0 A kinetic analysis could not be performed due to the high acid
1
Acknowledgements
concentration and speed of the reaction. However, a rough
estimation of the reaction rate was obtained from the comparison of
end-of-reaction times.
1 K. K. Sen Gupta, H. Samaddar, P. K. Sen and A. Banerjee, J. Org.
Chem., 1982, 47, 4511.
2 K. K. Sen Gupta, S. C. Kumar, P. K. Sen and A. Banerjee,
Tetrahedron, 1988, 44, 2225.
3 (a) C.-Z. Dong, M. Julia and J. Tang, Eur. J. Org. Chem., 1998, 1689;
We thank Dr A. C. Whitwood and Dr R. Sayer (University of
York) for performing the EPR and ATR measurements, the
EPSRC and Contract Chemicals Ltd for financial support
1
1
1
1
(
to RMHB) and the European Commission for a Marie Curie
Individual Research Fellowship (to GR).
(
b) W. J. Wilson and F. G. Soper, J. Chem. Soc., 1949, 3376.
4 To give two examples: in the bromination of nitrobenzene it was
found that the yield of 3-bromonitrobenzene was nil below 50% w/w
H SO , then rose sharply to 90% at 68% w/w H SO , and dropped to
References
1
For recent reviews on bromination, see (a) S. D. R. Christie, J. Chem.
Soc., Perkin Trans. 1, 1999, 737; (b) R. S. Brown, Acc. Chem. Res.,
2
4
2
4
15% at 70% w/w H SO (all data from ref. 4a). In the case of
2
4
1
997, 30, 131. On oxidation (c) M. Besson and P. Gallezot, Catal.
Today, 2000, 57, 127; (d ) S. Imamura, Ind. Eng. Chem. Res., 1999,
8, 1743; (e) For a monograph, see R. A. Sheldon and J. K. Kochi,
benzaldehyde, which can undergo both oxidation and bromination
reactions, it was found that 39% w/w H SO was the optimal
concentration to maximise 3-bromobenzaldehyde yield, provided
that the temperature was kept at 90–100 ЊC, since under those
reaction conditions koxidation > kbromination (see ref. 4b).
2
4
3
Metal Catalyzed Oxidations of Organic Compounds, Academic
Press, New York, 1981.
2
3
These salts are used to scavenge the bromide ions from the solution
so that the formation of molecular bromine is minimised, see (a)
T. J. Broxton, L. W. Deady, J. D. McCormack, L. C. Kam and
S. H. Toh, J. Chem. Soc., Perkin Trans. 2, 1974, 1769; (b) C. S. Reddy
and E. V. Sundaram, Tetrahedron, 1989, 45, 2109.
A. Ewenson, D. Itzhak, M. Freiberg-Bergstein, A. Shushan,
B. Croitoru, D. Beneish and N. Faza, WO 99/19275 (to Bromine
Compounds Ltd).
15 A specific study dealing with the kinetics of electrophilic aromatic
bromination under dilute acidic conditions showed that it is unlikely
ϩ
that “Br ” itself exists in solution, and that a better explanation
ϩ
would be the reaction of an {ArH ϩ H OBr } complex in the
2
rate-determining step. See H. M. Gilow and J. H. Ridd, J. Chem.
Soc., Perkin Trans. 2, 1973, 1321.
16 H. Y. Choi and D. Y. Chi, J. Am. Chem. Soc., 2001, 123, 9202.
17 For a discussion, see N. S. Isaacs, Physical Organic Chemistry,
Longman, Essex, 1987 pp. 740–743.
18 For recent studies on Ar–X bond scissions, see (a) L. Pause,
M. Robert and J.-M. Savéant, J. Am. Chem. Soc., 1999, 121, 7158;
(b) A. B. Pierini, J. S. Duca, Jr. and D. M. A. Vera, J. Chem. Soc.,
Perkin Trans. 2, 1999, 1003.
4
5
(a) J. J. Harrison, J. P. Pellegrinia and C. M. Selwitz, J. Org. Chem.,
1
981, 46, 2169; (b) A. Groweiss, Org. Process Res. Dev., 2000, 4, 30.
Bromination using HOBr generated from bromate and an inorganic
reducing agent such as NaHSO3 was also reported, see (a)
D. Kikuchi, S. Sakaguchi and Y. Ishii, J. Org. Chem., 1998, 63, 6023;
(
7
b) H. Ohta, Y. Sakata, T. Takeuchia and Y. Ishii, Chem. Lett., 1990,
33.
19 The spectra were recorded using a horizontally mounted trapezoid
germanium internal reflection element (IRE) (81 × 11 × 4 mm, 60Њ
bevel). Data acquisition of single-beam background spectrum of the
bare IRE was accomplished using the Omnic ESP software.
6
7
Herein the term acid bromate refers to a solution or a mixture
of a bromate salt and a strong concentrated Brønsted acid, e.g.
KBrO ϩ 50% w/w H SO .
Ϫ1
Measurements were performed at 2 cm resolution (256 scans).
3
2
4
For example, numerical methods have been used to determine the
equilibrium relationship between the various primary species in
bromination reactions. See A. N. Shevelkova, Y. I. Sal’nikov, N. L.
Kuz’mina and A. D. Ryabov, FEBS Lett., 1996, 383, 259.
W. R. Fawcett, G. Liu and A. A. Kloss, J. Chem. Soc., Faraday
Trans., 1994, 2697.
20 ADF 2.3.0, Theoretical Chemistry, Vrije Universiteit, Amsterdam;
(a) E. J. Baerends, D. E. Ellis and P. Ros, Chem. Phys., 1973, 2, 41;
(b) G. te Velde and E. J. Baerends, J. Comput. Phys., 1992, 99, 84; (c)
C. Fonseca Guerra, J. G. Snijders, G. te Velde and E. J. Baerends,
Theor. Chem. Acc., 1998, 99, 391.
21 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200.
22 A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
8
9
(a) G. Rothenberg and J. H. Clark, Green Chem., 2000, 2, 248;
(
4
b) G. Rothenberg and J. H. Clark, Org. Process Res. Dev., 2000,
, 270.
1
The identity of these products was determined by GCMS and H
NMR as (in descending order of quantity) dibromoacetophenone,
tribromoacetophenone, bromophenacyl bromide and dibromophen-
acyl bromide.
23 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822.
24 L. Versluis and T. Ziegler, J. Chem. Phys., 1988, 322, 88.
J. Chem. Soc., Perkin Trans. 2, 2002, 630–635
635