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Special points of emphasis


Reactions of Alkenes

Because the mechanism of electrophilic addition of halogens to alkenes passes through a triangular halonium ion intermediate, addition of halogens to alkenes occurs with anti addition. The key to predicting stereospecificity with the electrophilic additions is to have a clear picture of the intermediate after the first addition. When the first addition is a proton (as with electrophilic addition of HX and acid catalyzed hydration), the intermediate is a trigonal carbocation, so the halide or hydroxyl substituent can attack in the second step from either side. With addition of X2 the intermediate is a triangular halonium ion, so the second addition must approach from the reverse side. I know for a fact that the MCAT does expect you to know that addition of halogens occurs with anti- stereospecificity because I had the question on my own exam many years ago.

Reactions of Alkenes

Reactions of Conjugated Species

Electrophilic addition of halogen to conjugated diene does not result in the triangular halonium ion typical of addition to unconjugated alkenes. Instead, the first addition leads to an allylic carbocation intermediate. This means that either direct (1,2) or conjugate (1,4) addition will ultimately occur. You must remember that addition to a conjugated diene forms an allylic carbocation intermediate, which can lead to either direct (1,2) or conjugate (1,4) addition.

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