Abstract
A kinetic study was carried out on the solvolysis of o-nitrobenzyl bromide (o-isomer, 1) and p-nitrobenzyl bromide (p-isomer, 3), and o-nitrobenzoyl chloride (o-isomer, 2) in a wide range of solvents under various temperatures. In all of the solvents without aqueous fluoroalcohol, the reactions of 1 were solvolyzed at a similar rate to those observed for 3, and the reaction rates of 2 were about ten times slower than those of the previously studied p-nitrobenzoyl chloride (p-isomer, 4). For solvolysis in aqueous fluoroalcohol, the reactivity of 2 was kinetically more reactive than 4. The l/m values of the extended Grunwald–Winstein (G–W) equation for solvolysis of 1 and 2 in solvents without fluoroalcohol content are all significantly larger than unity while those in all the fluoroalcohol solvents are less than unity. The role of the ortho-nitro group as an intramolecular nucleophilic assistant (internal nucleophile) in the solvolytic reaction of 1 and 2 was discussed. The results are also compared with those reported earlier for o-carbomethoxybenzyl bromide (5) and o-nitrobenzyl p-toluenesulfonate (7). From the product studies and the activation parameters for solvolyses of 1 and 2 in several organic hydroxylic solvents, mechanistic conclusions are drawn.
Highlights
Ortho-substituted benzyl halides solvolyze somewhat more slowly than their para-isomers since nucleophilic displacement at the reaction center is reduced by the steric hindrance of a substituent group in the ortho-position [1,2]
We have previously reported our studies using the Grunwald–Winstein equation in investigations of the reaction mechanisms for the solvolyses of 5 and 6, which were known to have the possibility of intramolecular nucleophilic attack at the α-carbon (Equation (1)) [3]
The sensitivities (l/m = 2.16) to changes in NT and Yx of solvolysis of 1 in all of the solvents, except fluoroalcohol, are very similar to those for 6 [3] and 8 [20,21], which are shown to proceed by a bimolecular character, reflecting nucleophilic assistance from a solvent molecule paralleling the mechanism for 3 (l/m = 2.16)
Summary
Ortho-substituted benzyl halides solvolyze somewhat more slowly than their para-isomers since nucleophilic displacement at the reaction center is reduced by the steric hindrance of a substituent group in the ortho-position [1,2]. The solvolysis of benzoyl chloride has led to the carried out a mechanism series of studies on the4).solvolyses of benzoyl chloride and derivatives. Co-workers carried out a series of studies on the solvolyses of benzoyl chloride and derivatives. With bond running was proposed highly aqueous solvents carbonyl co-workers carried a ahead series studies onsolvolyses thefor solvolyses of benzoyl chloride. The solvolyses of 5 and 6 were found because of the internal assistance rather than the steric hindrance of the ortho position on the attack on the alkyl carbon from the presence of the ortho carbomethoxy group, to undergo solvolysis by a duality of mechanisms, i.e., intermolecular assistance from a solvent molecule and intramolecular assistance). The solvolyses of p-nitrobenzyl bromide (3) and 4 were studied in order to compare the isomeric difference between o- and p-isomers
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