Question

When alkenes are hydrated, the final product is an alcohol, but alkyne hydration produces carbonyls. Write a reaction for the hydration of 1-pentyne showing the formation of pentanal and a separate reaction showing the formation of 2-pentanone. Explain why the final product is not an alcohol.

Answer 1

When alkynes are hydrated under strongly acidic conditions, the reactions starts by the positive part of the addendum - the proton attacking an alkynyl carbon, giving rise to an enol, regenerating the acid catalyst. The enol formed is only an intermediate, as it immediately tautomerizes to form a carbonyl compound for increased stability.

Tautomers are rapidly interconverting constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen atom and a differently located double bond. The instability of such alpha, beta-unsaturated alcohols can be explained by the high electronegativity and nucleophilic nature of the hydroxyl O attached to the acidic sp2 hybridised carbon, which results in the electron withdrawal from the pi-bond through to the oxygen, after it loses its proton readily, thus converting it into a carbonyl with a C=O.

Similarly, the enols formed by the hydration of alkynes exist only as intermediates which interconvert to their more stable ketone or aldehyde forms.

Direct hydration of alkynes, by strong mineral acids lead to addition by Markonikov's rule while hydroboration or any such indirect hydrations result in the anti-Markonikov product. Depending upon the enol position, the final carbonyl product is determined. Similarly, acid-catalysed hydration of pent-1-yne gives pentan-2-one while hydroboration of pent-1-yne gives pentanal.