Page 792 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

H H H H H H H H H H 775
0.3 0.5 0.6 0.5 0.3
SECTION 9.1
0.5 0.5 0.5 0.5
Electrophilic Aromatic
0.6 0.6
0.6 Substitution Reactions
H H
α – 1.7β
0.33 0.33
+ α – 1.0β
α
0.33
charge α + 1.0β
distribution α + 1.7β
Fig. 9.2. Molecular orbitals and energy levels for the cyclohexa-
dienylium ion.

molecule, which is located centrally with respect to the aromatic ring. The N−O bond
distance is 1 07Å and the C−C bond distances are similar to those observed for the
3
radical cation. There are probably similar complexes in other EAS reactions. Recent
computational studies of the nitration of benzene describe the earliest energy minimum
as NO 2 + approaches benzene (in the gas phase) as being directed at the midpoint of a
particular C−C bond, as opposed to the center of the ring. 4
In order for a substitution to occur, a “ complex” must be formed. The term
complex is used to describe a cationic intermediate in which the carbon at the
site of substitution is bonded to both the electrophile and the hydrogen that is being
displaced. As the term implies, a bond is formed at the site of substitution. The
intermediate is a cyclohexadienylium cation. Its fundamental electronic characteristics
can be described in simple MO terms, as shown in Figure 9.2. The intermediate is
a4 electron delocalized system that is electronically equivalent to a pentadienyl
cation. There is no longer cyclic conjugation. The LUMO has nodes at C(2) and C(4)
of the pentadienyl structure and these correspond to the positions meta to the site
of substitution on the aromatic ring. As a result, the positive charge of the cation is
located at the positions ortho and para to the site of substitution. These electronic
features of the -complex intermediate are also shown by resonance structures.

E H E H E H
+
+
+

As we will see in Section 9.2, this pattern of charge distribution leads to the o,p-
or m-directing characteristics of various ring substituents.
There is considerable interest in the mechanism for conversion of the complex
into the complex. In particular, the question arises as to whether an electron transfer
occurs to yield a discrete cation radical–radical pair.

3 S. V. Rosokha and J. K. Kochi, J. Am. Chem. Soc., 123, 8985 (2001).
4
H. Xiao, L. Chen, X. Ju, and G. Li, Science in China B, 46, 453 (2003); P. M. Esteves, J. W. de Carneiro,
S. P. Cardoso, A. G. H. Barbosa, K. K. Laali, G. Rasul, G. K. S. Prakash, and G. A. Olah, J. Am. Chem.
Soc., 125, 4836 (2003).