有机化学Chap.pptVIP

axial bond: the set of six, straight up and down bonds in chair cyclohexane. Ring flip interconverts these bonds with the set of equatorial bonds. equatorial bond: the set of six, also “up and down”, but more or less in the plane of the ring in chair cyclo-hexane. These bonds are interconverted with the set of axial bonds through ring “flipping” of the chair. Summary: Substituted Cyclohexane Compounds In a sample of cyclohexane, the two identical chair conformers are present in equal concentration, and the hydrogens are all equivalent (50% equatorial 50% axial) due to rapid interconversion of the conformers. When the cyclohexane ring bears a substituent, the two chair conformers are not the same. In one conformer the substituent is axial, in the other it is equatorial. Due to steric hindrance in the axial location, substituent groups prefer to be equatorial and that chair conformer predominates in the equilibrium. Chair conformations are generally more stable than other possibilities. * Substituents 取代on chair conformers prefer to occupy equatorial positions due to the increased steric hindrance of axial locations. * The following equations and formulas illustrate how the presence of two or more substituents on a cyclohexane ring perturbs the interconversion of the two chair conformers in ways that can be predicted. We noted earlier that cycloalkanes having two or more substituents on different ring carbon atoms exist as a pair (sometimes more) of configurational stereoisomers. Now we must examine the way in which favorable ring conformations influence the properties of the configu- rational isomers. Remember, configurational stereoisomers are stable and do not easily interconvert, whereas, conformational isomers normally interconvert rapidly. In examining possible structures for substituted cyclohexanes, it is useful to follow two principles. ? Conformational Structures of Disubstituted Cyclohexanes 1,1-dimethylcyc