I have studied that in SN1 REACTION nucleophile prefers teritiary carbocation over primary and secondary , because tertiary carbocation is more Stable becuase of delocalisation of positive charge due to electron donating effect of alkyl groups . But wouldn't that make it less susceptible to nucleophilic attack since the +ve charge on carbocation has decreased? How does stability actually increases the nucleophilic attack I can't get the words but i hope you got my question
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2 Answers
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In $S_N1$ reactions, the rate-determining step is the formation of a carbocation by loss of a leaving group. Since the transition state and the carbocation intermediate are very similar in structure, the stability of the carbocation affects the rate of the $S_N1$ reaction: the more stable the formed carbocation, the lower the activation energy for its formation, and the more readily it is formed. Carbocations are stabilised by adjacent alkyl groups by hyper conjugation and not resonance delocalisation, as you suggested (except for allylic and benzylic carbocations). The more adjacent alkyl groups, the more stable the carbocation is. Although methyl and primary carbocations bear a greater positive charge on the central carbon, they are too unstable to be formed (no or limited stabilization = high activation barrier), whereas tertiary carbocations are stable enough to be formed.
It is important to note that stability of carbocations is relative, meaning even 'more stable' carbocations are still very electron-deficient and hence very electrophilic.
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Good to see that you pointed that out, actually a nucleophile prefers to provide electron cloud to a carbocation, but since the carbocation is more stable on tertiary carbon due to hyper conjugation,the carbocation will get rearranged to remain stablised, but its stability is kind of nothing , as compared to the product formed, an intermediate is always less stable then the real product, hence the attack will still occur, even if carbocation is stable.
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$\begingroup$ so nucleophile will attack whether the carbocation is stable or not but the final product will be much more stable with tertiary carbocation that's why it prefers it ? $\endgroup$– GautamCommented Oct 28 at 10:15
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$\begingroup$ Could you elaborate your answer? As it stands you are mixing several concepts and your justification for the observed behaviour is not very clear. $\endgroup$– PAEPCommented Oct 28 at 11:38