When $\ce{HBr}$ is added to 2-chloro-2-butene, the proton preferentially bonds to the $sp^2$ carbon that's not bonded to chlorine. Does this mean that hyperconjugation takes place between the lone pairs of chlorine and the positively charged $C3$? I would assume that since there is still a free rotation about the $\ce{C-Cl}$ bond and that chlorine is in a tetrahedral formation this would be the reason behind the regioselectivity.

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    $\begingroup$ The positive charge (in the intermediate) formed on C2 is stabilized by resonance with the lone pairs of chlorine. $\endgroup$ Commented Feb 28, 2018 at 6:07
  • $\begingroup$ @ApoorvPotnis: isn't the inductive effect stronger for halogens? Wouldn't it be a net destabilizing effect that chlorine induces? $\endgroup$
    – harry
    Commented May 25, 2021 at 23:28
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    $\begingroup$ @harry I am seeing this comment 3 years after it was posted and honestly, I don't know the answer. I have forgotten all chemistry I knew years ago. $\endgroup$ Commented Mar 19 at 17:50
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    $\begingroup$ @ApoorvPotnis I'm the person who asked the question. I now work in a completely different field and I don't understand anything from my own question XD $\endgroup$
    – Jin Bae
    Commented Mar 21 at 20:33

1 Answer 1


Hyperconjuation is the interaction of electrons in sigma orbitals (in $C-H$ or $C-C$) with an adjacent empty p orbital or $\pi$ orbital. What you are describing is not hyperconjugation, but resonance; more specifically, the chlorine shows the $+R$ effect.

Now if you draw the resonance structures of this molecule you will realize that the $C_3$ has a negative charge on it in one canonical form. This means, in the actual molecule, that $C_3$ carbon has a partial negative charge, which is why the hydrogen from $HBr$ preferentially bonds to that carbon.

A more acceptable explanation would be that, when the hydrogen bonds to $C_3$, the resultant intermediate carbocation (positive charge on $C_2$) is stabilized by the $+R$ effect of chlorine, resulting in a racemic mixture of $2-bromo-2-chlorobutane$ to form as the major product.


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