The expression "SN1" is the abbreviation of "unimolecular nucleophilic substitution reaction", one of the most common reactions in organic chemistry. As a fundamental reaction type, the mechanism of SN1 reaction has long been clear to the researchers, and the rate of the reaction concluded from the mechanism is quite simple. According to the mechanism, the rate of an "ideal" SN1 reaction is only proportional to the concentration of the substrate, which can be expressed as the following formula: rate=k[RX] ([RX] is the concentration of the substrate). However, experiments have shown some exceptions including some of which follow the formula at the early period of the reaction but change the rates later (O. T. Benfey, E. D. Hughes, and C. K.
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However, there are three types of deviations between the results and the theoretical ones which can not be neglected. One of them is the “reversibility of the initial heterolysis” (O. T. Benfey, E. D. Hughes, and C. K. Ingold, J. Chem. Soc. 2489) which makes the reaction a pure first-order one at the beginning but slower than it should be in the later period. One sort of examples are the hydrolysis reactions of diarylmethyl halides. For illustration, the hydrolysis of diphenylmethyl chloride is discussed here. The reason for the dropping rate at the later period is that the leaving group having disassociated may compete with the reactant to combine with the diphenylmethyl cation. At the beginning, although chloride ion is a better nucleophilic reagent than water, its concentration is so small that it can be neglected, while the concentration of water is so large that it does not appear in the rate formula as the solvent. During the process of the reaction, the concentration of chloride ion increases gradually and the effect of the competition between chloride ion and water becomes important enough to be taken into consideration in the later period, so the rate drops later. The case shows that the formula should be modified to meet more general cases, applying the steady-state approximation, the rate formula can be modified to be a more universal one taking no consideration of the …show more content…
However, if the leaving ion can decrease the rate, adding additional leaving ion can also decrease the rate of the reaction in the same way (Michael B Smith and Jerry March, 434). This is a common effect in the inorganic reactions called common-ion effect. Since there are many organic reactions occurring in organic solvents, the effect is not discussed in organic reactions often. However, some SN1 reactions like the hydrolysis discussed is proceeded in the solvents suitable and there are ions produced during the processes in which the common-ion effect should be taken into consideration. While other SN1s are not affected by the common-ion effect because no ion is involved in these reactions. The common-ion effect discussed is a qualitative effect, which makes the reaction rates with such effect difficult to express with formulas, but it is a kinetic effect to be considered which slows down the rate of a reaction in certain
However, there are three types of deviations between the results and the theoretical ones which can not be neglected. One of them is the “reversibility of the initial heterolysis” (O. T. Benfey, E. D. Hughes, and C. K. Ingold, J. Chem. Soc. 2489) which makes the reaction a pure first-order one at the beginning but slower than it should be in the later period. One sort of examples are the hydrolysis reactions of diarylmethyl halides. For illustration, the hydrolysis of diphenylmethyl chloride is discussed here. The reason for the dropping rate at the later period is that the leaving group having disassociated may compete with the reactant to combine with the diphenylmethyl cation. At the beginning, although chloride ion is a better nucleophilic reagent than water, its concentration is so small that it can be neglected, while the concentration of water is so large that it does not appear in the rate formula as the solvent. During the process of the reaction, the concentration of chloride ion increases gradually and the effect of the competition between chloride ion and water becomes important enough to be taken into consideration in the later period, so the rate drops later. The case shows that the formula should be modified to meet more general cases, applying the steady-state approximation, the rate formula can be modified to be a more universal one taking no consideration of the …show more content…
However, if the leaving ion can decrease the rate, adding additional leaving ion can also decrease the rate of the reaction in the same way (Michael B Smith and Jerry March, 434). This is a common effect in the inorganic reactions called common-ion effect. Since there are many organic reactions occurring in organic solvents, the effect is not discussed in organic reactions often. However, some SN1 reactions like the hydrolysis discussed is proceeded in the solvents suitable and there are ions produced during the processes in which the common-ion effect should be taken into consideration. While other SN1s are not affected by the common-ion effect because no ion is involved in these reactions. The common-ion effect discussed is a qualitative effect, which makes the reaction rates with such effect difficult to express with formulas, but it is a kinetic effect to be considered which slows down the rate of a reaction in certain