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At 1:57 -I effect of bromine is more than +M effect of bromine. So why is secondary cation more stable than primary cation? Secondary has to face -inductive effect of bromine.
Primary carbocation is pretty much a science fiction, it's too unstable and virtually never going to be a reasonable intermediate. There are a few examples and reactions that do form primary carbocations, but they are very rare. Back to you question though, just because Br is a halogen, doesn't mean it has a strong inductive effect. Br's electronegativity is about 2.9-3.0 while C is about 2.5-2.6. So, we're looking at the ΔEN ≈ 0.4-0.5 which is barely a polar bond. This way, the overall hyperconjugation and a weak resonance stabilization we have in this case is quite substantial to make that carbocation rather stable. That is, of course, *if* we get a carbocation there to begin with. We teach this mechanism as proceeding via a carbocation intermediate, but the experimental evidence strongly suggests it's a termolecular process that doesn't form a carbocation.
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At 1:57
-I effect of bromine is more than +M effect of bromine.
So why is secondary cation more stable than primary cation?
Secondary has to face -inductive effect of bromine.
Primary carbocation is pretty much a science fiction, it's too unstable and virtually never going to be a reasonable intermediate. There are a few examples and reactions that do form primary carbocations, but they are very rare.
Back to you question though, just because Br is a halogen, doesn't mean it has a strong inductive effect. Br's electronegativity is about 2.9-3.0 while C is about 2.5-2.6. So, we're looking at the ΔEN ≈ 0.4-0.5 which is barely a polar bond. This way, the overall hyperconjugation and a weak resonance stabilization we have in this case is quite substantial to make that carbocation rather stable. That is, of course, *if* we get a carbocation there to begin with. We teach this mechanism as proceeding via a carbocation intermediate, but the experimental evidence strongly suggests it's a termolecular process that doesn't form a carbocation.
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