6152gcyygbyhu64235-Geometric-Isomerism.pdf

Geometric isomerism

1. GEOMETRIC ISOMERISM
PREPARED BY:
MR.NADIM MRCHHIPA
ASSOCIATE PROFESSOR,
ASP & BRI, ADALAJ

2. DEFINITION
•Theisomerismwhichoccursduetodifferenceofthepositionsofthe
substituentsaboutadoublebondoraringduetorestrictedrotationiscalled
geometricisomerism.
•Theydonotrotatetheplaneofpolarisedlight(unlesstheyalsohappentobe
chiral),anddonothaveidenticalproperties.
•Conditionsforgeometricisomerism
Theremustbeacarbon-carbondoublebondinthecompounds.
Eachofthecarbonofthedoublebondmustbeattachedtotwodifferent
substituents

3. CIS-TRANS ISOMERSIM
Configurationoftheisomericbut-2-eneshowninfigure.Theydifferintheirnames
bytheprefixescis-(Latin:onsameside)andtrans-(Latinacross),whichindicate
thatmethylgroupareinsamesideoronoppositesideofthemolecule.
Theseformsarenotinterconvertibleduetorestrictedrotationofdoublebond.

4. Firstwewilldeterminewhichofthetwochairconformsofcis-1,4-dimethyl-
cyclohexaneismorestable.Onechairconformerhasonemethylgroupinan
equatorialpositionandonemethylgroupinanaxialposition.Theotherchair
conformeralsohasoneTherefore,bothchairconformersareequallystable.
Thismethodofdenotinggeometricisomerismworksbestwhenthealkeneisdi-
substituted.Infact,itwillalwaysworkwhenthealkeneisdi-substituted(andother
conditionsarefulfilled).Butthismethodcanfailwithtri-substitutedortetra-
substituted
alkenes.

5. For this cis/trans method of denoting to work, there must beat least one
identical group on each carbon of the double,bond. For example:

6. Cisisomer is less stable than trans isomer
• In cisisomer, two large groups on the separate carbons are always onthe same side.
Thus, these two groups are closer to each other and repeleach other. This is called
steric strain.
• On the other hand, in trans isomer the two large groups are on theopposite sides. So
they are far apart. Hence they don’t repel each other.So, the steric strain is far less.

7. E AND Z NOMENCLATURE
•FordenotingGeometricalisomersbycis/trans,isnotsufficientwhenthereare
morethantwodifferentsubstituentsonadoublebond.SodenotethemE/Z
nomenclatureisadopted.
•Ifthegroupofhighestpriorityonbothcarbonareonthesameside,thenitis
Z(Z=Zusammen=Together)isomer,iftheyareonoppositesides,thenitisE
(E=Entgegen=Opposite)isomer.
•ThelettersEandZarerepresentedwithinparanthesesandareseparated
fromrestofthenamewithahyphen.
•thegroupsattachedtoeachcarbonofthedoublebondareanalyzedand
thengivenprioritiesaccordingtoCahn-Ingold-Prelog(CIP)rules.

8. CIP RULES FOR E/Z NAMING CONVENTION
•Substituents on any one of the two double-bonded carbon atom is looked at.
•First, the atom which is directly attached to the double bond carbon is looked
at. This is the first atom. The group where first atom has higher atomic number
has higher priority.

9. •If, both groups are attached by the same first atom,then the atomic number of
the second atom (atomattached to first atom) is looked at.
•Similarly, if the second atoms are also same, third atoms are looked at.

10. Ifthefirstatomsoftwogroupshavethesamehigheratomicnumber
substituents,onewithmoresuchsubstituentisgivenhigherpriority.

11. If there is any double bond or triple bond within the group, it isconsidered at two
or three single bonds respectively. So:
Ifthereisaphenylgroupattachedtofirstatom,thenitisthoughtthatFirstatomis
attachedtothreecarbons.

12. Ifisotopesofsameelementarepresent,thehigherpriorityisgiventotheisotope
withhigheratomicmass.E.g.theDeuteriumisotope(H2orD)hasmorepriority
thanprotium(H1orH).TheC13isotopehasmoreprioritythanC12.

14. SYN-ANTI SYSTEM
•Thisisusedforcompoundswhichareoximesofaldehyde,hydrazonesand
Semicarbazide,inwhichcarbonisjoinedtonitrogenbydoublebondalsoexhibit
geometricalisomerism.
•SinceHandOHgroupcanarrangeonsamesideoroppositesidesofthedouble
bond.
•whenhydrogenandhydroxylgroupareonthesameside,theisomerisknownassyn
(analogoustocis)andwhenthesegroupsareontheoppositesides,theisomeris
knownasanti(analogoustotrans).

15. In Aldoximethe synisomer-in which –OH group of the oximeis on the
side of the hydrogen of the aldehyde carbon
In Ketoxime-specify the group with respect to which the oxime-OH
group is syn

17. DETERMINATION OF CONFIGURATION OF
GEOMETRICAL ISOMERISM
1.Dipole Moment
Cisisomer have higher dipole moment than trans-isomer. As in trans-isomer two
bond moments are opposed because of the symmetry of molecule, where sys
isomer being non-symmetrical has a finite dipole moment as bond moments are
not opposed.

18. 2.Melting/BoilingPoint
transisomerhavehigherMeltingandBoilingthancis-isomer.Asintrans-isomer
moleculesaremoresymmetricalandhencefitmorecloselyinthecrystallattice
ascomparedtothemoleculesofcisisomer.
IntermolecularforcesworkwellintransisomerandUshapeofcisisomercan
notfirperfectlyincrystallattice.Poorpackingisleadstopoorintermolecular
forces.Sotheyrequiredlessenergytobreak.

19. 3.Solubility.
•In general, solubility of a cisisomer is higher than that of the corresponding
trans isomer. This is due to the reason that the molecules of a cisisomer are less
tightly held in the crystal lattice.

20. 4.Stability
•Thetransisomerismorestablethancisisomerduetosterichindrance.
Intermolecularreactionsoccureasilywhenreactinggroupsareclosetogether.
Hence,thecisisomerwillformcyclicderivativesmorereadilyasagainst
transderivatives.Butthisreactionwilltakeplaceinonlythosecisisomersin
whichthesubstituent’sontwodoublebondedcarbonsarecapableof
intramolecularreactionwitheachother.

21. CONFORMATIONAL ISOMERISM
•Differentspatialarrangementofatomsthatcanbegeneratedorconverted
intooneanotherbyfreerotationaboutsinglebondisknownasconfirmations.
•Differentconfirmationofsamemoleculealsocalledconfirmers,rotamersor
conformationalisomers.
•Itcanbedeterminedbyuseofx-rayandelectrondifferenction,IR,Raman,
UVandNMR,etc.
•Confirmationscanbecalculatedbymethodcalledmolecularmechanics.

22. REPRESENTATION OF CONFORMATIONAL ISOMERS
WEDGE AND DASH SAWHORSE NEWMAN

23. TORSIONAL OR DIHEDRAL ANGLE (Ø)
•the angle created by two intersecting planes
•In case of ethane angel between HCC and CCH plane.

24. CONFORMATIONAL ISOMERISM IN ETHANE
•Ethane has two confirmation known as Staggered and Eclipsed.

25. CONTINUED……
•Thereisaenergybarrierofabout3kcal/mol.Thepotentialenergyofthe
moleculeisatminimumfromthestaggeredconfirmations,increasewith
rotation,andreachesamaximumataeclipsedconfirmation.SoEthane
moleculeexistasmoststable,eclipsedconfirmation.
•As3kcalenergybarrierisnortoolarge,evenatroomtemperaturerapid
interconversionbetweenstaggeredconfirmationoccursassinglebondpermits
freerotation.
•Theenergyrequiredtorotatetheethanemoleculeaboutthecarbon-carbon
bondiscalledtorsionalenergy.Thereasonforinstabilityofeclipsedorskew
confirmationistorsionalstainalsocalledeclipsedinteractionstrain.

26. WHY THE ECLIPSED CONFIRMATION IS HIGHER IN
ENERGY THAN STAGGERED CONFIRMATION
•ThereisasomestericrepulsionbetweentheHydrogenatomsoftheeclipsed
confirmationthatisreducedinstaggeredconfirmation.
•IneclipsedconfirmationelectroncloudofC-Hbondaremostnearerso
repulsionincreases.
•Thusrepulsionforcecausedtorsionalstraininmolecule,themorestrainmore
willbetheinternalenergyofmolecule,lessstability.

27. CONFORMATIONAL ISOMERISM IN N-BUTANE

28. CONTINUED…..
•Duetopresenceofmethylgroup,twonewpointsincluded:thereareseveral
staggeredconfirmationandonemorefactorbesidestorsionalstrainaffect
theconformationalstabilities.
•Therearefourdifferentconfirmationofn-Butane:
1)Fullystaggeredconfirmation,calledanti,transorantiperiplanner.Dihedral
angle,180°,wheremethylgrouparefarawayfromeachother.

29. 2)Gauchealsocalledsyn-clinical.Dihedralangle,60°and300°,wheremethyl
groupareclosertoeachotherthaninanti-confirmation.

30. 2)TwoeclipsedconfirmationknownasanticlinicalwithDihedralangle,120°and
240°.synperiplanarDihedralangle,0°,wheremethylgroupareclosesttoeach
otherandalsoknownasfullyeclipsed.
Anticlinical(Skew) synperiplanar(Fully eclipsed)

31. •Anticonfirmationfoundtobemoststable,comparedtogaucheby
0.9kcal.mol,botharefreefromtorsionalstrain.
•Ingaucheconfirmationmethylgrouparecrowdedtogether,thatcloserthan
theirsumofvanderwallsradii;underthesecondition,Vanderwallsforces
arerepulsiveandraisetheconformationalenergy.Sobecauseofthis
repulsionVanderwallsstraingeneratedandmoleculesbecomelessstable.
•Thisforcesnotaffectonlyrelativestabilitiesbutalsotheheightsofenergy
barrier,energymaximumisreachedwhentwomethylgroupswingpasteach
otherratherthanHydrogenaround5kcal/mol,butasitisnotsobigso
rotationcanhappensatevenroomtemperaturealso.

32. REACTIONS
•E2isananti-elimination.Theyarestereospecific.Thehydrogenandthe
halogenmustbeonoppositesidesofthemoleculebeforetheE2elimination
cantakeplace.Thismakessenseasboththebaseandtheleavinggroupare
negativelycharged.Thereforetheywouldtrytobeasfarapartaspossible.
Inaddition,theleavinggroupislargeandthereismoreroomfortheremoval
oftheadjacentprotonifitisontheoppositesidefromtheleavinggroup.
•Iftheanti-arrangementisnotpossible,syn-arrangementmaytakeplace.

34. CONFORMATIONAL ISOMERISM IN CYCLOHEXANE
•AllCinCyclohexaneisSP
3
Hybridized,Sotheywillattachedtoeachother
withbondangle109°notby120°asinplannerstructure.Sotheywill
appearasTwodifferentconfirmationin3D-ChairandBoat.AsCyclohexane
ringisfreeofAngleStrainandTorsionalStrain.

36. AXIAL AND EQUATORIAL BOND IN CYCLOHEXANE

37. HOW TO DRAW CYCLOHEXANE

39. RELATIVE STABILITY OF CONFIRMERS OF CYCLOHEXANE
Ring Flipping or Ring Inversion

40. 1. CHAIR CONFIRMATION
•C-Hbondsareperfectlystaggered,SoBondoppositionstrainisminimum.
•‘H’atomsonadjacentcarbonatomshaveenoughspacefortheir
accommodation,SoStericstrainisminimum.

41. Axial
hydrogens

42. •AsaresultofsimultaneousrotationaboutallC-Cbonds,achair
conformationofcyclohexanecaninterconverttoanotherchair
conformationbyaring-fliporring-inversion.
•Intheprocess,equatorialbondsbecomeaxialandviceversa

43. 2. BOAT CONFIRMATION
•1. Bond opposition strain: C-H bonds on the sides are eclipsed.
•2. Fp–Fpinteraction: Distance between two FpHsis 1.84Ao, These two
strains make boat conformation highly strained.
•It has 29.71kJ/molmore energy than chair conformation.
•Therefore boat conformation is less stable than chair conformation.

44. Steric
interactions

45. 3. TWIST OR SKEW BOAT CONFORMATION:
•Less torsional strain as compared to boat conformation.
•Flag pole Hsare away from each other.
•C2, C3, C5 and C6 become non-planer.
•Energy content : 6.696kJ less than boat but 23.02kJ more than chair.
•Therefore more stable boat but less stable than chair.

46. 4. HALF CHAIR CONFORMATION:
•Suffers from angle strain
•It has 46.04kJ more energy than chair conformation. Maximum energy content
than any other conformation. There it is least stable.

47. •Isolation of any conformation of CH is not possible because :
•At RT the average energy content of CH is more than sufficient to overcome
this small barrier.
•There exists a dynamic equilibrium between different conformations of CH.
•Chair <> Twist Boat <> Boat<> Half Chair
•Decreasing Order of Stability
Chair > Twist Boat > Boat > Half Chair

48. ATROPISOMERISM
•Biphenylsarecompoundswherebyaphenylringisconnectedtoanotherthrough
acentralσbond.Kindofconfirmationisomerism.
•Atropisomersarestereoisomersresultingfromhinderedrotationaboutoneor
moresinglebonds,wheretheenergybarriertorotationishighenoughtoallow
fortheisolationoftheconformers,Kindofenantiomer(fromGreek,a=notand
tropos=turn).
•Atropisomersare detectable by NMR if half lives exceed 10
-2
sec.
•Atropisomersare isolatable if the half-life is above 1000 sec.

49. •Polynuclear aromatic systems such as binolalso exist as enantiomers.
•Ifbulkygrouponorthopositionofbi-phenylorstrainedringstructuralfeatures.
Bulkysubstituentsorstrainedringsmayenhancethebarriertorotationbetween
twodistinctconformationstosuchanextentastoallowobservationof
atropisomers.
•Atropisomerismisalsocalledaxialchiralityandthechiralityisnotsimplya
centreoraplanebutanaxis.

50. CRITERIA FOR ATROPISOMERSIM
•Neitherringmusthaveplaneofsymmetry

51. 2
ND
CASE OF PLANE OF SYMMETRY
•Thesubstituentinorthopositionshouldbelargeenoughsoitcanrestrict
rotationaroundpivotalbond

52. 3
RD
CASE OF PLANE OF SYMMETRY
•In the third case neither ring is symmetric there is no plane of symmetry, and
many such compounds have been resolved. This corresponds to AB.....AB.

53. CONDITION FOR ATROPOISOMERISM
•1. A rotationally stable axis
•2. Presence of different substituents on both sides of the axis
•3. The configurationalstability of axially chiral biarylcompounds is mainly
determined by three following factors:
•i. The combined steric demand of the substituent in the combined steric demand of
the substituents in the proximity of the axis.
•ii. The existence, length and rigidity of bridges.
•iii. Atropisomerisationmechanism different from a merely physical rotation about the
axis, e.g. photo chemically or chemically induced processes.

54. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

55. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

56. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

57. NOMENCLATURE FOR ASSIGNING ATROPISOMERS
•Determining the axial stereochemistry of biarylatropisomerscan be accomplished
through the use of a Newman projection along the axis of hindered rotation.
•The ortho, and in some cases meta substituents are first assigned priority based on
Cahn–Ingold–Prelog priority rules.
•Starting with the substituent of highest priority in the closest ring and moving along
the shortest path to the substituent of highest priority in the other ring, the absolute
configuration is assigned P or Δfor clockwise and M or Λfor counterclockwise.

58. NOMENCLATURE FOR ASSIGNING ATROPISOMERS

61. STEREOSPECIFIC REACTION
•A reaction in which stereo chemically different molecules reacts differently is called a
stereospecific reaction. In this case the cis-and trans-stereoisomers give different
products.
•In Stereospecific reaction, stereoisomers can:
•Yields different stereoisomers as product.
•Reacts at different rate.
•Reacts with different paths to yield quite different kind of compounds as products.
•Its focused on reactants and their stereochemistry, as each stereoisomer behaves
specifically.
•It means, Reaction starts with one specific stereoisomer can yield a specific isomer
only as product.
•Stereospecificity towards enantiomers is called enantiospecificity.
•Stereospecificity towards distereomersis called distereospecificity.

62. Stereospecific'relatestothemechanismofareaction,thebest-knownexample
beingtheSN2reaction,whichalwaysproceedswithinversionofstereochemistry
atthereactingcentre.

63. STEREOSELECTIVEREACTION
•A stereoselectiveprocess is one in which one stereoisomer predominates over
another when two or more may be formed as per favorable reaction
pathway.
•If more than one reaction occur between a set of reactants under the same
conditions giving products that are stereoisomers and if one product forms in
greater amounts than the other, the overall reaction is said to be
stereoselective.
•Steroselectivitysolely concerns with the products, and their stereochemistry.
•Stereoselectivitytowards enantiomers is called enantioselective.
•Stereoselectivitytowards distereomersis called distereoslective.

67. C
C
H
CH
3
H
H
3C
C
C
CH
3
H
H
H
3C
CH
3
H Br
CH
3
Br H
CH
3
Br H
CH
3
H Br
CH
3
H Br
CH
3
H Br
+
Br
2
Br
2

68. CC CC anti-addition
CC CC
syn-addition