Paine, Stuart William; (1993) Mechanisms of nucleophilic substitution. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The 15N n.m.r. studies of various reactions that are believed to proceed via nucleophilic radical chain mechanisms did not show a C.I.D.N.P. effect. The reaction of sodium nitrite with p-nitrocumyl bromide in dimethyl sulphoxide is a new and interesting system. No C.I.D.N.P. effect was observed for this reaction when followed by 15N n.m.r. spectroscopy. The reaction of sodium nitrite with p-nitrocumyl bromide in dimethyl sulphoxide proceeds via a heterolytic mechanism to give the products α,p-dinitrocumene, p-nitrocumyl alcohol and p-nitro-α-methylstyrene respectively. The overall disappearance of p-nitrocumyl bromide can be expressed as follows; -(d[RBr]t/dt) = ks[RBr]t + k'3[RBr]t [NO2¯]t2 The unimolecular component of the overall rate of reaction can be attributed to the elimination of p-nitrocumyl bromide in dimethyl sulphoxide. The termolecular component is probably due to a variation in the rate constant for a bimolecular process caused by the addition of sodium perchlorate. There is strong evidence to support this premise as a very large termolecular rate constant was obtained when sodium perchlorate was excluded. The reaction of lithium azide with p-nitrocumyl bromide in dimethyl sulphoxide proceeds via a heterolytic mechanism to give the products p-nitrocumyl azide, p-nitrocumyl alcohol and p-nitro-α-methylstyrene respectively. The overall disappearance of p-nitrocumyl bromide can be expressed as follows: -(d[RBr]t/dt) = ks[RBr]t + k'2[RBr]t [N3¯]t. Moreover, the reaction of tetrabutylammonium azide with p-nitrocumyl bromide in dimethyl sulphoxide also obeys, within experimental error, concurrent first and second order kinetics. Mechanisms of Nucleophilic Substitution The unimolecular component of the overall rate of reaction can be attributed to the elimination of p-nitrocumyl bromide in dimethyl sulphoxide. The bimolecular kinetic component is best explained by a "loose" SN2/E2C mechanism. The lack of an isotope effect for the bimolecular component indicates that there is very little carbocation character in the rate determining step. The large kBr/kCI ratio obtained for this reaction is indicative of SN2 reactions in a dipolar aprotic solvents.

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