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3.12
DisubstitutedCycloalkanes:
StereoisomersStereoisomersareisomersthathave
sameconstitutionbutdifferent
arrangementofatomsinspace
IsomersConstitutionalisomersStereoisomers
1,2-DimethylcyclopropaneTherearetwostereoisomersof
1,2-dimethylcyclopropane.Theydifferinspatialarrangementofatoms.
1,2-Dimethylcyclopropanecis-1,2-Dimethylcyclopropanehasmethylgroups
onsamesideofring.trans-1,2-Dimethylcyclopropanehasmethylgroups
onoppositesides.
Relativestabilitiesofstereoisomersmaybe
determinedfromheatsofcombustion.
3371kJ/mol3366kJ/molvanderWaalsstrainmakescis
stereoisomerlessstablethantrans
3.13
ConformationalAnalysis
ofDisubstitutedCyclohexanes
cistransCH35219kJ/mol5212kJ/mollessstablemorestableTransstereoisomerismorestablethancis,but
methylgroupsaretoofaraparttocrowdeachother.H3CHHH3CCH3HH1,4-DimethylcyclohexaneStereoisomers
CH3Twoequivalentconformations;eachhasoneaxial
methylgroupandoneequatorialmethylgroupH3CHHHCH3HCH3HH3CHCH3Conformationalanalysisof
cis-1,4-dimethylcyclohexane
Twoconformationsarenotequivalent;moststable
conformationhasbothmethylgroupsequatorial.CH3H3CHHHH3CHCH3HH3CHCH3Conformationalanalysisof
trans-1,4-dimethylcyclohexane
cistrans5223kJ/mol5217kJ/mollessstablemorestableAnalogousto1,4inthattransismorestable
thancis.CH3CH3HHH3CCH3HH1,2-DimethylcyclohexaneStereoisomers
Twoequivalentconformations;eachhasoneaxial
methylgroupandoneequatorialmethylgroupCH3CH3HHHCH3HCH3HCH3HCH3Conformationalanalysisof
cis-1,2-dimethylcyclohexane
Twoconformationsarenotequivalent;moststable
conformationhasbothmethylgroupsequatorial.CH3H3CHHHCH3HCH3HH3CHCH3Conformationalanalysisof
trans-1,2-dimethylcyclohexane
cistrans5212kJ/mol5219kJ/molmorestablelessstableUnlike1,2and1,4;cis-1,3ismorestablethantrans.H3CCH3HHCH3H3CHH1,3-DimethylcyclohexaneStereoisomers
Twoconformationsarenotequivalent;moststable
confor
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