End 9/26, start 9/28 after this slide

(1)

End 9/26, start 9/28 after

this slide

(2)

Racemic Products

If optically inactive reagents combine to form a chiral

molecule, a racemic mixture is formed.

(3)

Optical Purity

a) Optical purity (o. p.) is sometimes called enantiomeric excess (e. e.).

b) One enantiomer is present in greater amounts.

observed rotation

rotation of pure enantiomer

× 100 o. p. =

(4)

Chirality of Conformers

a) If equilibrium exists between two chiral conformers, the molecule is not chiral.

b) Judge chirality by looking at the most symmetrical conformer.

c) Cyclohexane can be considered to be planar, on average.

(5)

Chirality of Conformational Isomers

The two chair conformations of cis-1,2-dibromocyclohexane are nonsuperimposable, but the interconversion is fast and the molecules are in equilibrium. Any sample would be racemic and, as such, optically inactive.

(6)

Fischer Projections

a) Flat representation of a 3-D molecule

b) A chiral carbon is at the intersection of horizontal and vertical lines.

c) Horizontal lines are forward, out of plane.

d) Vertical lines are behind the plane.

(7)

Fischer Projections (Continued)

(8)

Fischer Rules

a) Rotation of 180 ° in plane doesn’t change the molecule.

b) Rotation of 90 ° is not allowed.

(9)

180 ° Rotation

a) A rotation of 180° is allowed because it will not change the configuration.

(10)

90 ° Rotation

a) A 90° rotation will change the orientation of the horizontal and vertical groups.

b) Do not rotate a Fischer projection 90°.

(11)

Glyceraldehyde

a) The arrow from group 1 to group 2 to group 3 appears counterclockwise in the Fischer projection. If the

molecule is turned over so the hydrogen is in back,

the arrow is clockwise, so this is the (R) enantiomer of

glyceraldehyde.

(12)

Fischer Mirror Images

a) Fisher projections are easy to draw and make it easier to find enantiomers and internal mirror planes when the

molecule has two or more chiral centers.

CH

₃

H Cl

Cl H

CH

₃

(13)

Fischer (R) and (S)

a) Lowest priority (usually H) comes forward, so assignment rules are backward!

b) Clockwise 1-2-3 is (S) and counterclockwise 1-2-3 is (R).

c) Example:

(14)

Diastereomers: Cis-Trans Isomerism on Double Bonds

a) These stereoisomers are not mirror images of each other, so they are not enantiomers. They are diastereomers.

(15)

Diastereomers: Cis-Trans Isomerism on Rings

a) Cis-trans isomers are not mirror images, so these are diastereomers.

(16)

Diastereomers

a) Molecules with two or more chiral carbons b) Stereoisomers that are not mirror images

(17)

Two or More Chiral Carbons

a) When compounds have two or more chiral centers they have enantiomers, diastereomers, or meso

isomers.

b) Enantiomers have opposite configurations at each corresponding chiral carbon.

c) Diastereomers have some matching and some opposite configurations.

d) Meso compounds have internal mirror planes.

e) Maximum number of isomers is 2

ⁿ

, where n = the

number of chiral carbons.

(18)

(19)

Comparing Structures

(20)

Meso Compounds

a) Meso compounds have a plane of symmetry.

b) If one image is rotated 180°, then it can be superimposed on the other image.

c) Meso compounds are achiral even though they have chiral centers.

(21)

Number of Stereoisomers

a) The 2

ⁿ

rule will not apply to compounds that may

have a plane of symmetry. 2,3-dibromobutane has

only three stereoisomers: (±) diastereomer and the

meso diastereomer.

(22)

Properties of Diastereomers

a) Diastereomers have different physical properties, so they can be easily separated.

b) Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated.

c) Enantiomers are difficult to separate.

d) Convert enantiomers into diastereomers to be able to

separate them.

(23)

Diastereomers and Their

Physical Properties

(24)

Louis Pasteur

a) In 1848, Louis Pasteur

noticed that a salt of racemic (±)-tartaric acid crystallizes into mirror-image crystals.

b) Using a microscope and a pair of tweezers, he

physically separated the enantiomeric crystals.

c) Pasteur had accomplished the first artificial resolution of enantiomers.

(25)

Chemical Resolution of Enantiomers

React the racemic mixture with a pure chiral compound, such as tartaric acid, to form diastereomers, and then separate

them.

(26)

Formation of (R)- and

(S)-2-Butyl Tartrate

(27)

Chromatographic

Resolution of Enantiomers

(28)

Chapter 6 Lecture

Organic Chemistry, 9

^th

Edition

L. G. Wade, Jr.

Alkyl Halides;

Nucleophilic Substitution

(29)

Classes of Alkyl Halides

• Alkyl halides: Halogen is directly bonded to an sp

³

carbon.

• Vinyl halides: Halogen is bonded to an sp

²

carbon of alkene.

• Aryl halides: Halogen is bonded to an sp

²

carbon on a benzene ring.

C C H

H

H Cl vinyl halide C

H H

H

C H

H

Br

alkyl halide

I

aryl halide

(30)

Polarity and Reactivity

• Halogens are more electronegative than C.

• Carbon–halogen bond is polar, so carbon has partial positive charge.

• Carbon can be attacked by a nucleophile.

• Halogen can leave with the electron pair.

(31)

IUPAC Nomenclature

• Name as a haloalkane.

• Choose the longest carbon chain, even if the halogen is not bonded to any of those carbons.

• Use lowest possible numbers for position.

3

1 2 4

2-chlorobutane 4-(2-fluoroethyl)heptane

1 2 3 4 5 6 7

1 2

(32)

Examples

1 2 3 4 5 6 7 8 9

6-bromo-2-methylnonane

cis-1-bromo-3-fluorocyclohexane

(33)

Systematic Common Names

• The alkyl groups is a substituent on halide.

• It is useful only for small alkyl groups.

isobutyl bromide sec-butyl bromide

tert-butyl bromide

(34)

Common Names of Halides

• CH

₂

X

₂

is called methylene halide.

• CHX

₃

is a haloform.

• CX

₄

is carbon tetrahalide.

• Common halogenated solvents:

– CH

₂

Cl

₂

is methylene chloride.

– CHCl

₃

is chloroform.

– CCl

₄

is carbon tetrachloride.

(35)

Alkyl Halides Classification

• Methyl halides: Halide is attached to a methyl group.

• Primary alkyl halide: Carbon to which halogen is bonded is attached to only one other carbon.

• Secondary alkyl halide: Carbon to which halogen is bonded is attached to two other carbons.

• Tertiary alkyl halide: Carbon to which halogen is

bonded is attached to three other carbons.

(36)

Primary, Secondary, and

Tertiary Alkyl Halides

(37)

Types of Dihalides

• Geminal dihalide: Two halogen atoms are bonded to the same carbon.

• Vicinal dihalide: Two halogen atoms are bonded

to adjacent carbons.

(38)

Uses of Alkyl Halides

• Industrial and household cleaners

• Anesthetics

– CHCl₃ was used originally as a general anesthetic but it is toxic and carcinogenic.

– CF₃CHClBr is a mixed halide and is sold as Halothane.

• Freons are used as refrigerants and foaming agents.

– Freons can harm the ozone layer, so they have been replaced by low-boiling hydrocarbons or carbon dioxide.

• Pesticides such as DDT are extremely toxic to insects but not as toxic to mammals.

– Haloalkanes can not be destroyed by bacteria, so they accumulate in the soil to a level that can be toxic to

mammals, especially humans.

(39)

Dipole Moments

• Electronegativities of the halides:

F > Cl > Br > I

• Bond lengths increase as the size of the halogen increases:

C—F < C—Cl < C—Br < C—I

• Bond dipoles

C—Cl > C—F > C—Br > C—I

1.56 D 1.51 D 1.48 D 1.29 D

• Molecular dipoles depend on the geometry of the molecule.

(40)

Dipole Moments and Molecular Geometry

Notice how the four, symmetrically oriented polar bonds of the carbon tetrahalides cancel to give a molecular dipole moment of zero.

(41)

Boiling Points

• Greater intermolecular forces, higher b. p.

– Dipole–dipole attractions are not significantly different for different halides.