Influence of the Nucleophile on SN1 Reactions

The nucleophile has no effect on the reaction rate of SN1 reactions. However, the nucleophile does determine the structure of the product.

Fig.1SN1 reactions with different nucleophiles.

Therefore, it is important to distinguish between the rate-determining and the product-determining step. For instance, the reaction between 2-chloro-2-methylpropane and methanol yields 2-methoxy-2-methylpropane. However, in the presence of an equivalent amount of sodium azide, 1,1-dimethylethyl azide is formed. Obviously, sodium azide is a more effective nucleophile than methanol is. Its reaction with carbocations is therefore much more rapid than that of methanol would be.

Fig.2Rate-determining and product-determining step.

The reaction rate is determined by the individual reaction step that displays the transition state richest in energy. However, the product distribution arises from the energy ratio of the subsequent transition states and thus from the chemical properties of the nucleophile. The more efficient nucleophile induces a lower-energy transition state, which in turn determines the product distribution. In SN1 reactions, the rate-determining and the product-determining step can be clearly distinguished from one another. This may be used for controlling SN1 reactions.