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1、3.43.4The Shapes of Cycloalkanes:The Shapes of Cycloalkanes:Planar or Nonplanar?Planar or Nonplanar?assumed cycloalkanes are planar polygonsassumed cycloalkanes are planar polygonsdistortion of bond angles from 109.5 givesdistortion of bond angles from 109.5 givesangle strain to cycloalkanes with ri
2、ngs eitherangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentanesmaller or larger than cyclopentaneBaeyer deserves credit for advancing the ideaBaeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.of angle strain as a destabilizing factor.
3、But Baeyer was incorrect in his belief that But Baeyer was incorrect in his belief that cycloalkanes were planar.cycloalkanes were planar.Adolf von Baeyer(19th century)Adolf von Baeyer(19th century)Torsional strainTorsional strain strain that results from eclipsed bonds strain that results from ecli
4、psed bonds van der Waals strain(steric strain)van der Waals strain(steric strain)strain that results from atoms being too strain that results from atoms being too close togetherclose together angle strainangle strainstrain that results from distortion of bondstrain that results from distortion of bo
5、ndangles from normal valuesangles from normal valuesTypes of StrainTypes of Strain Measuring Strain in Cycloalkanes Measuring Strain in CycloalkanesHeats of combustion can be used to compareHeats of combustion can be used to comparestabilities of isomers.stabilities of isomers.But cyclopropane,cyclo
6、butane,etc.are not isomers.But cyclopropane,cyclobutane,etc.are not isomers.All heats of combustion increase as the numberAll heats of combustion increase as the numberof carbon atoms increase.of carbon atoms increase.Measuring Strain in Cycloalkanes Measuring Strain in CycloalkanesTherefore,divide
7、heats of combustion by numberTherefore,divide heats of combustion by number of carbons and compare heats of combustion of carbons and compare heats of combustion on a per CHon a per CH2 2 group basis.group basis.CycloalkaneCycloalkanekJ/molkJ/molPer CHPer CH2 2CyclopropaneCyclopropane2,0912,09169769
8、7CyclobutaneCyclobutane2,7212,721681681CyclopentaneCyclopentane3,2913,291658658CyclohexaneCyclohexane3,9203,920653653CycloheptaneCycloheptane4,5994,599657657CyclooctaneCyclooctane5,2675,267658658CyclononaneCyclononane5,9335,933659659CyclodecaneCyclodecane6,5876,587659659Heats of Combustion in Cycloa
9、lkanesHeats of Combustion in CycloalkanesCycloalkaneCycloalkanekJ/molkJ/molPer CHPer CH2 2According to Baeyer,cyclopentane shouldAccording to Baeyer,cyclopentane shouldhave less angle strain than cyclohexane.have less angle strain than cyclohexane.CyclopentaneCyclopentane3,2913,291658658CyclohexaneC
10、yclohexane3,9203,920653653The heat of combustion per CHThe heat of combustion per CH2 2 group is group isless for cyclohexane than for cyclopentane.less for cyclohexane than for cyclopentane.Therefore,cyclohexane has less strain thanTherefore,cyclohexane has less strain thancyclopentane.cyclopentane
11、.Heats of Combustion in CycloalkanesHeats of Combustion in CycloalkanesAdolf von Baeyer(19th century)Adolf von Baeyer(19th century)assumed cycloalkanes are planar polygonsassumed cycloalkanes are planar polygonsdistortion of bond angles from 109.5 givesdistortion of bond angles from 109.5 givesangle
12、 strain to cycloalkanes with rings eitherangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentanesmaller or larger than cyclopentaneBaeyer deserves credit for advancing the ideaBaeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.of angle st
13、rain as a destabilizing factor.But Baeyer was incorrect in his belief that But Baeyer was incorrect in his belief that cycloalkanes were planar.cycloalkanes were planar.heat of combustion suggests that angle strain heat of combustion suggests that angle strain is unimportant in cyclohexaneis unimpor
14、tant in cyclohexanetetrahedral bond angles require nonplanar tetrahedral bond angles require nonplanar geometriesgeometries3.53.5Conformations of CyclohexaneConformations of CyclohexaneAll of the bonds are staggered and the All of the bonds are staggered and the bond angles at carbon are close to te
15、trahedral.bond angles at carbon are close to tetrahedral.Chair is the most stable Chair is the most stable conformation of cyclohexaneconformation of cyclohexaneAll of the bond angles are close to tetrahedralAll of the bond angles are close to tetrahedralbut close contact between flagpole hydrogensb
16、ut close contact between flagpole hydrogenscauses van der Waals strain in boat.causes van der Waals strain in boat.180 pm180 pmBoat conformation is less stable Boat conformation is less stable than the chairthan the chairEclipsed bonds bonds gives torsional strain toEclipsed bonds bonds gives torsio
17、nal strain toboat.boat.Boat conformation is less stable Boat conformation is less stable than the chairthan the chairLess van der Waals strain and less torsional Less van der Waals strain and less torsional strain in skew boat.strain in skew boat.BoatBoatSkew boatSkew boatSkew boat is slightly more
18、stable Skew boat is slightly more stable than boatthan boatthe chair conformation of cyclohexane is thethe chair conformation of cyclohexane is themost stable conformation and derivativesmost stable conformation and derivativesof cyclohexane almost always exist in the of cyclohexane almost always ex
19、ist in the chair conformationchair conformationGeneralizationGeneralization3.63.6Axial and Equatorial Bonds in Axial and Equatorial Bonds in CyclohexaneCyclohexaneThe 12 bonds to the ring can be The 12 bonds to the ring can be divided into two sets of 6.divided into two sets of 6.Axial bonds point n
20、orth and southAxial bonds point north and south6 Bonds are axial6 Bonds are axialThe 12 bonds to the ring can be The 12 bonds to the ring can be divided into two sets of 6.divided into two sets of 6.Equatorial bonds lie along the equatorEquatorial bonds lie along the equator6 Bonds are equatorial6 B
21、onds are equatorial3.73.7Conformational InversionConformational Inversion(Ring-Flipping)in Cyclohexane(Ring-Flipping)in Cyclohexanechair-chair interconversion(ring-flipping)chair-chair interconversion(ring-flipping)rapid process(activation energy=45 kJ/mol)rapid process(activation energy=45 kJ/mol)a
22、ll axial bonds become equatorial and vice versaall axial bonds become equatorial and vice versaConformational InversionConformational InversionHalf-Half-chairchairHalf-Half-chairchairSkewSkewboatboatHalf-Half-chairchairSkewSkewboatboatHalf-Half-chairchairSkewSkewboatboat45 45 kJ/molkJ/mol45 45 kJ/mo
23、lkJ/mol23 23 kJ/molkJ/molmost stable conformation is chairmost stable conformation is chairsubstituent is more stable when equatorialsubstituent is more stable when equatorial3.83.8Conformational Analysis of Conformational Analysis of Monosubstituted CyclohexanesMonosubstituted Cyclohexanes5%5%95%95
24、%Chair chair interconversion occurs,but at any Chair chair interconversion occurs,but at any instant 95%of the molecules have their methyl instant 95%of the molecules have their methyl group equatorial.group equatorial.Axial methyl group is more crowded than an Axial methyl group is more crowded tha
25、n an equatorial one.equatorial one.MethylcyclohexaneMethylcyclohexaneCHCH3 3CHCH3 35%5%95%95%Source of crowding is close approach to axial Source of crowding is close approach to axial hydrogens on same side of ring.hydrogens on same side of ring.Crowding is called a 1,3-diaxial repulsion and Crowdi
26、ng is called a 1,3-diaxial repulsion and is a type of van der Waals strain.is a type of van der Waals strain.MethylcyclohexaneMethylcyclohexane40%40%60%60%Crowding is less pronounced with a small Crowding is less pronounced with a small substituent such as fluorine.substituent such as fluorine.Size
27、of substituent is related to its branching.Size of substituent is related to its branching.FluorocyclohexaneFluorocyclohexaneF FF FLess than 0.01%Less than 0.01%Greater than 99.99%Greater than 99.99%Crowding is more pronounced with a bulky Crowding is more pronounced with a bulky substituent such as
28、 substituent such as terttert-butyl.-butyl.terttert-Butyl is highly branched.-Butyl is highly branched.terttert-Butylcyclohexane-ButylcyclohexaneC(CHC(CH3 3)3 3C(CHC(CH3 3)3 3van der Waalsvan der Waalsstrain due tostrain due to1,3-diaxial1,3-diaxialrepulsionsrepulsionsterttert-Butylcyclohexane-Butylcyclohexane