Toluene isotopic exchange

It is, of course, tempting to discuss the more novel and interesting deviations. However, the problems encountered in the nitration and isotope exchange reactions do not detract from the precision of the empirical correlation for substitution in toluene. 

A graphical examination of the data for meta substitution in toluene is presented as Fig. 8. The deviations from a precise linear relationship are larger than for the reactions at the para position of toluene. As for para substitution, the largest discrepancies are detected for the nitration and isotopic exchange reactions, indicated in Fig. 8 by broken circles. 

Ostman and Olsson68 have determined the kinetic isotope effect in the isotopic exchange of thiophene, by comparing the rates of protodedeuteration and protodetritiation at both positions of the ring. The values obtained for the D/fcT ratios are 1.94 for the a position and 1.70 for the )3 position. Thus the ratio kD/kT seems to increase with increasing reactivity, a tendency already observed in the hydrogen exchange of benzene and toluene.64... 

The obvious problem now is to question why a dissociative 7 -complex substitution mechanism is required in addition to the associative mechanism to interpret these isotope exchange reactions. Inspection of the dissociative mechanism shows that if a substituent such as a methyl group is present in the molecule (i.e., toluene from benzene, or the pico-lines from pyridine), then it would appear difficult to form a a bond at the adjacent carbon positions in the picolines (18) owing to steric... 

Further acceleration of isotope exchange, by three to four powers of ten, can be achieved116-118 by working in liquid deuterium fluoride, particularly if it contains BF3.108-110 The properties attaching to deuterium bromide apply also to this system. For toluene, equilibrium is reached in 15 minutes,108 and, as in exchange in DBr, only the hydrogen atoms attached directly to the aromatic ring are replaced, even in presence of BF3. 

For the exchanges carried out in liquid ammonia, kinetic isotope effects kD kT of 2.3-2.5 have been obtained for reaction of benzene, toluene, and naphthalene and for the reactions of the 2 positions of furan and thiophene with -butoxide in dimethyl sulphoxide somewhat lower values, 1.5 and 1.3, respectively, were obtained591, but whether this was a solvent or a substituent effect is not apparent from the data. 

The exchange reaction between Np(VI) and Np(V) has been investigated using the isotopic method ( Np) and an extraction separation (Np(VI) with tri-butylphosphate or thenoyltrifluoroacetone in toluene) Cohen et al. have found, for the exchange in perchlorate media, a rate law... 

More rigorous evidence supporting the inversion in the transesteiifi-cation of sulfinates was provided by kinetic measurements in which the rate of the acid-catalyzed racemization of (+)-methyl p-toluene-sulflnate 65 in methanol was compared with that of isotopic methoxy-methoxy exchange under exactly the same conditions. 

Fig. 9.17. Partial LT-FAB mass spectmm of the reaction mixture containing the iridium complexes 1 and 2 in toluene. In addition to the changes in mass, the isotopic pattern changes upon exchange of Cl by Br. By courtesy of P. Hofmann, University of Heidelberg.

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