Organic Chemistry/Introduction to reactions/Alkyne hydration

Alkynes readily combine with water in the presence of acid (usually sulfuric acid) and mercury(II) salts (usually the sulfate is used) to form carbonyl compounds, in a process known as Kucherov's reaction. In the case of acetylene (ethyne) the product is acetaldehyde (ethanal), while other alkynes form ketones. Terminal alkynes reliably form methyl ketones and symmetric internal alkynes give single ketone products, but mixtures of ketones may form from unsymmetrical internal alkynes.

The intermediate product is an enol (an alkene with a hydroxyl group attached to a doubly bonded carbon), which then transforms to its keto tautomer through keto-enol tautomerisation.

Mechanism

Alkene hydration only requires acid catalysis and proceeds through the most stable carbocation available (Markonikov selectivity), but alkynes are less basic (less easily protonated), as they would generate less stable vinyl cations upon protonation. Both H⁺ and Hg²⁺ are catalysts in the hydration of alkynes. The Hg²⁺ additive presumably adds to the alkyne to form a more stable vinyl cation (β-stabilized) than would be afforded by protonation. As with Markovnikov selectivity in the case of alkenes, the most stable vinyl cation (usually the most highly substituted) is preferred.

Addition to either end of a symmetric alkyne (internal or acetylene/ethyne) gives the same cation and only one carbonyl product is formed, but addition to unsymmetrical alkynes gives two possible cations, and two products are possible. Terminal alkynes selectively produce a methyl ketone owing to a strong preference for the most highly substituted vinyl cation, but unsymmetrical internal alkynes can produce two different vinyl cations, whose stabilities may be similar if the alkyne's substituents are electronically and sterically similar, and therefore two different ketone products would be obtained in a mixture.

After the addition of water and O-deprotonation to an α-mercurated enol, the acid can effect an electrophilic substitution of the Hg²⁺ by α-C-protonation (ipso-protonation) and elimination of Hg²⁺ (demercuration), a process akin to electrophilic aromatic substitution (S_EAr).[example]. Both routes result in an enol that (usually) tautomerizes to the (usually) more stable keto form, again an acid-catalyzed process.

The α-mercurated enol may instead tautomerize to the (usually more stable) keto form in an acid-catalyzed step involving α-C-protonation and O-deprotonation, but this is mechanistically unproductive; this must be reversed by reprotonation of the carbonyl O to trigger the fragmentation and loss of Hg²⁺. Both routes start with α-C-protonation; whether this is followed by immediate demercuration or by O-deprotonation, O-reprotonation and demercuration (the "tautomer detour") is indistinguishable. The more direct C-protonation-demercuration route is the most step-economical and suffices to explain the alkyne hydration process. Although both enol and keto forms of the α-mercurated ketone likely co-exist, only the protonated version has mechanistic significance.

Note

• A specific S_EAr analogy to the H⁺-Hg²⁺ exchange involved in alkyne hydration is the acid-promoted demercuration of arylmercury(II) salts, triggered by ipso-protonation, while the mercuration of arenes, a substitution of H⁺ by Hg²⁺, would be an example of the reverse reaction.