The natural abundance carbon-13 NMR spectra of benzyl bromide, 1- and 2-bromomethylnaphthalenes, 6-methyl-2-bromomethylnaphthalene, 4-bromomethylbiphenyl, and 9-bromomethylanthracene have been obtained and assigned. At each carbon position chemical shift differences (Δδ) for the structural change ArCH → ArCHBr have been tabulated, and for formally conjugated positions significant downfield shifts occur. The magnitudes of Δδ are very dependent on the aryl group and disposition, and correlate with theoretical measures of the change in π-charge density for ArCH→ ArCH. This deshielding is considered to be associated with removal of π charge by C-Br hyperconjugative electron withdrawal, and parallels trends previously established by F NMR measurements. This removal of π charge is not inconsistent with predominant (ortho, para) electrophilic substitution in benzyl halides if in the transition state rotation in -CHX occurs to promote C-H hyperconjugative electron release, but necessarily impeding C-Br electron withdrawal.
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Bullpitt et al. (1976) studied this question.