Polar and dipolar substituent effects and transition state in the reaction between substituted anilines and nitrosobenzene. An evaluation of ortho and electrostatic effects.

概要

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The rate constants of the hydronium catalyzed attack of substituted anilines on nitrosobenzene, to give azobenzene, exhibit a large dependence on the substituent constant σ− (ρ−=−1.92). The relatively high value of ρ− compared to that obtained from the equilibrium of protonation of anilines (ρ−=−2.77) and the solvent effect on the rate constants support the idea that the transition state involves a large amount of nitrogen–nitrogen bond formation and therefore a strong charge development on the nitrogen and oxygen of the amino and nitroso groups, respectively. A good correlation between basicity and nucleophilic reactivity is obtained by the Brönsted equation (β=0.65, r=0.988) for the para and meta dipolar substituents. Aminobenzoates show a significantly higher reactivity than that calculated from the Brönsted line and this is attributed to an electrostatic participation of the carboxylato group. Nevertheless, this effect is somewhat compensated by repulsion between the carboxylato and the negatively charged oxygen of the nitroso group.
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