Note: Alkynes are less basic than alkenes and HgSO4 (Hg+2 is a Lewis acid) is added to ensure complete reaction.
Alkyne hydration reactions
Alkenes add the elements of one H20 molecule to give alcohol products. A similar reaction occurs with alkynes (one H20), but the products are ketones (HgSO4, H2SO4, H20) and aldehydes (hydroboration-oxidation).
(a) ketones from HgSO4, H2SO4, H20 (Markovnikov)
Note: Alkynes are less basic than alkenes and HgSO4 (Hg+2 is a Lewis acid) is added to ensure complete reaction.
(b) aldehydes from hydroboration-oxidation (anti-
Markovnikov hydration)
Note: Terminal alkynes (two p-bonds) can add two molecules of BH3 and each BH3 (three B-H bonds) can react with three alkyne molecules. Theresult is a complicated and very sterically hindered adduct which generally gives poor yields in the subsequent oxidation reaction. It is best to perform the reaction with a sterically hindered dialkyl borane so that only one of the two pbonds will add a B-H. Two possible hindered boranes are dicyclohexyl borane and disiamyl borane.
