4) The stability order of carbocations is exactly the order of SN1 reactivity for alkyl
halides and tosylates.
5) The order of stability of carbocations:3° > 2° ≈ Allyl ≈ Benzyl >> 1° > Methyl
R 3 C+ > R 2 CH+ ≈ H 2 C=CH–CH 2 + ≈ C 6 H 5 –CH 2 >> RCH 2 + > CH 3 +
6) Formation of a relatively stable carbocation is important in an SN1 reaction
⇒ low free energy of activation (∆G‡) for the slow step of the reaction.
i) The ∆G° for the first step is positive (uphill in terms of free energy) ⇒ the
first step is endothermic (∆H° is positive; uphill in terms of enthalpy).
7) The Hammond-Leffler postulate:
i) The structure of a transition state resembles the stable species that is nearest
it in free energy ⇒ Any factor that stabilize a high-energy intermediate should
also stabilize the transition state leading to that intermediate.
ii) The transition state of a highly endergonic step lies close to the products in
free energy ⇒ it resembles the products of that step in structure.
iii) The transition state of a highly exergonic step lies close to the reactants in free
energy ⇒ it resembles the reactants of that step in structure.