Opening of a protonated epoxide

The opening of the halonium ion closely resembles that of an epoxide in acidic medium:

Opening of protonated 1-methylcyclohexene oxide by methanol: the fast pathway goes through a transition state similar to a tertiary carbocation and gives the major product, with the methoxy group on the more substituted carbon

Epoxides are slightly basic, so in acidic medium a good number of molecules are protonated. The electrophilicity of the carbons bonded to the protonated oxygen increases considerably and the C–O bonds are weakened.

If the epoxide is unsymmetrical, cleavage of the C–O bond leading to the more substituted carbocation is the most favourable. The mechanism resembles S_N1, but the attack of the nucleophile (here, methanol) takes place almost at the same time as the C–O bond breaks: a free carbocation is never formed. That is why, although the mechanism resembles S_N1, the configuration of the attacked carbon is inverted.

The same happens with the halonium ion: the nucleophile attacks the carbon that best supports the positive charge, from the side opposite the halogen.

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