Question

Why does brom o- 1 butene and bromo 2 butene undergo resonance?

Answer 1

Well, it's not that THEY do...

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I assume you mean these:

But let's consider what this was originally...

1. The proton adds on (as strong acids do to `pi` bonds!) to the end carbon, so that a `2^@` carbocation forms, instead of a `1^@` one. This merely follows Markovnikov addition.

2. The `"Br"^(-)` could have taken option `(1)` (dashed arrow) and attacked at the cationic carbon... but internal resonance, option `(2)` (solid arrow), is faster than external nucleophilic attack.

Furthermore, since the resonance generates a `1^@` carbocation (which is known to be among the least stable of the substituted carbocations), the two arrows in option `(2)` are simultaneous, and NOT stepwise---no second intermediate forms.