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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?

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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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  • $\begingroup$ Wow I never thought of it like that... You seem to be knowledgeable about organic chemistry. I am writing jee. Could you tell me how I should study organic chemistry and the books I should use? $\endgroup$ – user103292 Jan 15 '16 at 8:15

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