Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

∆∆H #
from steric repulsions
in transition state
cis
trans
Bromination transition state
resembles a three-membered ring
∆∆H
from steric repulsions in
ground state
In general, the difference in enthalpy of the transition states for bromination was
greater than the enthalpy difference of the isomeric alkenes, This finding is consistent with
a cyclic somewhat closer together than in the alkene.
The kinetics of brominations are often complex, with at least three term making
contributions under given conditions:
Rate = k 1 [alkene][Br 2 ] + k 2 [alkene][Br 2 ] 2 + k 3 [alkene][Br 2 ][Br - ]
Br 2
+Br
Br 2
Br 2
Br
Br -
Br
C=C C=C C — C C—C—
Br 2
Relative reactivity of alkenes toward halogenation
Alkene Chlorination Bromination
1.00 1.00
1.15 0.12
63 27
50 17.5
58 57
11,000 13,700
The Hammett correlation for bromination of styrenes is best with σ + substituent
constants and, gives ρ = -4.8. The rates of reactions increase with solvent polarity and
with additional substitution of electron-releasing alkyl groups at the double bond. All of
these features are in accord with an electrophilic mechanism.
Chlorination generally exhibits second-order kinetics and usually leads to expulsion
of a proton from the chloronium ion intermediate.
R
R 2 C
Cl +
R Cl
R
R
C C C C + H +
R
R 2 C
R
H
Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 4