Additional Factors Affecting the Rate and Mechanism of Ring Opening Reaction

Abstract

The reactions of thiosulphate ion with some bromoalkylamines of structure RR'N-(CH2)n-Br, where n = 2,3,4,5 or 6 and R - R1 - H, methyl, ethyl or isopropyl, were studied kinetically in an aqueous solution at different pH, The kinetic results for these reactions in basic medium gave explicit evidence in favour of the formation of cyclic intermediate during the reaction course. The rate of these reactions depends mainly on the reactivity of the cyclic intermediate. It has been found that thereactivity of large ring intermediate is mainly controlled by the energy of ring strain relief. As the ring size becomes small (smaller than five members) the rate of the reaction becomes very fast and can't be explained on the bases of ring strain relief only, In order to explain this observation other factors may affect the reactivity of the cyclic intermediate are taken into account, such as the electron density at the nitrogen atom, the sterochemical situation at the reaction centre and the probability of a successful reaction. Taking into account these factors gave a better explanation for the high reactivity of small ring are given, On the other hand, it has been observed that the reaction in acidic medium proceed with a different mechanism in which the reaction occurs by a direct attackment of the nucleophile on the methylene group attached to the bromo group. The dependence of the rate constants on temperature were studied in order to estimate the thermodynamic parameters which were utilized in the discussion of the factors affecting the rate of ring opening reaction.