Unit II Geometrical isomerism

Pharmaceutical Organic Chemistry III B.Pharm IV SEM

1. Unit II
GEOMETRICAL
ISOMERISM
Lecture by
SOWMIYA PERINBARAJ, M.Pharm
Assistant Professor
Dept. of Pharmaceutical Chemistry
SVCP

2. INTRODUCTION
+In1875,Vant’tHoffandLeBelrecognisedanothertypeofstereoisomerismin
organicmoleculeshavingtwodifferentlysubstitutedatomstoeachotherby
meansofadoublebond(C=C).
+Thegeometricalisomerismariseswhenatomsorgroupsarearranged
differentlyinspaceduetorestrictedrotationofadoublebondinamolecule.
+Thecarbon-carbondoublebondsconsistofσbondandπbond.
+Thepresenceofπbondlocksthemoleculeinoneposition,thereforerotation
aroundtheC=Cbondisnotpossible.
+Geometricalisomerhavesamemolecularformulaandstructuralformula
+Butithasdifferentorientation(spatialarrangementofatoms)duetopresenceof
doublebondsorrings.
2

3. +In general Geometrical isomerism is shownby alkenesor their
derivatives in which two different atoms or groups are attached to
each carbon containing the double bond.
+Eg:Two different spatial arrangements of methyl groups about a
double bond in 2-butene give rise to the following geometrical
isomers.
+i.e.,cis-2-buteneandtrans-2-butene
3

4. NOMENCLATURE OF GEOMETRICAL
ISOMERISM
+To represent the various geometrical isomers following nomenclatures are used.
❖Cis-Trans nomenclature
❖E-Z nomenclature
❖Syn-Anti nomenclature
Cis-Trans nomenclature:
✓GeometricalisomerismiscommonlyknownasCis-Transisomerism.
Cis-isomer:CisisaLatinwordsanditmeans“thissideof”
✓Cisindicatesthattheidenticalatomsorgroupsareonthesamesideofthe
carbondoublebondorring.
4

5. ❖Trans isomer: Trans is a Latin word which means “the other side of”
Trans indicates that the identical atoms or groups are on opposite side of
the carbon double bond or ring.
+Thus,cis-transisomersarestereoisomers,thatispairsof
moleculeswhichhavethesamemolecularformulabutwhose
identicalatomsorgroupsareindifferentorientationinthree
dimensionalspace.
5

6. Examples:
2)3-HexeneCH3CH2CH=CHCH 2CH3
+Geometricisomerismispossiblebecauseeachdouble
bondedcarbonisattachedtotwodifferentgroups
(CH3CH2andH)attachedtoit.
6

7. Examples:
3)ButenedioicAcidCOOH-CH=CH-COOH
+Geometricisomerismispossiblebecauseeachdouble
bondedcarbonisattachedtotwodifferentgroups(COOH
andH)attachedtoit.
7

8. Properties:
➢Cis-transisomersdonotdifferentmuchinchemicalproperties.
➢Theydifferinphysicalpropertieslikeboilingpoint,meltingpoint,crystalstructure,
solubilityandrefractiveindex.
➢Incisisomerbecausethesimilargroupsareveryneareachother,VanderWaalsrepulsion
andStearichindrance(bulkygroups)makethemoleculemuchunstable.
➢Whereasintransisomer,similargroupsarediagonallyoppositetoeachother.Hencethere
isnosuchstericinteractionanditismorestable.
8

9. Properties:
➢Hencereactivityofcisisomerislittlehigherthantransisomer.
➢Dipolemomentstudiesisoneofthebestmethodtoidentifycis-trans
isomers.CisisomerhaslargerDPMandtransiszero.
9

10. Comparison between cis and trans
isomer
S.no Cis isomer Trans isomer
1
It has same connectivity and identical atoms
of same side.
It has same connectivity and identical atoms of
opposite side.
2 Steric interaction is present No steric interaction
3 Less stable isomer More stable isomer
4 Energy and reactivity is more Energy and reactivity is less
5 Cis isomer has polar molecule Trans isomer has less polar or non polar
6 Dipole moment value is higher Dipole moment value is zero
7 Melting point is comparatively low Melting point is comparatively high
8 Boiling point is comparatively high Boiling point is comparatively low
9 Solubilityis comparatively high Solubility is comparatively low
10

11. Limitations:
+Cis-Transnomenclatureisnotavailableformoleculeswhich
containfourdifferentconnectivesubstituents.
+Cis-transnomenclatureisalsonotapplicableformolecules
containingC=Nwhichcontainfourdifferentconnective
substituents.
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12. E-Z nomenclature
+Thissystemcanbeusedtospecifytheconfigurationaboutanycarbon-carbondouble
bondunambiguouslybyusingasetofpriorityrules.
+Thissystemisdevisedin1964byR.S.Cahn,C.K.IngoldandV.Prelog.
E-Configuration:
+EistakenfromGermanwordentgegenwhichmeans“opposite”
+Ifthetwogroupsofhighestpriorityareontheoppositesidesofthedoublebond,
thebondisassignedas“E”configuration.
Z-Configuration:
+ZistakenfromGermanwordzusammenwhichmeans“together”
+Ifthetwogroupsofhighestpriorityonthesamesidesofthedoublebond,thebond
isassignedas“Z”configuration.
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13. +TheE-Zsystemisbasedonasetof"priorityrules",whichallowto
rankanygroups.
+ThegeneralstrategyoftheE-Zsystemistoanalyzethetwogroupsat
eachendofthedoublebond.Ateachend,rankthetwogroups,using
theCIPpriorityrules.
+Then,seewhetherthehigherprioritygroupatoneendofthedouble
bondandthehigherprioritygroupattheotherendofthedoublebond
areonthesameside(ZfromGermanwordzusammen=together)or
onoppositesides(EfromGermanwordentgegen=opposite)ofthe
doublebond.
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14. +There are four CIP priority rules
Rule 1:
+Each atom is assigned a priority. Priority is based on atomic number; higher
the atomic number, the higher the priority.
Rule 2:
+For isotopes, the higher the atomic mass the higher the priority. For example,
deuterium (Hydrogen-2) has higher priority than protium (Hydrogen-1).
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15. Rule3:
+Ifprioritycannotbeassignedonthebasisofatomicnumberoratomicmassconsidering
thefirstatomofagroup,thenlookatthenextsetofatomsandcontinueuntilapriority
canbeassigned.
+Prioritycanbeassignedatthefirstpointofdifference.
+Iftheatomsdirectlylinkedtothedoublebondarethesame,thenthesecond,third,
fourth,etc.atoms(awayfromthedoublebond)arerankeduntiladifferenceisfound.
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16. Rule 4:
+In the case of double or triple bonds, atoms participating in
the double or triple bond are considered to be bonded to an
equivalent number of similar atoms by single bonds, that is,
atoms of double and triple bonds are duplicated or triplicated.
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17. +To assign E-Z system,first determine the groups of highest priority
on each carbon. If the two highest priority groups are on the same
side of the double bond, the configuration is Z. If they are on
opposite side of the double bond then the configuration is E.
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19. Syn-Anti Nomenclature
+Geometricisomerismisalsopossibleincompoundscontaining
oximes(C=N)andAzocompounds(N=N).
+Thus,Syn-Antinomenclatureisusedtoindicatethe
geometricalisomerismcontainingC=NorN=Nbondssuchas
oximesandazocompounds.
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20. Syn-Anti Nomenclature
+Inaldoxime,thesynoranticonfigurationisdeterminedbytheposition
ofthelonepairofelectronsonnitrogenatomwithrespecttoalkyl(R)
oraryl(Ar)group.
Synconfiguration:
+Inaldoxime,synconfigurationisoneinwhichthelonepaironnitrogen
andthealkylorarylgrouponsamesideorHandOHispresentin
sameside.
Anticonfiguration:
+Inaldoxime,anticonfigurationisoneinwhichthelonepaironnitrogen
andthealkylorarylgrouponoppositesideorHandOHispresentin
oppositeside.
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21. Example: Aldoxime
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22. Syn-Anti Nomenclature
+Inketoxime,theprefixessynandantiindicatewhichalkyl(R)oraryl
(Ar)groupofketoximeissyn(onthesameside)oranti(onthe
oppositesides)withrespecttotheOHgroup.
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23. Syn-Anti Nomenclature
+Azocompoundscontaintwolone
pairofelectronsoneachnitrogen
atom.
+Synconfiguration:
+Ifbothlonepairarepresentonsame
side,thenassignedasSyn
configuration.
+Anticonfiguration:
+Ifbothlonepairarepresenton
oppositeside,thenassignedasanti
configuration.
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24. Methods of
determination of
configuration of
geometrical isomers

25. Determination of configuration of
geometrical isomers
❑Several methodsare available to determine the configuration
❑Method can be selected depending upon nature of compound
❑Use of multiple methods gives more reliable results
❑Some of the methods are
1.Method of cyclisation
2.By converting into compounds of known
configuration
3.Method of optical activity
4.Methods based on physical properties
5.By stereoselective reaction
6.By stereospecific reaction

26. 1. Method of Cyclisation:
➢Thismethodisbasedonthefact,Intramolecularreactionsaremore
likelytotakeplaceifthereactinggroupsarecloser.
➢Eg:Incaseofmaleicacid,anhydrideisformedundermild
conditionsbecausethetwo–COOHgroupsarecloser(cis)but
fumaricaciddoesnotgiveananhydrideunderordinaryconditions.
➢Undervigorousconditionsitformsmaleicanhydride,sointhiscase
thetwo–COOHgroupsareinoppositedirection(trans).
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27. 2. By converting into compounds of known
configuration:
✓Thismethodisapplicableinthosecasesinwhichoneofthetwoisomerscan
beconvertedintoacompoundofknownconfiguration.
✓Assumingthatthereisnoisomerizationduringtheprocessofconversion,
thentheconfigurationofproductandthatofstartingmaterialwillbesame.
✓Eg:oneformoftrichlorocrotonicacidcanbehydrolyzedtogivefumaric
acid,sothisformoftrichlorocrotonicacidmustbethetans-isomer.
✓Theotherformofaciddoesnotgivefumaricacidonhydrolysisandforms
isocrotonicacidonreduction.
✓Hencethisformoftrichlorocrotonicacidandisocrotonicacidarecis-
isomers.
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29. 3. Method of optical activity:
+Ingeometricalisomersifoneisopticallyactiveand
otherisopticallyinactive,thenopticallyactive
compoundcanbeeasilyresolved.
+Eg:Inhexahydrophthalicacids-
+cisformpossessesaplaneofsymmetryandisoptically
inactiveandtransisactive.
+Transformofhexahydrophthalicacidcanberesolved.
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30. 4. Methods based on physical
properties
A)Dipolemoment:
+Thedipolemomentofcisformisgenerallyhigherthanthatoftrans
form
+Eg:Inthecaseof1,2dichloroethylenecisisomerisgenerallyhave
1.80Dandtransisomerhavezerodipolemoment.
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31. b) Melting point, boiling point, solubility, Density
& Refractive index
+The physical constants are varying in cis and trans isomer is
shown in following table
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33. Solubility:
+Cis-isomershavehighersolubilities.
+Maleicacid(cis)-79.0g/100mlat293K
+Fumaricacid(trans)-0.7g/100mlat293K
c) X-ray and electron diffraction
➢ConfigurationcanbedeterminedusingX-raycrystallography.
➢X-Raycrystallographicanalysisofsorbicacidgaveitstrans
configuration.
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34. d) Spectroscopic method:
+Spectroscopicmethodbasedupon,
transisomerwillhavehigherλ-max
thancorrespondingcisisomer.
+NMRspectroscopyalsousedto
distinguishgeometricalisomers,
becausetransvinylprotonsaremore
stronglycoupledtoeachthanothercis
vinylprotons.
+InIRSpectroscopy,Transisomeris
readilyidentifiedbytheappearanceof
acharacteristicbandnear970-960cm
-1
.
Nosuchbandisobservedinthe
spectrumofthecisisomer.
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35. 5) By stereoselective reaction
+Inthesereactions,onestereoisomerisformedinmoreamountthantheother.
+Hydrogenationofalkynewithsodiumorlithiuminliquidammoniagivesa
mixtureoftransandcisalkenesinwhichtransisomerpredominates.
+Ontheotherhandhydrogenationofalkyneusingpalladisedcharcoalgivescis
alkenewithasmallamountoftransalkene.
+Reactionswhichyieldpredominantlyonestereoisomerofseveralpossible
stereoisomersarecalledstereoselectivereactions.
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36. 6) Stereospecific reactions
+Inthesereactions,theformationof
productisspecificwithrespecttothe
stereoisomer.Thatisdifferent
stereoisomericstartingmaterialsgive
risetodifferentstereoisomeric
products.
+Reduction: The catalytic
hydrogenationofcisisomergivesa
mesocompound.
+A(+-)productisobtainedifstarting
materialistransisomer.
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37. Hydroxylation:
+ThehydroxylationofalkenesifcarriedoutbymeansofOsO4,
KMnO4,H2O2proceedsincisfashion.
+Maleicacid(cis)andfumaricacid(trans)givemesoand(+-)–tartaric
acidrespectively.
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38. Conformational isomerism
+Conformationalisomerismisaformofstereoisomerisminwhichthe
isomerscanbeinterconvertedjustbyrotationsaboutsinglebonds.
+Conformationalisomersorconformersaredifferentshapesofthe
samemoleculeresultingfromrotationaroundasingleC-Cbond.
+Theyarenotdifferentcompounds(i.e.theyhavethesamephysical
andchemicalproperties)andarereadilyinterconvertible.
+Itisunstableandexchangeinshorttimedurationtoanother
conformer.
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39. Twotypesofprojectionstructureareusedtostudythe
conformationalisomers.
1.Sawhorse projection:
•A representation of molecular structure, from an oblique angle.
•It is named after its similar look to sawhorse.
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40. 2. Newman projection:
+Inthisprojection,visualizestheconformationsofabondfrom
fronttoback.
+Thefrontatomrepresentedbyabondlineandtheback
carbonrepresentedasacircle.
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41. +Eclipsedconformation:Aconformationaboutacarbon-carbonsinglebond
inwhichtheatomsorgroupsononecarbonareascloseaspossibletothe
atomsorgroupsonanadjacentcarbon.
+Staggeredconformation:Aconformationaboutacarbon-carbonsinglebond
inwhichtheatomsorgroupsononecarbonareasfaraspossibletotheatoms
orgroupsonanadjacentcarbon.
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Eclipsed Staggered

42. +Dihedralangle:Ananglecreatedbytwointersectingplanes.
+Anticonformation:Aconformationaboutasinglebondinwhichtwo
groupsonadjacentcarbonslieatadihedralangleof180
0
+Gaucheconformation:Aconformationaboutasinglebondofanalkane
inwhichtwogroupsofadjacentcarbonslieatadihedralangleof60
0
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43. CONFORMATION OF ETHANE
+ThemolecularformulaofethaneisC2H6
+IfoneofthemethylgroupsisallowedtorotatealongtheC-Caxis
keepingtherestofthemoleculeundisturbed,aninfinitenumberofpossible
arrangementsoftherotatedmethylgroupwithrespecttotheundisturbed
methylgroupisgenerated.
+Eachofthesepossiblearrangementrepresentsaconformation.
+So,sixpossibleconformationsareobtained,outofwhichthreeare
staggeredforminwhichthetwohydrogenatomsonthedifferentcarbon
atomsareasfaraspossible(1,3,5).
+OtherThreeareeclipsedinwhichthetwohydrogenatomsonthedifferent
carbonareascloseaspossible(2,4,6).
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45. Energy barrier diagram of ethane
+Staggeredandeclipsedconformerhavedifferentenergy.
+Staggeredconformerhasleastenergyduetoleaststerichindranceand
itismoststableconformer.
+Eclipsedconformerhashighestenergyduetomaximumsteric
hindranceanditisleaststableconformer.
+Torsionalenergy:Eclipsedconformerhas3kcal/mole(12.6kJ/mol)
thanstaggered.Thisenergyisknownastorsionalenergy.
+Torsionalangle:Theanglebetweenthefrontandbackhydrogenis
dihedral(ortorsional)angle.
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47. Why staggered conformation of ethane is lower in
energy than the eclipsed conformation?
➢ThepotentialenergyofStaggeredconformationislessthaneclipsed
becauseinthelattercasethehydrogenatomsonthetwocarbon
atomsareclosetoeachotherandhenceexertarepulsiveforce
(stericrepulsionduetononbondedinteractionofhydrogen
atom).
Canbejustifiedwithtwoimportantreasons
1)Thefirstisthattheelectronsinthebondsrepeleachotherandthis
repulsionisatamaximumintheeclipsedconformation.
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48. 2)Thesecondisthattheremaybesomestabilizinginteractionbetween
theC–HσbondingorbitalononecarbonandtheC–Hσ*antibonding
orbitalontheothercarbon,whichisgreatestwhenσ*antibonding
orbitalisunfilled:thisonlyhappensinthestaggeredconformation.
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50. CONFORMATION OF BUTANE
+ThemolecularformulaofbutaneisC4H10
+Ithastwostructuralisomer➔n-butaneandiso-butane
+Whenrotatingthen-butaneattheaxisoftheC
2
-C
3
bondat60°
angle,itshowsdifferentconformationisomerismwiththerespectof
methylgroupspresentonbackandfrontsideofC
2
-C
3
axis.
+Butanehasfourconformationisomers:
1.Fullyeclipsed
2.Gauche
3.Partiallyeclipsed
4.Antibutaneconformer
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51. 1. Fully eclipsed conformer
+Inthisconformer,methylgroupsofbackcarbonandfrontcarbonare
minimumdistance(eclipse)toeachother(0°)ordihedralangle
betweenmethylgroupis0°
+Thisconformerhasmaximumsterichindranceduetopresenceof
methylgroupateclipseposition,soithashighestenergyandhence
leaststableconformer.
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52. 2. Gauche (staggered conformer)
+Inthisconformer,methylgroupsofbackandfrontcarbonare60°
angleordihedralanglebetweenmethylgroupis60°
+Thistypeofconformationismorestableandhaslessconformer
energyasthereisalittlesterichindrancebetweenthesame
moleculeslikeMe-MeandH-HandMe-H.
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53. 3. Partially eclipsed conformer
+Inthisconformer,hydrogenatomofbackandmethylgroupoffront
carbonare0°angleordihedralanglebetweenmethyl/methylgroupis
120°
+Thisstageappeared120°rotationfromfullyeclipsed.
+Thistypeofconformationismorestablethanfullyeclipsedandleast
stablethangauche(staggered)duetopresenceofsterichindrancebetween
thehydrogenandmethylgroup.
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54. 4. Anti (staggered) conformer
+Inthisconformer,methylgroupsofbackandfrontcarbonare180°
angleordihedralanglebetweenmethyl/methylgroupis180°
+Thisstageappeared180°rotationfromfullyeclipsed.
+Thistypeofconformationismoststablethanfullyeclipsed,gauche
(staggered)andpartiallyeclipsedconformerduetopresenceofleast
sterichindrancebetweenbothmethylgroup.
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55. Energy barrier diagram of butane
+Energyorder:
+Anti<Gauche<
partiallyEclipsed
<fullyeclipsed
+Stabilityorder:
+Anti>Gauche>
partiallyEclipsed
>fullyeclipsed
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57. Conformation of cyclohexane
+Cyclohexane is asix membered carbocycle with
the molecular formula of ➔C6 H12
+Cyclohexane exists mainly in fourconformations:
1)Chair Conformation
2)Half chair conformation
3)Twist boat conformation
4)Boat conformation
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58. 1. Chair conformation
+Cyclohexaneexistinpuckeredconformationwhenit
achievestetrahedralbondangle➔109.5ºandstaggered
conformation.
+Thereisnosterichindrance,soithasminimumenergy
andmaximumstability.
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60. 2. Half chair conformation
+It has both angle strain and torsional
angle, so less stable than chair form.
+Energy ➔50 KJ/mol
3. Twist boat conformation
+More stable than boat conformation by
about energy ➔22 KJ/mol but less
stable than chair conformation.
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61. 4. Boat conformation
+Eclipsed conformer
+Due to steric interaction between the non-bonding atom, it is
less stable than chair conformation and has highest energy.
+Energy ➔28 KJ/mol
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63. Axial and equatorial of cyclohexane
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64. Energy diagram of cyclohexane
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Energyorder:
chair<twist
boat<boat<
halfchair
Stabilityorder:
chair>twist
boat>boat>
halfchair

65. Stereoisomers in biphenyl compounds
(Atropisomerism)
+Atropisomerscanbedefinedasisomersthatcanbeisolatedduetopreventionor
restrictionofrotationaboutasinglebond,usuallybetweentwoplanar
moieties.
+Thetermatropisomerismcomesfromthewordsa,Greek➔not(absence)
+tropos,Greek➔turn.
+Bulkygrouponorthopositionofbi-phenylleadstostrainedringstructural
features.
+Bulkysubstituentsorstrainedringsmayenhancethebarriertorotation
betweentwodistinctconformationstosuchanextentastoallowobservationof
atropisomers.
65

66. +Atropisomerismisalsocalledaxialchiralityandthechiralityisnot
simplyacentreoraplanebutanaxis.
+SimplebiphenylcaneasilyrotatebyC-Cbondanditissymmetricso
simplebiphenylisachiral.
+C-Csigmabondisknownaspivotalbond.
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67. +Biphenylsubstitutedonorthopositioninmolecule1(Figure2),which
containsachiralaxisalongthebiphenyllinkage.
+Thebiphenylringsareperpendiculartoeachotherinordertominimize
stericclashesbetweenthefourorthosubstituentsmeaningthatrotation
aboutthebiphenylbondthroughpivotalbondisrestricted.
+Theinterconversionbetweenthetwoisomersisrestricted(slow)
thereforetwoisomersareseparateentitiesandcanresolvedtoitsseparate
enantiomers.
+Thefirstchiralityduetorestrictedrotationaboutasinglebondwas
describedbyChristieandKennerin1922,theysuccessfullyresolvedthe
enantiomersof6,6'-dinitrobiphenyl-2,2'-dicarboxylicacid.
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69. ConditionsofAtropisomerism(opticalactivity):
1.Arotationallystableaxis
2.Presenceofdifferentsubstituentsonbothsidesoftheaxis
3.Theconfigurationalstabilityofaxiallychiralbiarylcompoundsis
mainlydeterminedbythreefollowingfactors:
i.Thecombinedstericdemandofthesubstituentinthecombinedsteric
demandofthesubstituentsintheproximityoftheaxis.
ii.Theexistence,lengthandrigidityofbridges.
iii.Atropisomerisationmechanismdifferentfromamerelyphysicalrotation
abouttheaxis,e.g.photochemicallyorchemicallyinducedprocesses.
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70. Stereochemicalassignment:
+Determiningtheaxialstereochemistryofbiarylatropisomerscanbe
accomplishedthroughtheuseofaNewmanprojectionalongtheaxis
ofhinderedrotation.
+Theortho,andinsomecasesmetasubstituentsarefirstassigned
prioritybasedonCahn–Ingold–Prelogpriorityrules.
+Startingwiththesubstituentofhighestpriorityintheclosestringand
movingalongtheshortestpathtothesubstituentofhighestpriorityin
theotherring,theabsoluteconfigurationisassignedPorΔfor
clockwiseandMorΛforcounterclockwise.
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72. Stereoselective Reactions
+Areactionthatyieldspredominantlyonestereoisomer(oronepairof
enantiomers)ofseveralpossiblediastereomericpossibilitiesiscalleda
stereoselectivereaction.
+Inastereoselectivereaction,onestereoisomerisformed(ordestroyed)
morerapidlythananother,thusresultinginpredominanceofthefavoured
stereoisomerinthemixtureofproducts.
+Duetodifferenceseitherinthefreeenergiesofactivationofthereaction
orthermodynamicstabilitiesoftheproducts,oneisomerisformed
predominantly.
+A ➔B + C
+BisformedmorethanCwhereasBandCarestereoisomers.
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73. +Stereoselectivitycanbefurthersubdividedintoenantioselectivityand
diastereoselectivity.
+Enantioselectivityisdefinedastheformationofoneofthetwo
enantiomerspredominantlyorexclusively.
+Diasteroselectivityisdefinedastheformationofoneofthetwoormore
diastereomerspredominantlyorexclusively.
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74. Enantioselectivity:
+It is achieved by using chiral substrate, reagent, catalyst or solvent.
+Examples:
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76. Diastereoselectivity:
+It is most commonly achieved through the presence of steric
hindrance.
+Examples:
+4-methylcyclohexenone
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77. +Epoxidation of cyclic alkenes:
+4-methylcyclopentene
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78. Stereospecific Reactions
+Areactioninwhichstereochemicallydifferentreactantsgives
sterochemicallydifferentproductsiscalledastereospecific
reaction.
+Areactionorsynthesisinwhichaparticularstereoisomerreactsto
giveonespecificstereoisomeroftheproductiscalledasterospecific
reaction.
+Suchareactionissaidtodisplaystereospecificity.
+A ➔B
+C ➔D
+AandCarestereoisomers;BandDarestereoisomers.
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79. Examples: 1
+Additionofbrominetocis-2-butenegivesracemic2,3-dibromobutane,
whilethetransisomergivesmeso-2,3-dibromobutane.
+Thisreactionisstereospecificbecausedifferentstereoisomergives
differentstereoisomer.
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80. Examples: 2
+Theformationofepoxidesfromalkenesontreatmentwithperacidsisa
stereospecificreaction.
+Cis-2-butenegivescis-2,3-dimethyloxirane,whiletrans-2-butenegives
trans-2,3-dimethyloxirane.
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81. Examples: 3
+1,2-diolsmaybepreparedfromalkenesusingstereospecific
oxidativereaction.
+Reactionofcyclohexenewithosmiumtetroxidegivestheosmate
esterwhichmaybecleavedtogivetheproductwithtwohydroxyl
groupsonsamesideofthemolecule.
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82. Examples: 3
+Reactionofcyclohexenewithperacidgivesanepoxidewhichon
hydrolysisgivestheproductwithtwohydroxylgroupsondifferent
sideofthemolecule.
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83. Examples: 4
+Diels-Alderreactionisstereospecificallyciswithrespecttothe
dienophile.
+1,3-butadienereactswithmaleicacidtogivecis-1,2,3,6-
tetrahydrophthalicacid,whilefumaricacidgivestransisomer.
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84. The way to get
started is to quit
talking and
begin doing.
Walt Disney
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