Deuterium reaction with benzene

The kinetic isotope effect was introduced in Chapter 19. If a bond to deuterium is formed or broken in the rate-determining step of a reaction, the deuterated compound will react more slowly, usually by a factor of about 2-7. This effect is particularly valuable when C-H bonds are being formed or broken. In Chapter 22 we told you that the rate-determining step in the nitration of benzene was the attack of the electrophile on the benzene ring. This is easily verified by replacing the hydrogen atoms round the benzene ring with deuteriums. The rate of the reaction stays the same. 

To understand the electrocatalytic steps, a more detailed characterization of surface species and of the activity of surface sites for prospective catalysts is needed. Some adsorption results only on Pt, discussed earlier, indicate the existence of two differnet catalytic sites that contribute to benzene exchange reactions with deuterium (795,194). Adsorption and reaction on two different catalytic sites is not unknown in heterogeneous, gas phase catalytic reductions of olefinic and cyclic hydrocarbons (560). 

Flash vacuum pyrolysis of 2,5-tetramethylene Dewar benzene gives tetralin. However, the intermediacy of 4-m a-cyclophane and its benzvalene equivalent has been implicated in the reaction with deuterium labeling being consistent with the observations (Scheme 11.81). " ... 

Four nucleophilic aromatic substitution reactions with amines have been studied for deuterium isotope effects. Hawthorne (81) found that the rates of reaction of o- and p- nitrochlorobenzene with piperidine in xylene are not altered by substitution of deuterium for hydrogen on the amino group of the nucleophile. Similarly, 2,4-dinitrochlorobenzene reacts at identical rates with -butylamine and with iV,iV-dideuterio-n-butylamine containing 0.75% ethanol (76). Under the conditions of the rate measurements the deuterated amine does not undergo exchange with either the chloroform or the ethanol present. Finally, the reaction of trichloro-s-triazene with aniline shows no isotope effect with AT,JV-dideuterioaniline in benzene as well as in benzene saturated with deuterium oxide (82). 

B. Reactions of Benzene with Deuterium Oxide and with Deuterium-... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

B. Reaction of Benzene with Deuterium on Nickel Catalysts. 103... 

The outcome of this research naturally raised the question whether the catalysts investigated differed also in chemisorptive and catalytic properties. We therefore examined how the adsorption of CO and CO2, and the reaction of benzene with deuterium proceed on these catalysts. This study forms the subject of the present article. 

However, it will at any rate be clear now that the palladium, nickel, and iridium catalysts used in our experiments differ widely in surface characteristics, as is evident from the variations in chemisorptive behavior. An obvious question that may be asked now is whether the catalysts differ also in catalytic behavior. This induced us to study the reaction of benzene with deuterium on the nickel and iridium catalysts. 

The reaction of benzene with deuterium is an excellent means for studying the influence of the particle size on the catalytic activity. Working with a relatively simple mixture of reactants, one can study a number of reactions at the same time. 

Fig. 13. Schema for the experimental procedure of the reaction of benzene with deuterium.

The outcome of the experiments on nickel and on iridium shows that there is a marked contrast between these metals as regards the influence of particle size upon the course of the reaction of benzene with deuterium. Clearly, further experiments should be done along the same lines with other metals of group VIII of the periodic system. 

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