by the pair of electrons with which it migrates. It never leaves the carbon
skeleton.
- There two ways to remove a proton from the carbocation:
- Path (b) leads to the highly stable tetrasubstituted alkene, and this is the path
followed by most of the carbocations.
- Path (a) leads to a less stable, disubstituted alkene and produces the minor
product of the reaction.
- The formation of the more stable alkene is the general rule (Zaitsev’s rule) in
the acid-catalyzed dehydration reactions of alcohols.
Step 4
A −
(a)
A
H
(a)
Less stable
alkene
More stable
alkene
(minor product)
(major product)
CH 2 C C CH^3 + H
H 3 C
H
CH 3
+
(b)
(b)
H 3 C
CH 3
C
CCH 3
CH 3
H 2 C
CH 3
CCC H 3
CH 3
- Rearrangements occur almost invariably when the migration of an alkanide
ion or hydride ion can lead to a more stable carbocation.
H 3 C CCC H^3
CH 3
CH 3
H 3 C
H
+
2 o carbocation
methanide
migration H^3 C C
C CH 3
H 3 C
H
+
3 o carbocation^
H 3 C CCC H^3
H
H
H 3 C
H
+
2 o carbocation
hydride
migration H^3 C C
C CH 3
H 3 C
H
+
3 o carbocation^
