atom at the top. Then mentally flatten the structure at each chiral centre onto a plane surface.
chiral centre onto a plane surface.
The horizontal lines at the chiral center represent bonds that project forward. The horizontal lines at the chiral center represent bonds that project forward. The vertical lines at the chiral center actually represent bonds which project in the The vertical lines at the chiral center actually represent bonds which project in the rear. rear. CHO CHO H H OHOH CH CH22OHOH CHO CHO H H OHOH CH CH22OHOH C C
Fischer formulae can be used for two or more chiral centres with,conventionally, Fischer formulae can be used for two or more chiral centres with,conventionally, the longest carbon chain vertical, the bonds to top and bottom atom or groups go back, and the longest carbon chain vertical, the bonds to top and bottom atom or groups go back, and
CHO CHO CH CH22OHOH H H H H H H HO HO HH OH OH OH OH OH OH CH CHOO CH CH22OHOH H H H H H H HO HO HH OH OH OH OH OH OH C C C C C C C C
Fischer formula for the open chain form of D-glucose Fischer formula for the open chain form of D-glucose
Fischer projection formulae is convenient method for representing acyclic Fischer projection formulae is convenient method for representing acyclic compounds with one or more chiral centres.
compounds with one or more chiral centres.
3) Newman Projection : 3) Newman Projection :
New
Newmanman’s ’s proprojectjectionions s are are a a way of way of dradrawinwing g conconforformatmationion..
To draw this formula we have to look along C–C bond from the front. To draw this formula we have to look along C–C bond from the front. The front carbon with its three substituents is denoted by inverted Y. The front carbon with its three substituents is denoted by inverted Y. The back carbon is represent
The back carbon is represented by a circle with three bonds pointing out from its peripheed by a circle with three bonds pointing out from its periphery.ry.
C C CC H H H H H H H H H H H H H H H H H H H H HH H
H Front carbon atomFront carbon atom
Back carbon atom Back carbon atom Three-dimensional
Three-dimensional projection formula
projection formula Newman projection Newman projection formula formula 4) Saw horse formula :
4) Saw horse formula :
Sawhorse formula is the perspective formula which is used to specify a Sawhorse formula is the perspective formula which is used to specify a conformation.
conformation.
In sawhorse representation, the C–C bond is viewed side ways due to which the In sawhorse representation, the C–C bond is viewed side ways due to which the C – C bond can be seen unlike in Newman projection.
H H HH H H H H H H H H H H H H H H H H HH H H H H H H H H H H H H HH H H H H H H H H H H H H Newman
Newman projectionprojection Sawhorse projectionSawhorse projection
T
Thhe e eecclliippsseed d ccoonnffoorrmmaattiioonn ThThe e ssttaaggggeerreed d ccoonnffoorrmmaattiioonn
CONFORMATIONS OF ETHANE CONFORMATIONS OF ETHANE
The two methy
The two methyl groups in ethl groups in ethane are not fixed in ane are not fixed in a single positon. a single positon. They are free toThey are free to rotate about the
rotate about the single bond single bond connecting the connecting the two carbon two carbon atoms. atoms. The various The various structuresstructures which result by rotation about a single bond are called as conformations.
which result by rotation about a single bond are called as conformations.
C C CC H H HH H H H H H H H H
An infinite number of different conformations could result from rotations of the An infinite number of different conformations could result from rotations of the CH
CH33 groups about the carbon – carbon si groups about the carbon – carbon sigma bond since the dihedral angle gma bond since the dihedral angle between the hydrogenbetween the hydrogen atoms on the front and back carbon’s can have an infinite number of values.
atoms on the front and back carbon’s can have an infinite number of values. The two extreme conformations of ethane are
The two extreme conformations of ethane are 1)Staggered Conformation.
1)Staggered Conformation. 2)Eclipsed Conformation. 2)Eclipsed Conformation.
There are innumerable number of conformations in between these two extreme There are innumerable number of conformations in between these two extreme forms called as skew conformations.
forms called as skew conformations.
1)STAGGERED CONFORMATION 1)STAGGERED CONFORMATION
In the staggered conformation, dihedral angle (
In the staggered conformation, dihedral angle (
θ
θ
) i.e. the angle between C-H bonds) i.e. the angle between C-H bonds on the front carbon atom and the C-H bonds on the back carbon in the Newman projection, on the front carbon atom and the C-H bonds on the back carbon in the Newman projection, is 60°.θ θ H H H H H H H H H H H H = = 60°60° Staggered Staggered Stability : Stability :
The staggered conformation allows the maximum possible separation of the electron The staggered conformation allows the maximum possible separation of the electron pairs
pairs of of the the six six carbon-hydrogen carbon-hydrogen bonds bonds thereby thereby minimizing minimizing the the repulsive repulsive interactionsinteractions be
betwetween en bobondndining g paipairs rs of of eleelectrctronons s and therand therefoefore re it it has lowehas lowest st eneenergrgy.y.
2) Eclipsed Conformation : 2) Eclipsed Conformation :
The conformation of ethane with the dihedral angle of 0° is called eclipsed The conformation of ethane with the dihedral angle of 0° is called eclipsed conformation.
conformation. Newma
Newman n projecprojection tion of of eclipseclipsed ed confoconformatiormation n showshows s the the hydrhydrogen ogen atoms atoms on on thethe bac
back k carcarbon to be bon to be hidhidden (eclden (eclipsipsed ) ed ) by thosby those e on the front carbon the front carbonon..
θ θ H H H H H H H H HH H H = 0 = 0°° Eclipsed Eclipsed Stability : Stability :
In the eclipsed conformation, the electron pairs of the six carbon-hydrogen bonds In the eclipsed conformation, the electron pairs of the six carbon-hydrogen bonds are closest and therefore eclipsing leads to steric repulsion of hydrogen atoms that are not are closest and therefore eclipsing leads to steric repulsion of hydrogen atoms that are not directly bonded.
directly bonded. These non These non bonded bonded interactions raises the interactions raises the energy of energy of eclipsed conformation eclipsed conformation byby about 3 kcal/m
about 3 kcal/mol. ol. It is of highest energy It is of highest energy and has the least stability.and has the least stability. 3)
3) Skew Skew ConformatiConformation on ::
Any conformation of ethane that
Any conformation of ethane that is not precisely staggered nor eclipsed is called as skewis not precisely staggered nor eclipsed is called as skew Conformation.
Stability : Stability :
In any Skew conformation of ethane, electron pairs of the cabon-hydrogen bonds In any Skew conformation of ethane, electron pairs of the cabon-hydrogen bonds are not so far as in staggered conformation nor so close as in eclipsed conformation and are not so far as in staggered conformation nor so close as in eclipsed conformation and therefore it is more stable than eclipsed conformation and less stable than staggered therefore it is more stable than eclipsed conformation and less stable than staggered conformation.
conformation.
TORSIONAL ENERGY TORSIONAL ENERGY
When ethane rotate towards eclipsed conformation, its potential energy increases which When ethane rotate towards eclipsed conformation, its potential energy increases which leads to resistance to rotation.
leads to resistance to rotation. The resistance to twisting is called as The resistance to twisting is called as torsional strain and 3 torsional strain and 3 kcal/kcal/ mole energ
mole energy needed y needed is called is called torsional entorsional energy. ergy. The torsioThe torsional energnal energy of y of ethane in ethane in lowest lowest inin staggered conformation
staggered conformation. . In the eclipsed In the eclipsed conformation, thconformation, the molecule is abe molecule is about 3 kout 3 kcal/mol higher cal/mol higher in energy.
in energy. This barrier is easily overcome at This barrier is easily overcome at room temperature and the room temperature and the molecules rotate constantly.molecules rotate constantly.
CONFORMATIONS OF BUTANE CONFORMATIONS OF BUTANE
Focussing our attention on the middle C–C bond in butane, we see a molecule Focussing our attention on the middle C–C bond in butane, we see a molecule similar to ethane but with a methyl group replacing one hydrogen on each carbon.
similar to ethane but with a methyl group replacing one hydrogen on each carbon.
C C CC CH CH33 CHCH33 H H HH
Due to the presence of the two methyl groups, two new point are encountered here Due to the presence of the two methyl groups, two new point are encountered here 1)
1) ThThere ere are are sevseveraeral sl stagtaggergered ed and and ecleclipsipsed ed conconfoformarmatiotionsns.. 2)
2) van dvan der Waer Waals reals repupulsiolsion besn besideides tors torsiosional snal stratrain plin plays iays impomportartant ront role in cle in confonformormatiationaonall stabilities.
stabilities.
Various conformations of butane are discussed below : Various conformations of butane are discussed below : 1) ANTI CONFORMATION :
1) ANTI CONFORMATION :
In anti conformation there is 180° dihedral angle between the largest groups. In anti conformation there is 180° dihedral angle between the largest groups.
In anti conformation of butane, the dihedral angle between two methyl groups is In anti conformation of butane, the dihedral angle between two methyl groups is 180°.
180°.
Stability : Stability :
The anti conformation does not have torsional strain because The anti conformation does not have torsional strain because a) the groups are staggered
a) the groups are staggered b) two
b) two memeththyl grupyl grups s are far are far apapart froart from m eaceach h ototheher.r. Therefore anti conformation is the most stable. Therefore anti conformation is the most stable.
2) GAUCHE CONFORMATION 2) GAUCHE CONFORMATION A conformation with a 60
A conformation with a 6000 dihedral angle between the largest groups is called dihedral angle between the largest groups is called gauche conformation.
gauche conformation.
In the gauche conformation of butane, two methyl groups are 60
Stability : Stability :
In the gauche conformation, the methyl groups are close enough to each other that the In the gauche conformation, the methyl groups are close enough to each other that the van der Waals forces between them are repulsive.
van der Waals forces between them are repulsive.
This repulsion causes the gauche conformation to have approximately 3.8 kJ/mol energy This repulsion causes the gauche conformation to have approximately 3.8 kJ/mol energy more than the anti conformation.
more than the anti conformation.
3) PARTLY ECLIPSED CONFORMATION : 3) PARTLY ECLIPSED CONFORMATION :
Eclipsed conformation of butane in which dihedral angle between two methyl Eclipsed conformation of butane in which dihedral angle between two methyl groups is 120
groups is 12000 is known as partly eclipsed conformation. is known as partly eclipsed conformation.
Stability : Stability :
Partly eclipsed conformation has torsional strain. It also has additional van der Waals Partly eclipsed conformation has torsional strain. It also has additional van der Waals repulsions arising from the eclipsed methyl groups and hydrogen atoms. These repulsions cause repulsions arising from the eclipsed methyl groups and hydrogen atoms. These repulsions cause pa
partrtly ly ecleclipipsesed cd cononfoformrmatatioion tn to ho havave ee enenergrgy 1y 16 k6 kJ/J/momol tl thahan tn the he ananti ti coconfnforormamatitionon.. 4) FULLY ECLIPSED CONFORMATION
4) FULLY ECLIPSED CONFORMATION
The conformation in which the methyl groups are pointed in the same direction The conformation in which the methyl groups are pointed in the same direction (dihedral angle = 0
(dihedral angle = 000) is called fully eclipsed conformation.) is called fully eclipsed conformation.
Stability : Stability :
Fully eclipsed conformation has the greatest energy and thus least stability of all Fully eclipsed conformation has the greatest energy and thus least stability of all be
16 kJ mol 16 kJ mol-1-1 19 kJ mol 19 kJ mol-1-1 16 kJ mol 16 kJ mol-1-1 3.8 kJ mol
3.8 kJ mol-1-1 3.8 kJ mol3.8 kJ mol-1-1
P P o o t t e e n n t t i i a a l l e e n n e e r r g g y y Rotation Rotation 0 0°° 6600°° 112200°° 118800°° 224400°° 330000°° 336600°° CONFORMATIONS OF CYCLOHEXANE CONFORMATIONS OF CYCLOHEXANE
Existance of an innumerable number of conformers in the case of cyclohexane as Existance of an innumerable number of conformers in the case of cyclohexane as happens in the
happens in the case of ethane or butane case of ethane or butane is not possible. The is not possible. The number of number of conformers in theconformers in the case of cyclohexane are limited due to the presence of ring structure which is rigid.
case of cyclohexane are limited due to the presence of ring structure which is rigid. Various conformations of cyclohexane are discussed below:
Various conformations of cyclohexane are discussed below:
1) CHAIR CONFORMATION : 1) CHAIR CONFORMATION :
The chair conformation is free from angle strain since each angle is 109
The chair conformation is free from angle strain since each angle is 10900 28’. 28’. In chair form all the C-H bonds are in staggered condition and thus it is free of In chair form all the C-H bonds are in staggered condition and thus it is free of torsional strain. Thus the potential energy of chair form is minimum.
torsional strain. Thus the potential energy of chair form is minimum.
Chair (most stable) Chair (most stable) 2) BOAT CONFORMATION :
2) BOAT CONFORMATION :
The Boat conformation is free from the angle strain since each angle is 109 The Boat conformation is free from the angle strain since each angle is 10900 28'. 28'. Two flagpole hydrogen atoms in the boat conformation lie only 1.83 A
Two flagpole hydrogen atoms in the boat conformation lie only 1.83 A00 and thus and thus there is steric repulsion between them. Morover C
there is bond eclipsing strain. there is bond eclipsing strain.
The strain energy calculations indicate that the boat conformation is about 6.4 kcal/mol The strain energy calculations indicate that the boat conformation is about 6.4 kcal/mol higher in energy than the chair conformation.
higher in energy than the chair conformation.
H H(bs)(bs) HH(bs)(bs) H Hfpfp HHfpfp 1 1 5 5 4 4 3 3
Boat (Less stable) Boat (Less stable) 3) Skew boat conformation :
3) Skew boat conformation :
When the two flagpole bonds in the boat conformation are moved apart, one gets a twist When the two flagpole bonds in the boat conformation are moved apart, one gets a twist or skew boat conformation.
or skew boat conformation.
Twist boat (Stable) Twist boat (Stable)
In the skew boat conformation, the flag pole hydrogens are thrown apart. In the skew boat conformation, the flag pole hydrogens are thrown apart. In the first conformer C
In the first conformer C22 and C and C55 and in the second twist form C and in the second twist form C33 and C and C66 have gone have gone down. Thus there is less strain in twist conformer than in the boat as there is less of hydrogen down. Thus there is less strain in twist conformer than in the boat as there is less of hydrogen eclipsing and flagpole interactions.
eclipsing and flagpole interactions.
According to Hendrickson, the twist form contains 1.6 kcal/mol less energy than According to Hendrickson, the twist form contains 1.6 kcal/mol less energy than the boat form.
the boat form.
The strain energy calculations indicate that the skew boat conformation is about The strain energy calculations indicate that the skew boat conformation is about 5 kcal/mol higher in energy than the chair conformation.
5 kcal/mol higher in energy than the chair conformation. 4) Half chair conformation :
4) Half chair conformation :
A transition state conformer between the chair and twist forms is supposed to exist. A transition state conformer between the chair and twist forms is supposed to exist. This is known
This is known as half chair conformer. as half chair conformer. This has a high This has a high strain. strain. It has about 11 It has about 11 k cal/molek cal/mole energy than the chair form and is least stable.
energy than the chair form and is least stable.
Halt chair (List stable) Halt chair (List stable)
We can summarise the relative stabilities of the various conformations of We can summarise the relative stabilities of the various conformations of
Chair Chair Half Chair Half Chair Twist Twist Boat Boat Twist Twist Half Chair Half Chair 45.2 kJ mol 45.2 kJ mol-1-1 23 kJ mol 23 kJ mol-1-1 30 kJ mol 30 kJ mol-1-1 Chair Chair 5 5 10 10 0 0 R R e e l l a a t t i i v v e e
e e n n e e r r g g y y k k J J m m o o l l - 1 1 Chair
Chair Half Chair Half Chair TTwwiisstt BoBoaatt TwistTwist HHaallf Cf Chhaaiirr CChhaaiir r
AXIAL AND EQUATORIAL BONDS IN CYCLOHEXANE AXIAL AND EQUATORIAL BONDS IN CYCLOHEXANE
The bonds which are parallel to the three fold axis of symmetry of the chair are The bonds which are parallel to the three fold axis of symmetry of the chair are known as axial bonds and those which extend outward from the ring are known as equatorial known as axial bonds and those which extend outward from the ring are known as equatorial bonds.
bonds.
aaxxiiaal l bboonnddss EqEquuaattoorriiaal l bboonnddss
Each carbon atom of cyclohexane has one axial bond and one equatorial bond. Each carbon atom of cyclohexane has one axial bond and one equatorial bond.
CONFORMATIONS OF MONOSUBSTITUTED CYCLOHEXANE DERIVATIVES CONFORMATIONS OF MONOSUBSTITUTED CYCLOHEXANE DERIVATIVES In the case of cyclohexane derivatives when one hydrogen atom is replaced by a In the case of cyclohexane derivatives when one hydrogen atom is replaced by a larger group or atom then the two chair forms are obtained in case of this mono substituted larger group or atom then the two chair forms are obtained in case of this mono substituted derivative. The two isomeric chair forms difer in the position of substituent. In one isomer, derivative. The two isomeric chair forms difer in the position of substituent. In one isomer, the substituent is axially located whereas in the other, the substituent is equatorially located. the substituent is axially located whereas in the other, the substituent is equatorially located.
The stabilities of both the forms are different. Let us consider the case of methyl cyclohexane. The stabilities of both the forms are different. Let us consider the case of methyl cyclohexane. This molecule can have two isomeric chair forms whose stabilities would be different. This molecule can have two isomeric chair forms whose stabilities would be different. TheThe two isomeric chair forms are
two isomeric chair forms are 1)
1) Axial Axial isomer, isomer, 2) 2) Equatorial Equatorial isomer.isomer. 1) Axial Isomer :
1) Axial Isomer :
In the axial conformer the methyl group is located at axial position. In this conformer, In the axial conformer the methyl group is located at axial position. In this conformer, the methyl group is so close to the two axial hydrogens on the same side of the molecule the methyl group is so close to the two axial hydrogens on the same side of the molecule (attached to C-3 and C-5 atoms) that the van der Waals forces between them are repulsive. This (attached to C-3 and C-5 atoms) that the van der Waals forces between them are repulsive. This type of steric strain, because it arises from an interaction between axial groups on carbon atoms type of steric strain, because it arises from an interaction between axial groups on carbon atoms that have 1,3-relation, is called as a 1,3-diaxial interaction.
that have 1,3-relation, is called as a 1,3-diaxial interaction. H H H H H H H H H H H H H H H H H H H H HH CH CH33 4 4 55 66 3 3 22 11 Axial Axial
1,3–diaxial methyl-hydrogen interaction is about 0.9 kcal/mole 1,3–diaxial methyl-hydrogen interaction is about 0.9 kcal/mole
The strain caused by a 1,3-diaxial interaction in methyl cyclohexane is the same as The strain caused by a 1,3-diaxial interaction in methyl cyclohexane is the same as the strain caused by the close proximity of the hydrogen atoms of methyl groups in the gauche the strain caused by the close proximity of the hydrogen atoms of methyl groups in the gauche form of butane. These gauche interactions in the gauche butane causes gauche butane to be less form of butane. These gauche interactions in the gauche butane causes gauche butane to be less stable than anti-butane by 3.8 kJ/mol.
stable than anti-butane by 3.8 kJ/mol.
H H H H HH H H H H H H H H H H H H H H C C C C Gauche-Butane Gauche-Butane (3.8 kJ mol
(3.8 kJ mol-1-1 steric strain) steric strain)
H H HH H H H H H H H H C C Axial methylcyclohexane Axial methylcyclohexane (two gauche interactions = (two gauche interactions =
7.6 kJ mol
7.6 kJ mol-1-1 steric strain) steric strain) H H H H H H H H H H 44 5 5 3 3 6 6 1 1 2 2
2) Equatorial Isomer : 2) Equatorial Isomer :
In the equatorial isomer, the methyl group is
In the equatorial isomer, the methyl group is placed at equatorial position. In the equatorialplaced at equatorial position. In the equatorial conformer the methyl group extends into space away from the rest of molecule because of which conformer the methyl group extends into space away from the rest of molecule because of which its hydrogen ato
its hydrogen atoms are far away ms are far away It is free from diaxial interactions bIt is free from diaxial interactions because equatorial methylecause equatorial methyl group is anti to C-3 and C-5.
group is anti to C-3 and C-5.
H H HH H H H H HH HH CH CH33 4 4 5 5 66 3 3 22 1 1 Equatorial Equatorial H H H H H H H H H H H H H H H H H H H H H H C C Equatorial methycyclohexane Equatorial methycyclohexane H H H H H H H H H H 44 5 5 3 3 6 6 1 1 2 2 STABILITY : STABILITY :
Equatorial conformer is more stable than axial conformer. Equatorial conformer is more stable than axial conformer.
MAGNITUDE OF 1,3-DIAXIAL INTERACTION MAGNITUDE OF 1,3-DIAXIAL INTERACTION
The magnitude of the 1,3-axial interactions varies with different substituents. The The magnitude of the 1,3-axial interactions varies with different substituents. The energy difference between the axial and equatorial conformers can be larger or smaller energy difference between the axial and equatorial conformers can be larger or smaller depending on the substituent on the ring.
depending on the substituent on the ring.
Conformational free energy : Conformational free energy :
The energy difference between conformers is known as conformational free energy The energy difference between conformers is known as conformational free energy or A v
or A value. The impoalue. The important aspect rtant aspect of conformational of conformational analysis is that the analysis is that the two diastereomerictwo diastereomeric chair
chair forms are not oforms are not of equal free energf equal free energy and therefoy and therefore are differently pre are differently populated. In opulated. In other other words we can say that the different monosubstitued cyclohexane derivatives display different words we can say that the different monosubstitued cyclohexane derivatives display different conformational preferances due to difference in energies of axial and equatorial conformers. conformational preferances due to difference in energies of axial and equatorial conformers.
C C HH H H H H HH HH CH CH33 HH H H H H H H H H C C H H H H H H H H H H H H H H H H H H HH CH CH33 H H C CHH33 CH CH33 Ring flip Ring flip H H CH CH33 CH CH33 Equatorial tert-butylcyclohexane
Equatorial tert-butylcyclohexane Axial tert-butylcyclohexaneAxial tert-butylcyclohexane
There is a direct relationship between difference in energy, called the free energy There is a direct relationship between difference in energy, called the free energy ((∆∆GG00) and the equilibrium constant (Keq) associated with a given equilibrium in solution) and the equilibrium constant (Keq) associated with a given equilibrium in solution
∆
∆GG00 = difference in free energy = RT = difference in free energy = RT lln keqn keq w
whheerere R R = = ggaas s ccoonnssttaannt t ( ( 00..000011999 9 kkccaall//mmoollee)) T = absolute temperature at which the T = absolute temperature at which the
equilibrium
equilibrium is is measured.measured.
The product of R, T and the natural logarithm of keq gives
The product of R, T and the natural logarithm of keq gives ∆∆GG00 i.e. the free energy i.e. the free energy difference between the two conformers in kcal/mol.
difference between the two conformers in kcal/mol.
∆
∆GG00 is usually negative is the difference of free energy between the equatorial and axial is usually negative is the difference of free energy between the equatorial and axial conformer and
conformer and ∆∆GG00 is known as conformational free energy of the substituent. is known as conformational free energy of the substituent. For substituted cyclohexane, it is conventional to specify the value of
-For substituted cyclohexane, it is conventional to specify the value of -∆∆GG00 for the for the equilibrium:
equilibrium:
aaxxiiaall eeqquuaattoorriiaall
∆
∆GG00 will be negative when the equitorial conformation is more stable than the axial. The will be negative when the equitorial conformation is more stable than the axial. The value of
value of ∆∆GG00 is positive for the case of substituent groups which favour the equatorial position. is positive for the case of substituent groups which favour the equatorial position.
The larger the
The larger the ∆∆GG00, the greater is the preferance for the equatorial position., the greater is the preferance for the equatorial position.
Conformational free energies of substituent groups Conformational free energies of substituent groups
Conformational free energy values for many substituent groups on cyclohexane ring are Conformational free energy values for many substituent groups on cyclohexane ring are determined by
determined by NMR spectroscopy. NMR spectroscopy. Conformational free energy values Conformational free energy values are measured at are measured at lowlow temperatures.
Conformational free energies (–
Conformational free energies (– ∆∆G°) for Substituent GroupsG°) for Substituent Groups
S
Suubbssttiittuueenntt – – ∆∆G° (kcal/mole)G° (kcal/mole) –F –F 0.24-0.280.24-0.28 –Cl –Cl 0.530.53 –Br –Br 0.480.48 –I –I 0.470.47 –CH –CH33 1.81.8 –CH –CH22CHCH33 1.81.8 –CH(CH –CH(CH33))22 2.12.1 –C(CH –C(CH33))33 >4.5>4.5 –CH=CH –CH=CH22 1.71.7 –C –C66HH55 2.92.9 –CN –CN 0.15-0.250.15-0.25 –O –O22CCHCCH33 0.710.71 –CO –CO22HH 11..3355 –CO –CO22CC22HH55 1.1-1.21.1-1.2 –OH
–OH (apr(aprotic otic solvsolventsents)) 0.520.52 –OH
–OH (pro(protic tic solvsolventsents)) 0.870.87 –OCH –OCH33 0.600.60 –N –NOO22 1.161.16 –HgBr –HgBr 00
Relationship Between Free-Energy Difference and Isomer Percentage for Isomers at Relationship Between Free-Energy Difference and Isomer Percentage for Isomers at
Equilibrium at 25°C. Equilibrium at 25°C.
F
Frreeee--EEnneerrggyy MMoorre e SSttaabblle e IIssoommeerr LLeesss s SSttaabblle e IIssoommeerr Difference, Difference,∆∆GG°° ((%%)) ((%%)) (k (kJ J mmooll-1-1)) 0 0 5500 5500 1 1..77 6677 3333 2 2..77 7755 2255 3 3..44 8800 2200 4 4..00 8833 1177 5 5..99 9911 99 7 7..55 9955 55 1 111 9999 11 1 177 9999..99 00..11 2 233 9999..9999 00..0011 DISUBSTITUTED CYCLOHEXANE DISUBSTITUTED CYCLOHEXANE
The presence of two substituents on the ring of a cyclohexane allows for the The presence of two substituents on the ring of a cyclohexane allows for the possib
possibilitility y of of cis-cis-trantrans s isomeisomerism. rism. GeometGeometricarical l isomeisomerism rism in in variovarious us disubsdisubstituttituteded cyclohexane is discussed below,
cyclohexane is discussed below, 1,2- disubstituted cyclohexane : 1,2- disubstituted cyclohexane :
The planar representation of the cis and trans isomers of 1,2- disubstituted cyclohexane The planar representation of the cis and trans isomers of 1,2- disubstituted cyclohexane is as is as follows:-H H HH CH CH33 CHCH33 HH H H CH CH33 CH CH33 Ci
CIS FORM : CIS FORM :
The cis isomer has two identical e,a- and a,e- conformations. The cis isomer has two identical e,a- and a,e- conformations.
Consider cis-1,2- dimethyl cyclohexane, the two conformations are Consider cis-1,2- dimethyl cyclohexane, the two conformations are
CH CH33 CH CH33 HH33CCCHCH33 CHCH33 H H33CC eq,ax eq,ax
cis conformation has three butane gauche interactions. cis conformation has three butane gauche interactions.
TRANS FORM : TRANS FORM :
There are two possible chair conformations of trans-1,2- disubstituted cyclohexane. There are two possible chair conformations of trans-1,2- disubstituted cyclohexane. In one conformation, both the groups are axial; in the other both are equatorial.
In one conformation, both the groups are axial; in the other both are equatorial. The two chair conformations of 1,2- dimethyl cyclohexane are
The two chair conformations of 1,2- dimethyl cyclohexane are
CH CH33 CH CH33 ax, ax ax, ax CH CH33 CH CH33
The diaxial form of trans-isomer in the case of 1,2- dimethyl cyclohexane has four The diaxial form of trans-isomer in the case of 1,2- dimethyl cyclohexane has four but
butane ane gaugauche che inteinteractiractions ons whewhereas reas the the dieqdiequatouatorial form rial form has has onlonly y oneone, , that between thethat between the methyl
methyl groups. groups. Thus, in Thus, in the case of 1the case of 1,2- dimethy,2- dimethyl cyclohexanl cyclohexane, the dieque, the diequitorial trans isomitorial trans isomer iser is more stable than the cis isomer by about 1.8 kcal/mole.
more stable than the cis isomer by about 1.8 kcal/mole.
1.3 - disubstituted cyclohexane 1.3 - disubstituted cyclohexane
1,3- disubstituted cyclohexane exist in diastereomeric cis and trans forms whose 1,3- disubstituted cyclohexane exist in diastereomeric cis and trans forms whose pla
H H H H CH CH33 CH CH33 HH HH CH CH33 CH CH33 Ci
Cis-s-1,1,3-3-DiDimemeththylylcycyclclohohexexananee TranTrans-s-1,1,3-3-DiDimemeththylylcycyclclohohexexananee CIS FORM :
CIS FORM :
There are two possible conformations of CIS form. In one conformation, both the There are two possible conformations of CIS form. In one conformation, both the groups are axial whereas in the other, both are equatorial.
groups are axial whereas in the other, both are equatorial.
The two possible conformations of cis-1,3- dimethyl cyclohexane are given below. The two possible conformations of cis-1,3- dimethyl cyclohexane are given below.
CH CH33 CH CH33 ax, ax ax, ax CH CH33 CH CH33
The diequatorial form of cis-1,3-dimethyl cyclohexane is more stable than the diaxial The diequatorial form of cis-1,3-dimethyl cyclohexane is more stable than the diaxial conformation by abou
conformation by about 5.4 kcal/mole. t 5.4 kcal/mole. Thus diequatorial form is Thus diequatorial form is most preferred one.most preferred one.
TRANS FORM : TRANS FORM :
The trans isomer has two identical e,a and a, e conformations. The trans isomer has two identical e,a and a, e conformations.
Consider trans-1,3- dimethyl cyclohexane with identical conformations. Consider trans-1,3- dimethyl cyclohexane with identical conformations.
CH CH33 CH CH33 ax, eq ax, eq CH CH33 CH CH33
The trans isomer of 1,3- dimethyl cyclohexane has two butane gauche interactions. The trans isomer of 1,3- dimethyl cyclohexane has two butane gauche interactions. Thus the cis isomer is more stable by about 1.8 kcal/mole than the trans isomer. Thus the cis isomer is more stable by about 1.8 kcal/mole than the trans isomer.
1.4 disubstituted cyclohexane : 1.4 disubstituted cyclohexane :
The planar representation of the cis-trans isomers of 1,4- disubstituted cyclohexane The planar representation of the cis-trans isomers of 1,4- disubstituted cyclohexane is as follows is as follows H H HH H H33CC CHCH33 H H H H CH CH33 Ci
Cis-s-1,1,4-4-DiDimemeththylylcycyclclohohexexananee TranTrans-s-1,1,4-4-DiDimemeththylylcycyclclohohexexananee H
H33CC
CIS FORM : CIS FORM :
The cis isomer has two identical e,a and a,e conformation. The cis isomer has two identical e,a and a,e conformation.
Consider the two identical conformations of cis-1,4- dimethyl cyclohexane. Consider the two identical conformations of cis-1,4- dimethyl cyclohexane.
CH CH33 CH CH33 ax, ax ax, ax CH CH33 CH CH33 TRANS FORM : TRANS FORM :
There are two possible conformations of trans form of 1,4- disubstituted There are two possible conformations of trans form of 1,4- disubstituted cyclohexane,
cyclohexane,
In one conformation both the groups are axial; in other both are equatorial. In one conformation both the groups are axial; in other both are equatorial. Consider the two conformations of trans-1,4- dimethyl cyclohexane. Consider the two conformations of trans-1,4- dimethyl cyclohexane.
CH CH33 ax, ax ax, ax CH CH33 CH CH33 H H33CC
The diequatorial conformation is more stable
The diequatorial conformation is more stable than diaxial conformation and it representsthan diaxial conformation and it represents the structure of at least 99% of the molecules at equilibrium. The diaxial conformation has four the structure of at least 99% of the molecules at equilibrium. The diaxial conformation has four gauche interactions.
Conformations and Energies of the Dimethylcyclohexanes Conformations and Energies of the Dimethylcyclohexanes
IIssoommeerr CCoonnffoorrmmaattiioonn NoNo. . oof f ggaauucchhee IInntteerraaccttiioonn iinntteerraaccttiioonnss kkccaall..//mmoollee e,a e,a cciiss--11,,22 33 22..77 a,e a,e ee,,ee 11 00..99 trans-1,2 trans-1,2 aa,,aa 44 33..66 aa,,aa 44 55..44 cis-1,3 cis-1,3 ee,,ee 00 00 e,a e,a ttrraannss--11,,33 22 11..88 a,e a,e e,a e,a cciiss--11,,44 22 11..88 a,e a,e ee,,ee 00 00 trans-1,4 trans-1,4 aa,,aa 44 33..66 PROBLEM : PROBLEM :
a) Write structural formulas for the two chair conformations of cis-1-isopropyl-4-methyl a) Write structural formulas for the two chair conformations of cis-1-isopropyl-4-methyl cyclohexane.
cyclohexane. b) Are these two b) Are these two conformations equivaconformations equivalent ? c) If not, which wolent ? c) If not, which would beuld be more stable? d) Which would be the preferred conformation at equilibrium?
more stable? d) Which would be the preferred conformation at equilibrium? SOLUTION : SOLUTION : CH CH33 CH CH33 CH CH CH CH33 CH CH33 C CHH CCHH33 CH CH33
PRO
PROBLEM BLEM ::
a) Write the two conformations of cis-1,2-dimethylcyclohexane. b) Would these two a) Write the two conformations of cis-1,2-dimethylcyclohexane. b) Would these two conformations have equal potential energy? c) What about the two conformations of conformations have equal potential energy? c) What about the two conformations of cis-1-terta-butyl-2-methylcyclohexane? d) Would the two conformations of cis-1-terta-butyl-2-methylcyclohexane? d) Would the two conformations of trans-1,2-dimethylcyclohexane have the same potential energy?
dimethylcyclohexane have the same potential energy?
a) a) CH CH33 CH CH33 CH CH33 H H33CC b) b) YesYes c) c) C(CH C(CH33))33 CH CH33 H H33CC C(CH C(CH33))33 d) d) CH CH33 H H33CC CH CH33 CH CH33
CONFORMATIONAL EFFECTS ON STABILITY CONFORMATIONAL EFFECTS ON STABILITY
The free energies of acyclic diastereoisomers usually differ. Generally meso forms are The free energies of acyclic diastereoisomers usually differ. Generally meso forms are more stable than d
more stable than dll pairs. pairs. This is illustrated by considThis is illustrated by considering meso isomer and its active ering meso isomer and its active diastereodiastereo isomer in their most stable conformations.
isomer in their most stable conformations.
L L S S L L S S
Most stable conformation of meso and active isomers Most stable conformation of meso and active isomers
M M L L L L S S MM S S M M M M M
Meessoo AAccttiivvee
L denotes the largest substituent, M the medium sized substituent, and S the small L denotes the largest substituent, M the medium sized substituent, and S the small substituent in the two isomers.
The gauche interactions observed in the meso form are The gauche interactions observed in the meso form are
2L-M+2L-S+2M-S+2L-M+2L-S. 2L-M+2L-S+2M-S+2L-M+2L-S. The gauche interactions observed in the active form are
The gauche interactions observed in the active form are 2M-S+2L-S+M-M+S-S. 2M-S+2L-S+M-M+S-S. T
The he ddififfefererencnce be betetweween en ththe te two wo foformrms is iss 2M-2M-S-(S-(M-MM-M+S-+S-S).S).
In general, the crossed steric interactions between two groups of unequal size are In general, the crossed steric interactions between two groups of unequal size are less than the sum of interactions between the groups of like size i.e.(M-M+S-S)> 2M-S, provided less than the sum of interactions between the groups of like size i.e.(M-M+S-S)> 2M-S, provided that interactions are purely steric in origin.
that interactions are purely steric in origin.
Thus it is follows from the above discussion that meso isomer is more stable than Thus it is follows from the above discussion that meso isomer is more stable than the active isomer.
the active isomer.
GEOMETRICAL ISOMERISM GEOMETRICAL ISOMERISM
Geometrical isomerism is another type of stereoisomerism arising out of different spatial Geometrical isomerism is another type of stereoisomerism arising out of different spatial arrangement of groups attached to double bonds or rings in which stereoisomers are not readily arrangement of groups attached to double bonds or rings in which stereoisomers are not readily interconvertible.
interconvertible.
GEOMETRICAL ISOMERS : GEOMETRICAL ISOMERS :
Geometrical isomers are stereoisomers which differ in spatial arrangement of atoms Geometrical isomers are stereoisomers which differ in spatial arrangement of atoms or groups attached to double bonds or rings and this phenomenon is known as geometrical or groups attached to double bonds or rings and this phenomenon is known as geometrical isomerism. isomerism. CH CH33 CHCH33 C C CC H H HH CH CH33 C C CC H H H H CH CH33 Geometrical isomers Geometrical isomers H H HH C Cll CCll HH H H Cl Cl Cl Cl
RESTRICTED ROTATION AND THE DOUBLE BOND RESTRICTED ROTATION AND THE DOUBLE BOND
Substituents attached to
Substituents attached to the C = the C = C C can’t rotate freely since there can’t rotate freely since there is large energyis large energy bar
barrierier r to to rotrotatioation n assoassociaciated with ted with the grouthe groups ps joijoined by ned by a a doudouble bondble bond..
REASON FOR HINDERED ROTATION : REASON FOR HINDERED ROTATION :
The C = C double bond consist of a
The C = C double bond consist of a
π
π
bond and a bond and aσ
σ
bond; it is difficult to rotate bond; it is difficult to rotate the substituents 180the substituents 18000, since the, since the
π
π
bond must be broken, a reaction which requires about bond must be broken, a reaction which requires about 264264 kJ/mol of kJ/mol of energy. energy. Such a Such a rotation will rotation will seldom happen seldom happen at room at room temperature. temperature. TheThe inability of an olefinic double bond to rotate at room temperature is called hindered rotation. inability of an olefinic double bond to rotate at room temperature is called hindered rotation.
GEOMETRIC ISOMERISM IN OLEFINS GEOMETRIC ISOMERISM IN OLEFINS
There is hindered rotation about any carbon-carbon double bond but not all show There is hindered rotation about any carbon-carbon double bond but not all show geometric isomerism.
geometric isomerism. Geometric isomerism Geometric isomerism is only is only observed whobserved when there is en there is a certaina certain relationship among the groups attached to the doubly bonded carbons.
relationship among the groups attached to the doubly bonded carbons.
The requirement for geometric isomerism is shown in following olefin. The requirement for geometric isomerism is shown in following olefin.
Y Y AA C C CC Z Z BB
The requirement for geometric isomerism is A and B must be different groups, as The requirement for geometric isomerism is A and B must be different groups, as must Y and Z; However, either A or B can be same as Y or Z.
must Y and Z; However, either A or B can be same as Y or Z.
Thus, on this basis, we find that propylene, 1-butene and isobutylene do not show Thus, on this basis, we find that propylene, 1-butene and isobutylene do not show isomerism. isomerism. C C C C H H HH CH CH33 HH C C C C H H HH Et Et HH C C C C H H HH CH CH33 CHCH33 No geometric No geometric isomerism isomerism P
Prrooppyylleennee 11--BBuutteennee IIssoobbuuttyylleennee
Geometric isomerism can’t exist if either carbon carries two identical groups. Geometric isomerism can’t exist if either carbon carries two identical groups.
C C C C d d o isomerism o isomerism c c
CIS-TRANS ISOMERS CIS-TRANS ISOMERS
The prefixes cis and trans work well to specify the geometric isomers. The prefixes cis and trans work well to specify the geometric isomers. • CIS ISOMER :
• CIS ISOMER :
The geometric isomer in which similar groups are present on the same side of the The geometric isomer in which similar groups are present on the same side of the double bond is refererd as cis isomer.
double bond is refererd as cis isomer.
C C C C Cl Cl HH H H Cl Cl Cis-1,2-dichloroethene Cis-1,2-dichloroethene • TRANS ISOMER : • TRANS ISOMER :
The geometric isomer in which similar groups are present on the opposite side of The geometric isomer in which similar groups are present on the opposite side of the double bond is referred as trans isomer.
the double bond is referred as trans isomer.
C C C C Cl Cl HH H H ClCl trans-1,2-dichloroethene trans-1,2-dichloroethene
DRAWBACKS OF CIS-TRANS NOMENCLATURE DRAWBACKS OF CIS-TRANS NOMENCLATURE
Cis-trans nomenclature fails to specify the configurations of following compounds. Cis-trans nomenclature fails to specify the configurations of following compounds.
C C C C H H33CC HH Br Br C C C C H H Cl Cl Br Br C Cll H H33CC C C C C Cl Cl Cl Cl Br Br H H C C C C Cl Cl Cl Cl Br Br H H E-Z
E-Z SYSTEM SYSTEM OF OF NOMENCLATURENOMENCLATURE
E-Z system of nomenclature for geometric isomers have been developed after E-Z system of nomenclature for geometric isomers have been developed after Cahn-Ingold-Prelog convention for chiral carbon atoms.
Ingold-Prelog convention for chiral carbon atoms.
In order to assign E-Z nomenclature to geometric isomer : In order to assign E-Z nomenclature to geometric isomer : 1)
1) The tThe two gwo grouroups atps attachtached to ed to eaceach carh carbobon of tn of the dhe doubouble bole bond arnd are arre arranganged in ed in ordorder of er of prio
priorityrity.. 2)
2) If thIf the two e two grogroups oups of higf highest hest priprioriority arty are toge togethether on er on the sthe same same side oide of the f the doudouble bble bondond then the configuration is called as the Z isomer.
Br Br CHCH33 C C CC Cl Cl HH Z Z 1 1 11 2 2 22 Together Together (Z)-1-bromo-1-chloropropene (Z)-1-bromo-1-chloropropene Br Br CH CH33 C C CC Cl Cl H H E E 1 1 1 1 2 2 2 2 Opposite Opposite (E)-1-bromo-1-chloropropene (E)-1-bromo-1-chloropropene H H33CC CHCH33 C C CC H H HH CH CH33 CH CH33 H H H H > > > > H H33CC CH CH33 C C CC H H H H
((ZZ))--22--BBuutteennee ((EE))--22--BBuutteennee CH CH33 CH CH33 F F H H > > > > Br Br F F C C CC H H Cl Cl Z Z Br Br H H C C CC Cl Cl (I) (I) CH CH33 Br Br HH C C CC Cl Cl (II) (II) CH CH33 H H C C CC (III) (III) CH CH33 CCHH C22CHH22CHCH33 CH CH22CHCH33
When an alkene has more than one double bond, the stereochemistry about each When an alkene has more than one double bond, the stereochemistry about each double bond can be specified with E and Z nomenclature.
double bond can be specified with E and Z nomenclature.
Br Br 1 1 1 1 1 1 3-bromo-(3Z,5E)-octadiene 3-bromo-(3Z,5E)-octadiene
The E-Z nomenclature can
The E-Z nomenclature can also be used to also be used to designate cyclic compounds. designate cyclic compounds. When the twoWhen the two higher priority groups are on the same side of the ring then the compound is called as Z isomer higher priority groups are on the same side of the ring then the compound is called as Z isomer and when these groups are on opposite sides of the ring, the compound is called as E isomer. and when these groups are on opposite sides of the ring, the compound is called as E isomer.
H H HH CH CH33 HH H H CH CH33 CHCH33 CH CH33 Z-1
The Z or E isomers are not necessarily always the ones which woul
The Z or E isomers are not necessarily always the ones which would be called cisd be called cis or trans isomers respectively under the old nomenclature because E and Z nomenclature or trans isomers respectively under the old nomenclature because E and Z nomenclature depends on the priorities of the groups attached to the unsaturated carbon atoms. depends on the priorities of the groups attached to the unsaturated carbon atoms.
Cl Cl ClCl C C CC H H Br Br Cl Cl Br Br H H Cl Cl > > > > Cl Cl Cl Cl C C CC H H Br Br (E)-1-Bromo-1,2-dichloroethene (E)-1-Bromo-1,2-dichloroethene (cis-1-bromo-1,2-dichloroethene) (cis-1-bromo-1,2-dichloroethene) (Z)-1-Bromo-1,2-dichloroethene (Z)-1-Bromo-1,2-dichloroethene (trans-1-bromo-1,2-dichloroethene) (trans-1-bromo-1,2-dichloroethene)
OLEFINS AND CHIRALITY OLEFINS AND CHIRALITY
The compounds which show cis-trans isomerism with one double bond are not The compounds which show cis-trans isomerism with one double bond are not chiral because the four groups are in one plane.
chiral because the four groups are in one plane.
When the compound contains odd number of cumulative double bond When the compound contains odd number of cumulative double bond (three,five,etc) then orbital overlap causes the four groups to occupy one plane and hence (three,five,etc) then orbital overlap causes the four groups to occupy one plane and hence cis-trans isomerism is observed in such compounds.
cis-trans isomerism is observed in such compounds.
When the compound contains even number of cumulative double bonds and when When the compound contains even number of cumulative double bonds and when the both sides are disymmetric then optical activity is possible in such compounds.
the both sides are disymmetric then optical activity is possible in such compounds.
MULTIPLE DOUBLE BONDS : MULTIPLE DOUBLE BONDS :
If a molecule has more than one double bond, each substituted properly so as to If a molecule has more than one double bond, each substituted properly so as to give geometrical isomerism, then the number of possible geometric isomers of it will be give geometrical isomerism, then the number of possible geometric isomers of it will be
2
2nn. . Thus fouThus four geometric r geometric isomers should isomers should exist if thexist if there are two ere are two such doubsuch double bonds. le bonds. The four The four isomers of 5-cyclohexyl-2,4-pentadien-1-0
isomers of 5-cyclohexyl-2,4-pentadien-1-0ll are shown below; these are designated as are shown below; these are designated as trans-trans,cis-cis-,trans-cis,and cis-trans. trans-trans,cis-cis-,trans-cis,and cis-trans. H H C C CC H H H H C C CC H H CHCH22OHOH H H C C CC H H H H C C CC H H CH CH22OHOH T
H H C C CC H H H H C C CC H H CH CH22OHOH H H C C CC H H H H C C CC H H CHCH22OHOH T
Trraannss--cciiss CCiiss--ttrraannss
Geometric isomers of 5-cyclohexyl-2,4-pentadien-1-0 Geometric isomers of 5-cyclohexyl-2,4-pentadien-1-0ll Vitamin A has five double bonds thus the total number of possible isomers is 2 Vitamin A has five double bonds thus the total number of possible isomers is 255=32.=32.
PROPERTIES OF GEOMETRICAL ISOMERS PROPERTIES OF GEOMETRICAL ISOMERS
A pair of geometric isomers can be referred as diast
A pair of geometric isomers can be referred as diastereomers. ereomers. Thus as far as chemicalThus as far as chemical and physical properties are concerned, geometric isomers show the same relationship to and physical properties are concerned, geometric isomers show the same relationship to each other as do the other diastereomers.
each other as do the other diastereomers. Chemical Properties :
Chemical Properties :
The chemical properties of geometrical isomers are not identical, however, since their The chemical properties of geometrical isomers are not identical, however, since their structures are neither identical nor mirror images; they react with the same reagents, but at different structures are neither identical nor mirror images; they react with the same reagents, but at different rates.(Under certain conditions-especially in biological systems-geometrical isomers can very rates.(Under certain conditions-especially in biological systems-geometrical isomers can very widely in their chemical behaviour).
widely in their chemical behaviour). Physical Properties :
Physical Properties :
Geometrical isomers have different physical properties such as melting points, Geometrical isomers have different physical properties such as melting points, boi
boiling ling poipoints, nts, refrarefractivctive e indiindices, ces, solusolubilibilities, ties, densdensities ities etc.etc.
They can be distinguished from each other on the basis of their physical properties. They can be distinguished from each other on the basis of their physical properties. On the basis of the differences in physical properties they can be separated.
On the basis of the differences in physical properties they can be separated.
INTERCONVRSION OF GEOMETRICAL ISOMERS INTERCONVRSION OF GEOMETRICAL ISOMERS
The most straight forward way of interconverting geometrical isomers is by heating. The most straight forward way of interconverting geometrical isomers is by heating. The cis and tran
The cis and trans isomers can be intercos isomers can be interconverted nverted at higher temperatures oat higher temperatures or by irradiationr by irradiation with light of suitable wavelength.
with light of suitable wavelength.
The interconversion of isomers involves the breaking of the
The interconversion of isomers involves the breaking of the
π
π
bond bond of of the the carbon- carbon-carbon double bond followed by rotation about the carbon-carbon-carbon-carboncarbon double bond followed by rotation about the carbon-carbon
σ
σ
bond and subsequent bond and subsequent reformation of a newOPTICAL ISOMERISM OPTICAL ISOMERISM
ORDINARY AND PLANE POLARIZED LIGHT ORDINARY AND PLANE POLARIZED LIGHT
The nature of light is such that no purely verbal description can adeqately represent The nature of light is such that no purely verbal description can adeqately represent all of its properties. However one of the oldest and most successful attempts at a description all of its properties. However one of the oldest and most successful attempts at a description of this phenomenon treats light as a form of energy which is transmitted in waves.
of this phenomenon treats light as a form of energy which is transmitted in waves. When an ordinary light is passed through a Nicol pr
When an ordinary light is passed through a Nicol prism, it is converted into plane polaism, it is converted into plane polarizedrized light.
light. Plane polarized lighPlane polarized light can be defined as tht can be defined as the light whose waves ve light whose waves vibrate in one directioibrate in one directionn (plane).
(plane).
When ordinary light from a source with an infinite number of planes is passed through a When ordinary light from a source with an infinite number of planes is passed through a Ni
Nicocol pl prirismsm, o, onlnly a y a sisingngle le plplanane ie is as allllowowed ed to to ememerergege..
O
Orrddiinnaarry y lliigghhtt PPllaanne e ppoollaarriizzeed d lliigghhtt
OPTICAL ACTIVITY OPTICAL ACTIVITY
Certain substances have ability to rotate the plane of polarized light Certain substances have ability to rotate the plane of polarized light
are called as optically active compounds and this phenomenon of rotating the plane of are called as optically active compounds and this phenomenon of rotating the plane of po
polarlarizeized d liglight ht is is calcalled led opopticticallally y actactiviivity.ty.
If the substance does not rotate the plane of polarized light, it is considere
If the substance does not rotate the plane of polarized light, it is considered to be opticallyd to be optically inactive.
inactive.
DEXTRO AND LAEVO ROTATORY SUBSTANCES DEXTRO AND LAEVO ROTATORY SUBSTANCES
Optically active compound may rotate the plane of polarized light to the right or to the Optically active compound may rotate the plane of polarized light to the right or to the left.
left.
The substances which rotate the plane of plane polarized light to the right are called as The substances which rotate the plane of plane polarized light to the right are called as dextrorotatory and are designated as d or (+).
dextrorotatory and are designated as d or (+).
The substances which rotate the plane of plane polarized light to the The substances which rotate the plane of plane polarized light to the left are called as laevorotatory and are designated as l (–).
