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1、3.4The Shapes of Cycloalkanes:Planar or Nonplanar?,assumed cycloalkanes are planar polygons distortion of bond angles from 109.5 givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing fact
2、or. But Baeyer was incorrect in his belief that cycloalkanes were planar.,Adolf von Baeyer (19th century),Torsional strain strain that results from eclipsed bonds van der Waals strain (steric strain)strain that results from atoms being too close together angle strainstrain that results from distorti
3、on of bondangles from normal values,Types of Strain,Measuring Strain in Cycloalkanes,Heats of combustion can be used to comparestabilities of isomers. But cyclopropane, cyclobutane, etc. are not isomers. All heats of combustion increase as the numberof carbon atoms increase.,Measuring Strain in Cycl
4、oalkanes,Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a per CH2 group basis.,CycloalkanekJ/molPer CH2 Cyclopropane2,091697 Cyclobutane2,721681 Cyclopentane3,291658 Cyclohexane3,920653 Cycloheptane4,599657 Cyclooctane5,267658 Cyclononane5,933659 Cyclod
5、ecane6,587659,Heats of Combustion in Cycloalkanes,CycloalkanekJ/molPer CH2 According to Baeyer, cyclopentane should have less angle strain than cyclohexane. Cyclopentane3,291658 Cyclohexane3,920653 The heat of combustion per CH2 group is less for cyclohexane than for cyclopentane. Therefore, cyclohe
6、xane has less strain than cyclopentane.,Heats of Combustion in Cycloalkanes,Adolf von Baeyer (19th century),assumed cycloalkanes are planar polygons distortion of bond angles from 109.5 givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane Baeyer deserves credit for
7、advancing the ideaof angle strain as a destabilizing factor. But Baeyer was incorrect in his belief that cycloalkanes were planar.,heat of combustion suggests that angle strain is unimportant in cyclohexane tetrahedral bond angles require nonplanar geometries,3.5Conformations of Cyclohexane,All of t
8、he bonds are staggered and the bond angles at carbon are close to tetrahedral.,Chair is the most stable conformation of cyclohexane,All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat.,180 pm,Boat conformation is less stable
9、than the chair,Eclipsed bonds bonds gives torsional strain toboat.,Boat conformation is less stable than the chair,Less van der Waals strain and less torsional strain in skew boat.,Boat,Skew boat,Skew boat is slightly more stable than boat,the chair conformation of cyclohexane is themost stable conf
10、ormation and derivativesof cyclohexane almost always exist in the chair conformation,Generalization,3.6Axial and Equatorial Bonds in Cyclohexane,The 12 bonds to the ring can be divided into two sets of 6.,Axial bonds point north and south,6 Bonds are axial,The 12 bonds to the ring can be divided int
11、o two sets of 6.,Equatorial bonds lie along the equator,6 Bonds are equatorial,3.7Conformational Inversion(Ring-Flipping) in Cyclohexane,chair-chair interconversion (ring-flipping) rapid process (activation energy = 45 kJ/mol) all axial bonds become equatorial and vice versa,Conformational Inversion
12、,Half-chair,Half-chair,Skewboat,Half-chair,Skewboat,Half-chair,Skewboat,45 kJ/mol,23 kJ/mol,most stable conformation is chair substituent is more stable when equatorial,3.8Conformational Analysis of Monosubstituted Cyclohexanes,5%,95%,Chair chair interconversion occurs, but at any instant 95% of the
13、 molecules have their methyl group equatorial. Axial methyl group is more crowded than an equatorial one.,Methylcyclohexane,5%,95%,Source of crowding is close approach to axial hydrogens on same side of ring. Crowding is called a 1,3-diaxial repulsion and is a type of van der Waals strain.,Methylcyc
14、lohexane,40%,60%,Crowding is less pronounced with a small substituent such as fluorine. Size of substituent is related to its branching.,Fluorocyclohexane,F,F,Less than 0.01%,Greater than 99.99%,Crowding is more pronounced with a bulky substituent such as tert-butyl. tert-Butyl is highly branched.,tert-Butylcyclohexane,van der Waalsstrain due to1,3-diaxialrepulsions,tert-Butylcyclohexane,
