Page 582 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The steric dependence is imposed by the bulky trimethylamine leaving group. In the 563
2
1
TS for anti elimination, steric repulsion is increased as R and R increase in size.
When the repulsion is sufficiently large, the TS for syn elimination is preferred. SECTION 5.10
Elimination Reactions
+
N (CH ) D
3 3
R 2 R 1 R 2 R 1 1
R 1 H H R D
R 2 D D H N (CH ) R 2 H
+
H H 3 3
– B – B
anti TS syn TS
Another aspect of the stereochemistry of elimination reactions is the ratio of E-and
Z-products. The proportion of Z-and E-isomers of disubstituted internal alkenes formed
during elimination reactions depends on the identity of the leaving group. Halides
usually give mainly the E-alkenes. 299 Bulkier groups, particularly arenesulfonates, give
higher proportions of the Z-alkene. Sometimes, more Z-isomer is formed than E-
isomer. The preference for E-alkene probably reflects the unfavorable steric repulsions
present in the E2 transition state leading to Z-alkene. High Z:E ratios are attributed
to a second steric effect that becomes important only when the leaving group is large.
The conformations leading to E-and Z-alkene by anti elimination are depicted below.

OSO Ar OSO 2 Ar
2
H R R H
E-alkene Z-alkene
R H R H
H H
– B – B
1 2

When the leaving group and base are both large, conformation 2 is favored because it
permits the leaving group to occupy a position removed from the -alkyl substituents,
while also maintaining an anti relationship to the -hydrogen. Anti elimination through
a TS arising from conformation 2 gives Z-alkene.

5.10.4. Dehydration of Alcohols

The dehydration of alcohols is an elimination reaction that takes place under
acidic rather than basic conditions and involves an E1 mechanism. 300 The function
of the acidic reagent is to convert the hydroxyl group to a better leaving group by
protonation.

+
H + –H O –H
2
RCHCH R' RCHCH R' RCHCH 2 R' RCH CHR'
2
2
+
OH O H 2
299 H. C. Brown and R. L. Kliminsch, J. Am. Chem. Soc., 87, 5517 (1965); I. N. Feit and W. H. Saunders,
Jr., J. Am. Chem. Soc., 92, 1630 (1970).
300
D. V. Banthorpe, Elimination Reactions, Elsevier, New York, 1963, pp. 145–156.