The strength of a nucleophile is a measure of how fast it reacts to produce the

Question

The strength of a nucleophile is a measure of how fast it reacts to produce the substitution product. For substitution reactions that proceed via an SN2 mechanism, the table below shows the ratio of the rate of substitution of different nucleophiles to the reaction rate when the nucleophile is water: Propose a reasonable explanation for the trend in increasing rate of substitution for the nucleophiles represented above. (Consider energetic and entropic factors that may facilitate the attachment of the nucleophile and its stability in the surrounding solvent). The rate of substitution reactions that proceed via an SN1 mechanism is not affected by either the concentration or the strength of the nucleophile. How would you explain this?

Explanation / Answer

- The strength of nucleophile in a solvent depends on how free it is. In a polar protic solvent, nuclophile tends to get solvated. Smaller the nulecophile, better is its solvation and less available it will be for the substitution reaction. Thus, as size increases from F- to I-, the rate of substitution with nucelophile goes up with F- the smallest is better solvated than others and have a lower rate of reaction. When the nucleophile can stabilize its charge well like in CN- its rate of reaction for a nucleophilc substitution reaction would increase.

- In case of an SN1 reaction, the rate determining step is formation of carbocation by loss of the leaving group. The nucleophile comes in the later step which is the fast step and thus the type of nucleophile used does not affect the rate of reaction.

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