Benzene, isotopic exchange

Only within the past few years have serious attempts been made to estimate quantitatively the differences in reactivity between thiophene and benzene and between the 2- and 3-position of thiophene. Careful investigation on the acid-induced exchange of deuterium and tritium have shown that the ratios of the exchange rates in the 2- and 3-positions are 1045 61 for deuterium and 911 60 for tritium in 57% by weight aqueous sulfuric acid at 24.6°C. A kinetic isotope effect in the isotopic exchange has been found to be k-r/kr, = 0.51 0.03 in the 2-position and kr/kjy — 0.59 0.04 in the... 

As an example consider the exchange reaction between benzyl iodide and free iodide ions, Eq. (3-43).6 Mixing ethanolic solutions of benzyl iodide and radio-labeled potassium iodide started the isotopic exchange. At different times, samples were withdrawn and added to a benzene-water mixture, causing separation into two layers. The aqueous layer contained the iodide ions, and the organic layer contained the benzyl iodide. The activity of 1311 was determined in each layer. 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

Farkas and Farkas (32) examined the kinetics of the exchange and hydrogenation of benzene catalyzed by platinized platinum foil at room temperature. The occurrence of isotope exchange was detected by the thermal conductivity technique. They reported (i) that the exchange reaction was only a little faster than hydrogenation and (ii) that exchange... 

M-Values Observed for the Hydrogen Isotope Exchange in Benzene... 

The hydrogenation of benzene and its alkyl-substituted derivatives takes place stepwise (Scheme 11.5). On the basis of instrumental and isotope exchange studies, the involvement of tt-adsorbed (8, 9) and cr-adsorbed (10) species was suggested 96-98... 

Considering the effect of substituents on overall reactivity, the trifluoroacetylation of 2-methylthiophene at position 5 is 600 times faster than that of thiophene. The methyl group at positions 3, 4 and 5 of thiophene has been shown to increase the rate of hydrogen-isotope exchange at position 2 by factors of 340, 12 and 200 respectively the values alternate as in the benzene series (71PMH(4)55)... 

A value of kA is available from measurements of hydrogen isotope exchange (kx) corrected for primary and secondary isotope effects. This is illustrated in Scheme 8 for the detritiation of tritiated benzene. In the lower part of the scheme, the reacting isotope of the benzenonium ion intermediate (9-t) is indicated as a superscript on the rate constant kv and a secondary isotope effect is neglected. 

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... 

In one of the earlier studies, Strohmeier and Mittnacht found 98) that the exchange of benzenechromium tricarbonyl with labeled benzene is essentially slower than the corresponding reaction with (7r-C7Hg)Cr(CO)3. Kinetics of the isotopic exchange reaction... 

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