Page 400 - Mechanism and Theory in Organic Chemistry

Electrophilic Aromatic Substitution 387

with a single 7~ bond as shown in 68 and 69.1e1 The second possibility for 66 is 70,
a o complex in which the electrophile has formed a u bond with one carbon of the
aromatic ring.ls2

rr complex n complex o complex
68 69 70
There is an abundance of evidence that both rr complexes and u complexes
exist as stable species. For example, nmr studies have shown that the CH, protons
of the ethyl fluoride-boron trifluoride complex absorb at slightly lower fields in
the presence of toluene. Thus a new complex, which includes toluene and in which
the CH2 group bears more positive charge than it does in the absence of toluene, is
formed. However, the aromatic protons of toluene absorb at almost the same
frequency in the presence of BF,-FCH2CH3 as in its absence;ls3 thus, the new
complex is probably that shown in 71.

Another example is the complex that benzene forms with iodine. The infrared
spectrum in a frozen nitrogen matrix shows that in the complex, the benzene
symmetry in the ring plane is not altered. The 7~ complex 72, with the iodine
axial, has been proposed as the structure.ls4

Sigma complexes have also been observed in the nmr. For example, when
m-xylene is dissolved in HF + SbF, at -35OC, the proton magnetic resonance
spectrum shown in Figure 7.7 is obtained. The peak at 4.7 ppp downfield from
TMS is due to two parafinnic protons. The structure that best fits the spectrum

'" (a) G. A. Olah, S. Kobayashi, and M. Tashiro, J. Amr. Chem. Soc., 94, 7448 (1972); (b) D. V.
Banthorpe, Chem. Rev., 70, 295 (1970).
'" G. A. Olah and Y. K. Mo, J. Amer. Chem. Soc., 94, 9241 (1972).
le3 T. Oyama and R. Nakane, J. Org. Chem., 34, 949 (1969).
le4 L. Fredin and B. Melander, J. Amer. Chem. Soc., 96, 1672 (1974).