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My book (NCERT class 12 Chemistry) asks

Predict the order of reactivity of the four isomeric bromobutanes in SN1 and SN2 reactions.

The answer given for SN1 is

1-bromobutane < 1-bromo-2-methylpropane < 2-bromobutane < 2-bromo-2-methylpropane

My confusion is between (I) 1-bromobutane and (II) 1-bromo-2-methylpropane. The book explains

the carbocation derivative derived from (II) is more stable than derived from (I) because of a greater electron donating inductive effect of (CH3)2CH- group.

However, I have been taught that hyperconjugation is (usually) a stronger effect than induction. Since the carbocation from (I) would have a greater number of alpha hydrogens, that should be more stable.

However, the carbocation in (II) can rearrange to form a tertiary carbocation, whereas (I) can only rearrange to secondary. How significant will rearrangement be when considering the reaction rates? As an additional example, how will reaction rates of 2-bromopentane and 1-bromo-3-methylbutane (where tertiary carbocation is formed after two rearrangements) compare?

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  • $\begingroup$ The stability of a carbocation is governed with a mixture of contributions from these electronic effects and not just a single one of them. The carbocation formed in (I) has 2 alpha Hydrogens while the one in (II) has 1 alpha Hydrogen + an electron donating group through +Inductive effect. There is not much loss of Hyperconjugative stability while there is extra stability added from +I effect of the methyl group. $\endgroup$
    – Prajwal Tiwari
    Jul 8, 2022 at 9:00
  • $\begingroup$ Also, since the book specifically mentions path of SN1 mechanism here, clearly carbocation formation has to be considered(usually we won't consider a primary carbocation forming and rather proceed with an SN2 path) and (if) a carbocation will be formed, it will rearrange into a more stable one. The (I) would rearrange to give a secondary and the (II) would rearrange to give a tertiary carbocation. We can look it in this way as well and still it points to (II) being more stable $\endgroup$
    – Prajwal Tiwari
    Jul 8, 2022 at 9:03
  • $\begingroup$ @PrajwalTiwari Is going from 2 hyperconjugating structures to 1 not a significant loss of hyperconjugative ability? Besides, the methyl group has one of the weakest +I effect, and inductive effects drop significantly with distance. When I say "carbocation in (II)" I mean the primary carbocation formed initially, since the book uses stability arguments on that without considering rearrangement. Whether (II) gives SN1 at a faster rate and whether the carbocation initially formed from (II) is more stable are two different questions here. $\endgroup$
    – Naman Mishra
    Jul 9, 2022 at 3:52

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