MOLECULAR REARRANGEMENTS
7.8A REARRANGEMENTS DURING DEHYDRATION OF SECONDARY
ALCOHOLS
H 3 C
OH
C
CCH 3
CH 3
H 3 C H^3 C
CH 3
C
CCH 3
CH 3
H 2 C
CH 3
C
CCH 3
CH 3
85% H 3 PO 4
80 oC
3,3-Dimethyl-2-butanol 2,3-Dimethyl-2-butene
(major product)
2,3-Dimethyl-1-butene
(minor product)
+
Step 1
H 3 C
O H
H O
H
H
OH+ 2
OH
H
CCC H 3
CH 3
H 3 C
+ H 3 C CCC H^3
CH 3
H 3 C
+
Protonated alcohol
Step 2
OH+ 2
OH
H
H 3 C CCC H^3
CH 3
H 3 C
H 3 C CCC H^3
CH 3
H 3 C
H
+ +
A2o carbocation
- The less stable, 2° carbocation rearranges to a more stable 3° carbocation.
Step 3
H 3 C CCC H^3
CH 3
CH 3 CH 3
H 3 C
H
+
A 2o carbocation
H 3 C CCC H^3
H 3 C
H
δ+ +
δ+
+
(less stable)
Transition state
H 3 C C C CH^3
H 3 C
H
+
A 3o carbocation
(more stable)
- The methyl group migrates with its pair of electrons, as a methyl anion, –:CH 3 (a
methanide ion).
- 1,2-Shift:
- In the transition state the shifting methyl is partially bonded to both carbon atoms
