A Guidebook to Mechanism in Organic Chemistry

Nucleophilic Substitution at a Saturated Carbon Atom

By contrast, vinyl halides such as CH^CH—CI and halogeno-
benzenes are very unreactive. The reason in each case is that some
overlap occurs between the p orbital on chlorine and an orbital on
the adjacent carbon atom:

The effect is to make this carbon atom more negative and hence less

readily attacked by, e.g.^9 OH, and also to introduce a certain amount

of double bond character into the linkage between carbon and chlorine

(confirmed by the slightly shorter C—CI bond distances observed in

the above compounds compared with Me-CHa—CI); features that

will discourage attack by either SN\ or Sjy2 mechanisms.

The influence of steric factors on mechanism is particularly observed

when substitution takes place at the /J-position. Thus in the series

Me

• \
  CH,—CH,—Hal Me—CH,—CH,—Hal CH—CH,—Hal
  Me
  Me
  \
  Me—C—CH,—Hal
  Me^