The ion-pair mechanism and bimolecular displacement at saturated carbon. VI. Racemization and radio-bromide exchange for substituted 1-phenylbromoethanes; solvent effects

Abstract

. J. Chem. 65, 363 (1987).Racemization and radio-bromide exchange kinetics for I-phenylbromoethanes in acetonitrile and in nitromethane using tetrabutylammonium bromide are reported. The results, together with those previously reported for acetone solutions, provide direct empirical support for the ion-pair mechanism for nucleophilic substitution at saturated carbon. Changing the substituents on the phenyl from the 4-nitro through to the 3,4-dimethyl substrate and the solvent from acetone to the more polar acetonitrile and nitromethane shifts the transition state for bromide substitution from an early to a late stage of the equilibria series substrate intimate ion pair various solvated ion pairs free or dissociated ions. For all the substrates in acetone and, for the species giving the less stable carbocations, in acetonitrile and nitromethane, both racemizations and exchanges are bimolecular. In the latter solvents, the substrates giving the more stable carbocations show mixed kinetics. The ion-pair mechanism for nucleophilic substitution at saturated carbon was presented in some detail in the previous I paper in this series (8). It was pointed out that while there was I much direct empirical evidence for the involvement of ion pairs, Iespecially for solvolysis in reactions at or near the unimolecular 1 extreme (3, 9), perhaps the best direct empirical evidence for I their involvement in bimolecular substitutions was provided in that paper. Chiral 1 -phenylbromoethanes were prepared (10) with the following substituents on the phenyl group: 4'-nitro, 4'-bromo, unsubstituted, 3'-methyl, 4'-methyl, and 3',4'-dimethyl. The rates of racemization of chiral substrate and of radio-bromidebromide exchange on racemic substrate were determined in acetone containing lithium bromide. If the Hughes-Ingold mechanisms apply, the rate constants of racemization and exchange should be equal for a unimolecular process whereas for a bimolecular one the ratio should be 2.00: 1 .OO since the inversion of one half of the substrate results in a racemic mixture. In fact Hughes and Ingold reported that, for the parent compound, 1 -phenylbromoethane itself, the rate of racemization was, within experimental error, indeed twice the rate of bromide exchange (2), an observation we previously confirmed (1 1). With the 4-methyl compound, by contrast, the racemization to exchange rate constant ratio (k,,,/k,,,~) was found to average 2.36: 1.00. Both racemization and exchange were bimolecular, first order in each of alkyl bromide concentration and bromide activity.2 These results are very difficult to accommodate with the Hughes-Ingold mechanisms but present no problem with the ion pair cheme (Scheme 1). Apparently a portion of the initially formed contact ion pair is being sufficiently solvent separated for some of the carbocations to turn over with respect to the bromide ion before collapsing to starting material with partial racemization. The observed rate ratio means that racemization via the internal pathway is more than one third as fast as...

β-Deuterium kinetic isotope effects for identity processes: bromide ion substitution at 1-bromo-1-arylethanes and 2-bromooctane

“…1 clearly indicates, this apparent concentratior dependence arises from mixed kinetics (1,6). For such cases first-and second-order components of the reaction can be sep. arated by plotting the observed rate versus the concentration o. the nucleophile, or its salt.…”

“…Hughes et al (3) followed radio-iodide exchange with 2-iodooctane, and radio-bromide exchanges with 1-bromo-1-phenylethane and 2-bromopropanoic acid in the mid1930s. Others have also used radio tracers to follow exchanges (4)(5)(6)(7)(8)(9). We employed the electrodeposition of silver halide at a silver anode, a method developed and extensively used by Beronius (3, to follow radio-halide exchanges (6,7).…”

The ion-pair mechanism and bimolecular displacement at saturated carbon. VII. Racemization of substituted 1-phenylbromoethanes; ionic strength effects

“…either interfered with the silver bromide electrodeposition, were themselves incompletely dissociated, or both (4), so that attempts to work at constant ionic strength were abandoned; all subsequent kinetics were done at the same fixed concentrations ranging from 5.00 to 100.00 x M in bromide ion. With the extension of the study to the more polar solvents acetonitrile and nitromethane, we found that tetrabutylammonium bromide, Bu4NBr, was apparently completely dissociated over the concentration range used: plots of observed racemization rate versus Bu4NBr concentration were linear (3). (In acetone, Bu4NBr is partially associated, as is LiBr in the two more polar solvents.…”

“…Consequently, the ability of the medium to support ionic charges, its ionizing power, and hence its ionic strength, can be important. Obviously, varying the concentration of the bromide ion, that is of M+BrP, to establish reaction order (2) and to permit separation of uniand bimolecular components (2,3), changes the ionic strength, and hence ionizing power, unless inert electrolyte is used to maintain constant ionic strength. say racemization and bromide-radio-bromide exchange rates, or of one substrate with another, are made at the same M+Br-concentrations and temperatures, fixed ionic strengths may not be required.…”

“…In acetonitrile and nitromethane, those substrates that are able to form more stable carbocations, the 3,4-dimethyl-and the 4-methylphenylbromoethane, racemize at a rate that increases linearly with Bu4NBr concentration but with a large positive intercept (3). Equation [I] thus applies; kl is the intercept that is the first-order or unimolecular rate constant while k; is the slope or second-order rate constant for racemization.…”