SN1 vs SN2 in low concentration of nucleophile

Why is SN1 mechanism favoured by low concentration of nucleophilic reagent? It is a first order reaction so shouldn't the reagent be in excess? Also higher the concentration of reagent faster the reaction right?




Let us assume that the competing reaction is a Sn2 based pathway.


Sn2=k[substrate][nucleophile]


Sn1=k'[substrate]


Relative rate of products formed=(ie.. Sn1 to Sn2 product)=k'/(k[nucleophile]


Sn1 Reaction doesnot depend on concentration of the nucleophile , so more the concentration of nucleophile, the relative rate of Sn1 product diminishes.


Hence ,when the competing reaction is a Sn2 pathway, lesser concentration of nucleophile would favour the Sn1 pathway.


Hope this helps.


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