Page 791 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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Br1A
CHAPTER 9
Aromatic Substitution

Br1

3.18 A
3.36 A

C4 C3
C3A C2
C2A C1

Br4a
Br1

Br2 Br4

3.23 A 3.20 A
3.29 A
3.24 A

C2B C6B C5B
C3B
C4B C7B C1B C4B
C7B C1B
C6B C5B C2B C3B
Fig. 9.1. Structures of benzene-Br 2 (top) and toluene-Br 2 (bottom) complexes. Reproduced from Chem.
Commun, 909 (2001), by permission of the Royal Society of Chemistry.

has structural data become available. The structures of the Br complexes with benzene
2
and toluene have been examined by X-ray crystallography at low temperature. The
Br molecule is nearly perpendicular to the ring and located between two specific
2
carbons, as opposed to being associated with the delocalized electron density. For
toluene, there are two complexes with the Br being associated with the ortho and para
2
1
carbons. This is significant because these are also the preferred sites for substitution,
and the structures indicate that an aspect of position selectivity is present at the
complex stage. These structures are shown in Figure 9.1.
Structural information is also available on the complex between mesitylene and
+ 2
the nitrosonium ion, NO . In this case there appears to be a high degree of charge
transfer and the complex is essentially between the aromatic radical cation and the NO
1 A. V. Vasilyev, S. V. Lindeman, and J. K. Kochi, Chem. Commun., 909 (2001); S. V. Rosokha and
J. K. Kochi, J. Org. Chem., 67, 1727 (2002).
2
E. K. Kim and J. K. Kochi, J. Am. Chem. Soc., 113, 4962 (1991).