Solutions to Problems Stereochemistry, Building Bridges to Knowledge

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SolutionstoProblemsStereochemistryBuildingBridgestoKnowledge

PhotooftheYangpuSuspensionBridgeinShanghai,China

1. Ameasuredmassofanopticallyactivecompoundisdissolvedin

anaproticsolventmakingatotalvolumeof50.0mL.Whenthissolutionisplacedina10.0cmpolarimetertube,theobservedrotationis+2.79o.Ifthespecificrotationis+93og-1dm-1mL,calculatethemassoftheopticallyactivesampleinthepolarimetertube.

+93o g−1 dm−1 mL = +2.79o

10.0 cm x 1 m100 cm

x10 dm1m

x x g50.0 mL

2

x=1.5g

2. Specifytheabsoluteconfigurationofthechiralcenters,ifany,in

thefollowingcompounds.Indicatewhetherthecompoundisopticallyactive.a.

S Opticallyactive Chiralcenter Themoleculeischiral

b.

Noplaneofsymmetry Themoleculeischiral Opticallyactive

+93o g−1 dm−1 mL x 1.0 dm x x g50.0 mL

= +2.79o

C

Br

CH2CH3H3C

H

CC CCH3

CH3H

H

3

c.

Planeofsymmetry Themoleculeisachiral Opticallyinactive

d.

R Opticallyactive Chiralcenter Themoleculeischiral

e.

Planeofsymmetry Themoleculeisachiral opticallyinactive

CC CCH3

CH3H

H

CH3

H

O

4

f.

opticallyactive Noplaneofsymmetry Twochiralcenters Themoleculeischiral

g.

Nitrogenmoleculesarenotchiralbecauseoftheumbrellaeffect;therefore,themoleculeisopticallyinactive.

Br

BrH

H

CH3 HCH2CH3

N

..

5

h.

Opticallyactive Noplaneofsymmetry Twochiralcenters Themoleculeischiral

i.

C

C

C

CH2OH

H

H

OH

OH

O

H

C

C

H

H

CH2CH3

CH2CH3

Br

Br

6

Themoleculeisopticallyinactive,becauseofinternalcompensation.

Themoleculepossessesaplaneofsymmetry. Themoleculehastwochiralcenters. Themoleculeisachiral.

J,

opticallyactive

C

H

CH2CH3

CH2CH2CH3CH3

7

3. Showhowdifferencesinthesolubilityofdiastereomericsaltsmaybeusedtoresolveopticallyactivealcohols,ROH.Hint:

C

C

O

O

O

+ ROH

C

C

O

OH

OR

O

phthalic anhydride an acid-ester

8

9

4. Inthefollowingreaction,determineifthechiralcenterinthereactantshasbeendisruptedinsynthesizingtheproducts:

(a)

Thereisnodisruptiontothechiralcenter.

(b)

Thereisnodisruptiontothechiralcenter

(c)

Thereisadisruptiontothechiralcenter

CCH3 COOH

H

OH

+ ROHH+

CH3 C

H

OH

COOR + H2O

C

CH3

COOH

H

+

CH3

C

H

+CNCH3CH2 H2O HCl2 + NH4ClCH3CH2

C

CH3

H

+

CH3

C

H

+CH3CH2 CH3CH2Br NaOH OH NaBr

10

(d)

Thereisdisruptiontothechiralcenter5. Thespecificrotationofanopticallyactivedextrorotatory

compoundis+56o.Calculatethepercentagecompositionofamixtureofenantiomersofthiscompoundwhoserotationis-22o.

Letx=thefractionofthedextrorotatorycompoundThen,1-x=thefractionoflevorotatorycompoundx(+56o)+(1-x)(-56o)=-22o56x-56+56x=-22112x=56–22112x=34x=34/112x=0.30thefractionofthemixturethatisdextrorotatory1-x=0.70thefractionofthemixturethatislevorotatory

6. Ascertainwhetherthefollowingstructuresaresuperimposable

ontheirenantiomers:

(a)

Thismoleculedoesnotsuperposeonitsmirrorimage.

+ C CH2CH3

CH3

H

ClAlCl3

C

H

CH2CH3

CH3HCl+

CH3

CH3

Br

Br

H

H

11

(b)

Thismoleculedoesnotsuperposeonitsmirrorimage.

(c)

Thismoleculesuperposesonitsmirrorimage.

(d)

Inthestaticmodel,themoleculedoesnotsuperposeonitsmirrorimage,buttheotherlonepaircanbeprotonated;therefore,themoleculeisopticallyinactive.

CH3

CH3

HH

C

C

CH3

CH3

OH

OH

H

H

OH3C

H

..+

12

(e)

Thismoleculedoesnotsuperposeonitsmirrorimage.

(f)

Thismoleculesuperposesonitsmirrorimage.

(g)

Thismoleculesuperposesonitsmirrorimage.

NCH3CH2

CH3

CH2CO2CH3

H

+ Cl-

Cl Cl

H H

Br

Br

H

H

13

7. Calculatethenumberofstereoisomerspossibleforthefollowingcompound:

2n=25=32isomers8. Usingchemicalequations,suggestapathwayforresolvingthe

followingenantiomers:

RS

SHO

HO

H

H

HH

H

OHCH3H3CCH2

C

CH3

CH3CH2

H

N

H

CH3

C

CH3

CH2CH3

H3C

H

H

N

14

9. Determinewhichcompoundsarenonsuperimposable

(dissymmetric)ontheirmirrorimages:

a.

Thismoleculeisnonsuperposableonitsmirrorimage.

C C CCl

Cl

H H

15

b.

Thismoleculeisnonsuperposableonitsmirrorimage.

c.

Thismoleculeissuperposableonitsmirrorimage.

d.

Thismoleculeissuperposableonitsmirrorimage.

CH3

H3C

C

C

COOH

COOH

OH

OH

H

H

C C CH

H

Cl

H

16

e.

Thismoleculeisnonsuperposableonitsmirrorimage.10. AcompoundwiththemolecularformulaofC7H16wasfoundtobe

opticallyactive.WhenC7H16istreatedwithBr2andlight,theresultingmonobrominatedmixtureisopticallyinactive,butresolvable.

(a) SuggeststructuresforC7H16andtheresulting

monobrominatedproduct.

(b) WriteamechanismfortheconversionoftheopticallyactiveC7H16totheopticallyinactivemonobrominatedmixture.

1.

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2.

3.

Step3maybeenvisionedinthefollowingmanner:

18

11. Followingaretheidentifiedchiralcentersinanisatin,C15H20O8,andtheabsoluteconfiguration(R/Sdesignation)ateachchiralcenter.