DIFFERENTIAL REACTIVITY DIFFERENTIAL REACTIVITY

5) DIFFERENTIAL REACTIVITY

Since enantiomers react with chiral compounds at different rates, it may be possible Since enantiomers react with chiral compounds at different rates, it may be possible to effect a partial separation by stopping the reaction before it goes to completion.

to effect a partial separation by stopping the reaction before it goes to completion.

R & S NOMENCLATURE R & S NOMENCLATURE

The R, S convention is governed by three rules which are quite logical and easy to The R, S convention is governed by three rules which are quite logical and easy to remember; these rules set a priority sequence for the four substituents, A, B, D, E, about an remember; these rules set a priority sequence for the four substituents, A, B, D, E, about an asymmetric carbon.

asymmetric carbon.

Rule-1 :

Rule-1 : The priorities assigned to the four substituents on an asymmetric carbon depend The priorities assigned to the four substituents on an asymmetric carbon depend upon the atomic number of the attached atom. Greater the atomic number of atom, greater upon the atomic number of the attached atom. Greater the atomic number of atom, greater is the priority. [For example, the halogens have the priority I > Br > Cl > F ; other obvious is the priority. [For example, the halogens have the priority I > Br > Cl > F ; other obvious pri

prioriorities woties would be Cl > O uld be Cl > O > N > N > C.]> C.]

Let us now consider the configurational assignment for bromochlorofluoromethane, Let us now consider the configurational assignment for bromochlorofluoromethane, which has the atomic priorities Br > Cl > F > H. Visualize the molecule in such a way that which has the atomic priorities Br > Cl > F > H. Visualize the molecule in such a way that the atom with lowest priority projects behind the paper ; the remaining three substituents the atom with lowest priority projects behind the paper ; the remaining three substituents are arranged in order of priority either clockwise or counterclockwise as follows

are arranged in order of priority either clockwise or counterclockwise as follows Cl

Cl C C Br

Br FF H H

Cl Cl C C

Br F Br

H H

Clloocckkwwiissee CCoouunntteerrcclloocckkwwiissee ((RR) ) ccoonnffiigguurraattiioonn ((SS) ) ccoonnffiigguurraattiioonn

The clockwise direction is assigned the R configuration (Latin: rectus, right) and The clockwise direction is assigned the R configuration (Latin: rectus, right) and the counterclockwise direction is assigned the S configuration (Latin: sinister, left).

the counterclockwise direction is assigned the S configuration (Latin: sinister, left).

Rule–2 :

Rule–2 : When two or more substiuents on the asymmetric carbon have atoms with the When two or more substiuents on the asymmetric carbon have atoms with the same atomic number, then Rule 2 applies; precedence is given to the substituent with the same atomic number, then Rule 2 applies; precedence is given to the substituent with the highest atomic number in the second atom from the asymmetric center.

highest atomic number in the second atom from the asymmetric center.

For example, with sec-butyl alcohol, two of the adjacent atoms are C. However, For example, with sec-butyl alcohol, two of the adjacent atoms are C. However, the C of -CH

the C of -CH₂₂-CH-CH₃₃ is bonded to C, whereas the C of -CH is bonded to C, whereas the C of -CH₃₃ is bonded only to H; therefore, is bonded only to H; therefore, -CH

-CH₂₂-CH-CH₃₃ takes priority order for sec-buty1 alcohol is OH > C takes priority order for sec-buty1 alcohol is OH > C₂₂HH₅₅ > > CHCH₃₃ > > H.H.

C C OH OH

H H

CHCH33CHCH22 CHCH33

sec

sec-Butyl alcohol-Butyl alcohol Rule–3 :

Rule–3 : If the second atom from the asymmetric center has an identical atomic number in two If the second atom from the asymmetric center has an identical atomic number in two groups, then the total of the atomic numbers attached to the first carbon are c

groups, then the total of the atomic numbers attached to the first carbon are considered. With thisonsidered. With this rule a double bond, -C=A, is treated as -CA

rule a double bond, -C=A, is treated as -CA₂₂; for example, -C=O would be treate; for example, -C=O would be treated asd as ^C^C O O O O..

Similarly a triple bond, C

Similarly a triple bond, C ≡≡ A, would be treated as -CA A, would be treated as -CA₃₃..

With this rule, the carboxy1 (-COOH) with combined atomic numbers of 48 would With this rule, the carboxy1 (-COOH) with combined atomic numbers of 48 would take precedence over an aldehyde (-CH=O) with combined atomic numbers of 33; similarly take precedence over an aldehyde (-CH=O) with combined atomic numbers of 33; similarly -CH=O would take precedence over -CH

-CH=O would take precedence over -CH₂₂OH (combined atomic numbers = 18). Thus inOH (combined atomic numbers = 18). Thus in the case of glyceraldehyde, the priorities are

the case of glyceraldehyde, the priorities are – OH > -CH=O > CH

– OH > -CH=O > CH₂₂OH > H.OH > H.

The phenyl substituent is treated as -C-C

The phenyl substituent is treated as -C-C₃₃ with combined atomic numbers of 36; thus with combined atomic numbers of 36; thus ph

phenyenyl wol would uld taktake pre prioriority ity ovover ier isopsopropropy1y1, -C, -CH(CH(CHH₃₃))₂₂, with combined atomic numbers of 25., with combined atomic numbers of 25.

By applying these rules to some common substituents, one obtains the following sequence By applying these rules to some common substituents, one obtains the following sequence (group of highest priority first): I, Br, Cl, SO

(group of highest priority first): I, Br, Cl, SO₂₂R, SOR, SR, SH, F, OCOR, OR, OH, NOR, SOR, SR, SH, F, OCOR, OR, OH, NO₂₂,, NHCOR, NR

NHCOR, NR ₂₂, NHR, NH, NHR, NH₂₂, CCI, CCI₃₃, COCI, CO, COCI, CO₂₂R, CONHR, CONH₂₂, COR, CHO, CR , COR, CHO, CR ₂₂OH, CHOHR,OH, CHOHR, CH

CH₂₂OH, CR OH, CR ₃₃, C, C₆₆HH₅₅, CHR , CHR ₂₂, CH, CH₂₂R, CHR, CH₃₃, D, H., D, H.

CHO CHO C C HO

HO CHCH22OHOH H H

COOH COOH C C

HH33CC NH NH22 H H R

R-Glyceraldehyde-Glyceraldehyde

S - Alanine S - Alanine

If more than one asymmetric carbon is present, then each configuration is specified, If more than one asymmetric carbon is present, then each configuration is specified,

C C C C

OH OH COOH COOH

HOOC HOOC

H H

H H HO HO

2( R R), (), ( R R)-Tartaric acid)-Tartaric acid

COCO22HH C C C C COCO22HH

OH OH H

H H HO

CHO CHO C C

C C

CHCH22OHOH OH OH H

H HO H

C C

OH OH H

2((RR)), 3, 3((RR))--DDiihhyyddrrooxxyy 22((SS)), , 33((RR)), , 44((RR)), , 5 5--ssuucccciinniic c aacciidd TTeettrraahhyyddrrooxxyy- - ppeennttaannaall

H H

OH OH

HH33CC

H H

CHCH33 C

Cyycclloohheexxaannedediiooll--11((RR)),, 11((SS)),,33((SS))--DDiimmeetthhyyll 2

2((SS) ) oor r mmeessoo-- ccyycclloohheexxaannee cyclohexanediol-1,2

cyclohexanediol-1,2

STEREOCHEMISTRY STEREOCHEMISTRY

QEUSTIONS ASKED IN SET EXAMINATIONS QEUSTIONS ASKED IN SET EXAMINATIONS

1) NatNaturaural chl choleolestesterol rol has has 8 c8 chirhiral cal cententresres. It . It can can thetherefrefore ore exiexists sts as 2as 2⁸⁸ i.e. 256 possible i.e. 256 possible isomer. The number of diastereomers of natural cholesterol are

isomer. The number of diastereomers of natural cholesterol are

Chapter No. 7 : Concept of Chirality (Stereochemistry)

Chapter No. 7 : Concept of Chirality (Stereochemistry) 5858 4)

4) In thIn the resoe resolutlution oion of 1-pf 1-phenhenyleylethythylamlamine uine usinsing [S] - (g [S] - (–) ma–) maleileic acidc acid, the c, the compompounound obd obtaintaineded by

by rerecrcrysystatallllizizatatioion n of of ththe e mimixtxturure e of of ththe e didiasasteterereomomereric ic sasalt lt is is [R[R] ] - - 1 1 - - phphenenylyletethyhyl l amammomoniniumum - [S] - maleate.

- [S] - maleate. The other component of the mixture is more soluble & remains The other component of the mixture is more soluble & remains in solution.in solution.

The configuration of the more soluble salt The configuration of the more soluble salt

is-A) [S] - 1- phenylethylammonium - [S] - maleate A) [S] - 1- phenylethylammonium - [S] - maleate B) [S] - 1 - phenylethylammonium - [R] - maleate B) [S] - 1 - phenylethylammonium - [R] - maleate C) [R] - 1 - phenylethylammonium - [R] - maleate C) [R] - 1 - phenylethylammonium - [R] - maleate D) [R] -1 - phenylethylammonium - [S] - maleate D) [R] -1 - phenylethylammonium - [S] - maleate 5)

5) ThThe che chiriral mal mololececulule ame amonong tg the he fofollllowowining ag arere,,

CH33CH H

H H

H OHOH

OH OH

CH33CH CHCH33

O O

CHCH33 O O

((ii)) ((iiii)) ((iiiiii))

A) ) AAlll l tthhrreeee BB) ) ((iiii) ) & & ((iiiiii)) C

C) ) ((ii) ) & & ((iiii)) DD) ) OOnnlly y ((iiii)) 6)

6) ThThe coe confnfigigururatiation on of of ththe coe compmpouound nd shshowown bn beloelow iw is :s : CHO

CHO

OH OH CHCH22OHOH

OH OH H

H H H

A) ) 22SS, , 33SS BB) ) 22SS, , 33R R C

C) ) 22RR, , 33SS DD))22RR, , 33R R

7) ChChoooose thse the coe correrrect sct stattatemeement rnt regaegardirding tng the che compompououndnds shs showown ben belowlow::

CHCH33 Br

Br H H

H H

Br Br

CHCH33

C C C C H

H CHCH33

HOOC

HOOC HH

((II)) ((IIII))

A) ) BBootth h ccaan n bbe e rreessoollvveedd BB) ) BBootth h ccaan n nnoot t bbe e rreessoollvveedd

C) Compound (i) can be resolved but compound (II) can not be resolved as it has a plane C) Compound (i) can be resolved but compound (II) can not be resolved as it has a plane of symmetry passing through the olefinic carbons & also through all three substituents.

of symmetry passing through the olefinic carbons & also through all three substituents.

D) Compound (II) can be resolved but compound (I) can not be resolved as it has plane D) Compound (II) can be resolved but compound (I) can not be resolved as it has plane of symmetry after rotation of lower carbon by 120

of symmetry after rotation of lower carbon by 120^o^o..

8) ThThe ae absbsololutute ce cononfifiguguraratition on of of ththe ce comomp gp giviven en is is ::

C C H

H OHOH

CHCH22OHOH HH33CC

CHCH22OHOH

CC22HH55

A)) 22SS,, 33EE BB)) 22SS,, 33ZZ C

C))22RR,, 33EE DD)) 22RR,, 33ZZ 9)

9) ChChoooose tse the he cocorrrrecect ret relalatition on of of (i(ii) & i) & (i(iiiii) w) witith (h (i)i)

CHCH33

( ii ) ( ii ) (

( i )i )

CHCH33

C CHH33

( iii ) ( iii ) CHCH33

C CHH33

A) Both (ii) and (iii) are enantiomers of (i) A) Both (ii) and (iii) are enantiomers of (i) B) Both (ii) and (iii) are diastereomers of (i) B) Both (ii) and (iii) are diastereomers of (i)

10)

10) The The geogeometry metry of tof the twhe two do doubouble ble bondonds in s in the the givgiven men molecolecule ule isis

HH33CC

Cl Cl

Br Br

CHO CHO

COOH COOH

CHCH33

A)) 22ZZ,, 55ZZ BB)) 22ZZ,, 55EE C

C)) 22EE,, 55EE DD)) 22EE,, 55ZZ 11)

11) ChaChair forir form of cym of cycloclohexhexane haane has got ths got these maese many gany gauchuche inte interaceractiotions:ns:

A))66 BB))22

C))1122 DD)) 00

12)

12) MentMenthol hhol has thras three asymee asymmetrimetric centc centres. Hres. How maow many epny epimers imers (E) an(E) and diad diastereostereomersmers (D) are possible for it?

(D) are possible for it?

A) ) EE3 3 ; ; DD88 BB) ) EE3 3 ; ; DD66 C

C) ) EE1 1 : : DD88 DD) ) EE2 2 ; ; DD66 13)

13) In thIn the come compoupound snd showhown beln below thow the geoe geometry metry of thof the two e two doudouble bble bond ond is:is:

CHCH33 CHCH22ClCl

CHO CHO

CHCH22SHSH HH33COOCCOOC

CHCH22OHOH A

A) ) 22ZZ, , 55ZZ BB) ) 22ZZ, , 55EE C

C) ) 22EE, , 55EE DD) ) 22EE, , 55ZZ 14)

14) ββ-methyl-D-glucoside has five asymmetric centres. How many epimers (E) and-methyl-D-glucoside has five asymmetric centres. How many epimers (E) and diastereoisomers (D) are possible for this molecule ?

diastereoisomers (D) are possible for this molecule ? HO

HO HO

O O MeO Me HO

OH OH A

A) ) EE--5 5 ; ; DD--3300 BB) ) EE--5 5 ; ; DD--3311 C

C) ) EE--4 4 ; ; DD--3322 DD) ) EE--4 4 ; ; DD--3300

15)

15) The cThe chair hair form form of cyof cyclohexclohexane hane has lowas lower ener enthalpy thalpy than than boat boat form form because because the bthe boat hoat hasas 1

2 2

4 4

3 3 5 6 5

A) The bowsprit flag pole interaction.

B) Eclipsing interaction at C

B) Eclipsing interaction at C₁₁/C/C₂₂ and C and C₄₄/C/C₅₅

C) Both bowsprit flagpole interaction & eclipsing interactions C

C) Both bowsprit flagpole interaction & eclipsing interactions C₁₁/C/C₂₂ and C and C₄₄/C/C₅₅ D) Both bowsprit flagpole interactions & eclipsing interactions at C

D) Both bowsprit flagpole interactions & eclipsing interactions at C₂₂/C/C₃₃ and C and C₅₅/C/C₆₆..

16)

16) The sThe specpecific rific rotaotatiotion of a mn of a moleoleculcule in a 5 ce in a 5 cm pom polarilarimetmeter tuer tube is +be is +22^o^o. What will be its. What will be its rotation in 10 cm. tube?

rotation in 10 cm. tube?

A) -4

A) -4⁰⁰ B) 20B) 20⁰⁰

C) +4

C) +4⁰⁰ D) -2D) -2⁰⁰

17)) Select Select the apthe approppropriate sriate stereoctereochemichemical relaal relationstionship ahip among mong the fothe followillowing twng twoo compounds.

compounds.

CHCH33

OH OH

And And

CHCH33

OH OH A

A) ) SSttrruuccttuurraal l iissoommeerrss BB) ) CCiiss--ttrraanns s iissoommeerrss C

C) ) CCoonnffoorrmmaattiioonnaal l iissoommeerrss DD) ) R R & & S S iissoommeerrss..

18)

18) ThThe come compopounund shod shown bwn beloelow on tw on treareatmetment wnt with bith base wase willill CHCH33

CHCH33 CHCH33 H

H OO

A) ) UUnnddeerrggo o eeqquuiilliibbrriiuumm BB) ) BBe e rreeccoovveerreed d uunncchhaannggeedd C

C) ) UUnnddeerrggo o eeppiimmeerriissaattiioon n aat t CC--55 DD) ) UUnnddeerrggo o eeppiimmeerriissaattiioon n aat t CC--77..

19)

19) ChooChoose thse the corre correct coect confignfiguraturationaional noml nomenclenclature ature for thfor the fole followilowing mng molecuolecule:le:

CH CHOO C C

C C

CHCH22OHOH OH OH

H H

H HO H

HO CC

OH OH H

A) ) 22RR, , 33SS, , 44SS BB) ) 22SS, , 33RR, , 44SS C

C) ) 22RR, , 33SS, , 44RR DD) ) 22RR, , 33RR, , 44SS 20)

20) Axial mAxial methyl ethyl group group is less is less stable stable 1.8 k1.8 kcal/mol cal/mol than than the eqthe equatorial uatorial countcounterpart derpart due to ue to 1,3- 1,3-diaxial interactions between CH

diaxial interactions between CH₃₃ and H and H CHCH33 CHCH33

CHCH33

For the given molecule, the energy estimated will be, For the given molecule, the energy estimated will be, A

A) ) 33..6 6 kkccaall//mmooll BB) ) MMoorre e tthhaan n 33..6 6 kkccaall//mmooll C

C) ) 55..4 4 kkccaall//mmooll CC) ) MMoorre e tthhaan n 55..4 4 kkccaall//mmooll 21)

21) The The corcorrect rect statstatemeement rent regargardinding thg the come compopoundunds shs shown own isis

OH OH HOOC

HOOC H H

COOH COOH

OH OH

H H

HH33CC H H

CC22HH55 COOH COOH

((II)) ((IIII))

A) Both can be resolved A) Both can be resolved B) Both cannot be resolved B) Both cannot be resolved

C) Compound (I) can be but (II) can’t be resolved as it has plane of symmetry passing C) Compound (I) can be but (II) can’t be resolved as it has plane of symmetry passing through both olefinic carbons and all four substituent on these carbons.

through both olefinic carbons and all four substituent on these carbons.

D) Compound II can be but (I) can’t be resolved as it has plane of symmetry after D) Compound II can be but (I) can’t be resolved as it has plane of symmetry after rotation of lower carbon by 120

rotation of lower carbon by 120⁰⁰

22)

22) In thIn the coe compoumpound nd showshown thn the coe configunfiguration ration of tof the the three hree doubdouble ble bonds onds isis CH

CHOO

CHCH22 CHCH33

A) ) 22EE, , 66EE, , 99EE BB) ) 22ZZ, , 66ZZ, , 99EE C

C) ) 22EE, , 33ZZ, , 99EE DD) ) 22ZZ, , 66ZZ, , 99ZZ..

23)

23) PrePredicdict tht the ree relatilation on betbetween ween folfollowlowing ing two two paipairsrs

Ph Ph

H H H

O H

O H H

CHCH33

H H

&

^H^H

Ph Ph CHCH33

OH OH OH OH HO

HO HOHO

H H H H

& &

OH HO OH

HO Ph Ph

O O H H

H H

H H CHCH33 CHCH33

Ph Ph

((II)) ((IIII))

A) Both pairs are enantiomeric A) Both pairs are enantiomeric B) Both pairs are diastereomeric B) Both pairs are diastereomeric

C) Pair (i) is enantiomeric while pair (ii) is diastereomeric.

D) Pair (i) is diastereomeric while pair (ii) is enantiomeric.

24)

24) NatuNatural choral cholestelesterol on hrol on hydrydrogeogenatination affon affordords choles cholestanstanol as one ool as one of the prof the producduct.t.

H H

H HO H

H H

As this compound has nine asymmetric centre it can exists as 2

As this compound has nine asymmetric centre it can exists as 2⁹⁹ i.e.512 possible i.e.512 possible

25)

(ii) cis- 1, 2-dimethylcyclopropane (ii) cis- 1, 2-dimethylcyclopropane (iii) trans–1, 2- dimethylcyclopropane.

(iii) trans–1, 2- dimethylcyclopropane.

29)

29) ThThe core correcrect cont configfigururatiation oon of the df the dououble bble bononds at Cds at C₂₂, C, C₄₄ & & CC₆₆ in the following compound in the following compound is given

is given by-HH33CC

Ph Ph

Cl HH CHCH33

COOH COOH A

A) ) 22((ZZ)), , 44((ZZ)), , 66((EE)) BB) ) 22((ZZ)), , 44((EE)), , 66((ZZ)) C

C) ) 22((ZZ)), , 44((EE)), , 66((EE)) DD) ) 22((EE)), , 44((ZZ)), , 66((EE))

30)

30) ^Cl^Cl

Cl Cl Ph

Ph H H

Cl Cl Cl Cl CHCH33 H

CHCH33

can be represented by:

A) A)

CHCH33 HH33CC

Cl Cl

Ph Ph

Cl Cl

H B)B)

CHCH33 Cl

Cl Cl

Ph Ph

H H CHCH33

C) C)

CHCH33

HH33CC HH

Ph Ph Cl

Cl Cl

D) D)

CHCH33

H H

Cl Cl

Cl Cl Ph

Ph HH33CC

31)

31) WhiWhich ch one one of of the the folfollowlowing ing is is optopticalically ly inainactivctive?e?

A) A)

Cl HH

H ClCl

B) B)

Cl H Cl

H H Cl

C) C)

Cl Cl

H H H

Cl Cl

D) D)

Cl Cl

H H H

Cl Cl 32)

32) How maHow many steny steroisroisomeomers are posrs are possiblsible for estrae for estradiodiol havinl having the folg the followlowing string structuucture?re?

CHCH33 OH OH

H H

H H H

HO HO

H H

A)) 55 BB)) 3322

C))1166 DD)) 1100

33)

33) WhWhen en ooptpticicalally ly acactitivvee ^CH^CH33 ^(CH^(CH22))33 ^C^C ÔÔC^COÔC^CH^H33 CHCH22CHCH33

CHCH33

is hydrolysed with aqueous acid is hydrolysed with aqueous acid

it would give:

A) ) ((--) ) aallccoohhooll BB) ) ((++) ) aallccoohhooll C)

C) ((±±) ) aallccoohhooll DD) ) nn--ooccttaannooll

34)

34) ChooChoose the se the correct correct geomegeometrical rtrical relationelationship bship between etween variovarious grous groups in ups in the githe given mven moleculolecule:e:

Cl Cl Br

A) C1 and Br trans; Cl & i-Pr trans; Br & i-Pr trans A) C1 and Br trans; Cl & i-Pr trans; Br & i-Pr trans B) Br & i-Pr cis; Cl & i-Pr- trans; Br & Cl trans

B) Br & i-Pr cis; Cl & i-Pr- trans; Br & Cl trans C) Cl & Br - cis; Cl & i-Pr (trans); Br & i-Pr cis C) Cl & Br - cis; Cl & i-Pr (trans); Br & i-Pr cis D) Cl & Br - trans; Cl & i-Pr cis; Br & i-Pr cis.

D) Cl & Br - trans; Cl & i-Pr cis; Br & i-Pr cis.

35)

35) SelecSelect the cot the correct grrect geomeeometry otry of the twf the two douo double bble bondonds in ths in the folle followinowing molg moleculeecule::

CHHOO II

OCHOCH33

CHCH33 HH33CC

Cl Cl A

A)) 22ZZ,, 55ZZ BB)) 22EE,, 55EE C

C)) 22EE,, 55ZZ CC)) 22ZZ,, 55EE 36)

36) The sThe specipecific rofic rotatiotation of a n of a compcompounound mead measuresured in a d in a 5 cm p5 cm polarolarimeteimeter tubr tube is +2e is +2⁰⁰ atat the given concentration, temperature & wavelength. Its specific rotation measured the given concentration, temperature & wavelength. Its specific rotation measured in 10 cm tube under similar condition will be

in 10 cm tube under similar condition will be A

A)) ++44°° BB))--44°°

C)) --22°° DD)) ++22°°

STEREOCHEMISTRY STEREOCHEMISTRY

Answer Key Answer Key

1) ) CC 22) ) CC 33) ) BB 44) ) AA 55))DD 66) ) AA 77) ) CC 8

8) ) AA 99) ) CC 1100) ) CC 1111) ) DD 1122) ) AA 1133) ) CC 1144) ) AA 1

155) ) DD 1166) ) DD 1177) ) BB 1188) ) CC 1919) ) CC 2200) ) CC 2211) ) CC 2

222) ) BB 2233) ) CC 2244) ) CC 2255) ) CC 2266) ) DD 2727) ) DD 2288) ) BB 2

299) ) AA 3300) ) DD 3311) ) CC 3322) ) BB 3333) ) CC 3344) ) DD 3355) ) CC

In document Stereochemistry (Page 53-67)