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Joined 2026-09-21 Message
Incoming links: Halohydrin
Bromohydrin 2026-10-20
A halohydrin with bromine on the carbon adjacent to OH. Its relative stereochemistry can select between properly aligned epoxide closure and semipinacol rearrangement.
The oxygen substituent changes position through a halohydrin–epoxide sequence. First, hydroxide attacks the acetate carbonyl of , giving a tetrahedral intermediate. Collapse expels the substrate alkoxide, and proton transfer gives plus acetate. This is saponification, breaking the acyl–oxygen bond rather than replacing bromide directly with hydroxide.
A small equilibrium population of undergoes intramolecular SN2 reaction at the adjacent primary carbon: the O lone pair attacks and its C–Br pair leaves with bromide. The result is 2,2-Dimethyloxirane. Addition of HBr protonates the ring oxygen. bromide then attacks preferentially at the more substituted carbon, where the protonated ring has greater carbocation character, breaking that C–O bond and leaving oxygen attached to the former primary carbon:The middle formula denotes a three-membered epoxide, not an open-chain ether. Acidic opening of this highly unsymmetrical epoxide has the opposite regioselectivity to a simple basic nucleophilic opening at its less substituted carbon.
Finally, the primary alcohol oxygen attacks acetyl chloride. The tetrahedral intermediate collapses with chloride departure, and triethylamine removes the O–H proton and captures HCl. The product is . oxygen is carried through the epoxide to the other carbon; it is not necessary to invent a methyl migration or a carbon-skeleton rearrangement.
Semipinacol rearrangement 2026-10-20