Page 514 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

The computed structure of bromonium ions from alkenes such as 2-methylpropene 495
are highly dependent on the computational method used and inclusion of correlation
is essential. 78 CISD/DZV calculations gave the following structural characteristics. SECTION 5.3
Addition of Halogens
Br +
CH 3 1.99 Å
80 o
CH 1.47 Å
3

Another study gives some basis for comparison of the halogens. 79 QCISD(T)/6-
311(d p) calculations found the open carbocation to be the most stable for C H F +
2 4
+
and C H Cl but the bridged ion was more stable for C H Br . The differences
+
2 4 2 4
were small for Cl and Br.
+ F F + + Cl Cl + + Br Br +
0.0 27.7 0.0 3.3 2.4 0.0
+
Relative energy in kcal/mol of open and bridged [C H X] ions
2 4
AIM charges for the bridged ions were as follows (MP2/6-311G(d p)). Note the very
different net charge for the different halogens.

–0.34 +0.16 +0.33
F + H Cl + H Br + H
H H H
H H H H H H
C +0.27 C +0.06 C –0.02
H +0.20 H +0.18 H +0.18

MP2/6-311G(d p) calculations favored open carbocations for the ions derived
from cyclohexene. On the other hand, the bridged bromonium ion from cyclopentene
was found to be stable relative to the open cation.

+ F + + Cl + + Br + +
F Cl Br + Br
Br

0.0 25.2 0.0 8.6 0.0 7.0 4.2 0.0
Relative stability of open and bridged cations in kcal/mol
Ref. 79

This result is in qualitative agreement with an NMR study under stable ion conditions
that found that the bromonium ion from cyclopentene could be detected, but not the
one from cyclohexene. 80 Broadly speaking, the computational results agree with the
F < Cl < Br order in terms of bridging, but seem to underestimate the stability of the
bridged ions, at least as compared to solution behavior.

78
M. Klobukowski and R. S. Brown, J. Org. Chem., 59, 7156 (1994).
79 R. Damrauer, M. D. Leavell, and C. M. Hadad, J. Org. Chem., 63, 9476 (1998).
80
G. K. S. Prakash, R. Aniszefeld, T. Hashimoto, J. W. Bausch, and G. A. Olah, J. Am. Chem. Soc., 111,
8726 (1989).