Alkyne Reactions cont'd
For each of the reactions on the left, predict the major organic product. Pay particular attention to the regiochemistry of the reaction and the possibility of carbocation rearrangement.Click the mouse on the reactant molecule to view the answer;
click on the reagent to review the mechanism and regiochemistry of the reaction;
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The hydrolysis of an alkyne in the presence of Hg2+ ion results in the formation of an enol in which the hydroxyl group is bonded to the alkyne carbon which would form the most stable carbocation (Markovnikov addition); the intermediate enol rapidly reverts to give the corresponding carbonyl compound.
The reaction of an alkyne with BH3 in THF proceeds through the formation of an organo-borane intermediate with the boron bonded adjacent to the alkyne carbon which would form the most stable carbocation (anti-Markovnikov addition). Work-up with alkaline peroxide results in the formation of an anti-Markovnikov enol> which rapidly reverts to the corresponding carbonyl compound.
Terminal alkynes are slightly acidic and a powerful base such as sodium amide (sodium previously dissolved in liquid ammonia) will react with these compounds to give alkyne anions, which are powerful nucleophiles. The most common reaction in which these nucleophiles are utilized involves reaction with alkyl halides to displace the halogen and form a new alkyne with a longer carbon chain. This mechanism is shown in the similar reaction which is shown below.
The hydrolysis of an alkyne in the presence of Hg2+ ion results in the formation of an enol in which the hydroxyl group is bonded to the alkyne carbon which would form the most stable carbocation (Markovnikov addition); the intermediate enol rapidly reverts to give the corresponding carbonyl compound.
The reaction of an alkyne with BH3 in THF proceeds through the formation of an organo-borane intermediate with the boron bonded adjacent to the alkyne carbon which would form the most stable carbocation (anti-Markovnikov addition). Work-up with alkaline peroxide results in the formation of an anti-Markovnikov enol> which rapidly reverts to the corresponding carbonyl compound.
Terminal alkynes are slightly acidic and a powerful base such as sodium amide (sodium previously dissolved in liquid ammonia) will react with these compounds to give alkyne anions, which are powerful nucleophiles. The most common reaction in which these nucleophiles are utilized involves reaction with alkyl halides to displace the halogen and form a new alkyne with a longer carbon chain.