Reaction Review 10 - The Addition of H-X or X2 halogenation to Alkynes PDF

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Rxn Review #10 – The addition of H-X or X2 (halogenation) to Alkynes These reactions are electrophilic additions and are similar to the same types of reactants used with alkenes. Markovnikov’s rule is once again important with these reactions. The reactions are slower than their analogs with alkenes since vinyl carbocations or bridged vinylic ions are formed as intermediates (less stable than the carbocations formed in the additions to alkenes). First, consider some reactions of H-X with terminal alkynes:

How about H-X with internal alkynes? The first addition dictates the Markovnikov positions for the second addition (if two addns). If alkyne is not symmetric, two different stereoisomeric products are produced:

Halogenation of Alkynes (addition of X2) Also very similar in mechanism to halogenations of alkenes (but slower). Proceeds by bridge and ANTI mechanism in most cases. Vinylic carbocation intermediate only observed in situations where it’s stabilized by adjacent groups (i.e. phenyls… see below).

Examples:...