Answer to Question #85847 in Organic Chemistry for anuj

Question

Addition of water to an alkyne does not give a diol

Answer 1

The addition of the elements of water across the triple bond of an alkyne leads to the formation of aldehydes and ketones. Water addition to terminal alkynes leads to the generation of aldehydes, while nonterminal alkynes and water generate ketones.

These products are produced by rearrangement of an unstable enol (vinyl alcohol) intermediate. The term “enol” comes from the en in “alkene” and ol in “alcohol,” reflecting that one of the carbon atoms in vinyl alcohol has both a double bond (alkene) and an OH group (alcohol) attached to it. A vinyl group is 

and a vinyl alcohol is

Water adds across the triple bond of an alkyne via a carbocation mechanism. Dilute mineral acid and mercury(II) ions are needed for the reaction to occur.

The first step of the mechanism is an acid‐base reaction between the mercury(II) ion (Hg ²⁺) and the π system of the alkyne to form a π complex.

The π complex is converted into a single bond between one or the other of the carbons of the triple bond and the mercury (II) ion, with the resulting generation of a carbocation.

A molecule of water is attracted to the carbocation to form an oxonium ion

The oxonium ion loses a proton to stabilize itself.

The vinyl alcohol precursor that results is converted into vinyl alcohol (enol) by reaction with a hydronium ion (H ₃O ⁺).

Vinyl alcohols (enols) are unstable intermediates, and they undergo rapid isomerization to form ketones. Such isomerization is called keto‐enol tautomerism.

In a similar fashion, the less‐stable intermediate generates an aldehyde.