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

Jun 11, 2018

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

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 ${2}^{\circ}$ carbocation forms, instead of a ${1}^{\circ}$ one. This merely follows Markovnikov addition.

2. The ${\text{Br}}^{-}$ could have taken option $\left(1\right)$ (dashed arrow) and attacked at the cationic carbon... but internal resonance, option $\left(2\right)$ (solid arrow), is faster than external nucleophilic attack.

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