Advices For Studying Organic Chemistry

Figure 5.21 cis-1,2-Dimethylcyclohexane exists as two rapidly interconverting chair
conformations (c) and (d).
i) The two conformational structures (c) and (d) are mirror-image structures
but are not identical.
ii) Neither has a plane of symmetry ⇒ each is a chiral molecule ⇒ they are
interconvertible by a ring flip ⇒ they cannot be separated.
iii) Structures (c) and (d) interconvert rapidly even at temperatures considerably
below room temperature ⇒ they represent an interconverting racemic form.
iv) Structures (c) and (d) are not configurational stereoisomers ⇒ they are
conformational stereoisomers.

CH 3

CH 3

CH 3

CH 3

Top view

Not a symmetry plane

Chair view Ring-flip

Mirror plane

cis-1,2-Dimethylcyclohexane

(interconvertible enantiomers)

Nota symmetry plane

H 3 C

CH 3

CH 3

H 3 C

4. In general, it is possible to predict the presence or absence of optical activity in
   any substituted cycloalkane merely by looking at flat structures, without
   considering the exact three-dimensional chair conformations.

5.14 RELATING CONFIGURATIONS THROUGH REACTIONS IN

WHICH NO BONDS TO THE STEREOCENTER ARE BROKEN

1. Retention of configuration: