9.8 : アルキンからアルデヒドおよびケトンへ: 酸触媒による水和

Acid-catalyzed hydration of alkenes involves the addition of water across the π bond in the presence of a strong acid, like sulfuric acid, to form alcohols.

Similarly, alkynes also undergo acid-catalyzed hydration; however, they form aldehydes and ketones instead.

For example, acetylene gives acetaldehyde, whereas other terminal alkynes form methyl ketones.

Unsymmetrical internal alkynes undergo an acid-catalyzed hydration to give a mixture of ketones; in contrast, symmetrical internal alkynes yield a single ketone.

The addition of water to terminal alkynes follows Markovnikov's rule. However, for internal alkynes the addition is non-regioselective.

Since alkynes are less reactive than alkenes towards the addition of water, a mercuric salt, like mercuric sulfate, is used as a catalyst to facilitate the reaction.

Analogous to the oxymercuration of alkenes, the mechanism of acid-catalyzed hydration of alkynes begins with the nucleophilic attack by one of the alkyne π bonds on the mercuric ion to form a bridged-mercurinium ion intermediate.

Next, water, being a nucleophile, attacks the more substituted carbon from the opposite side of the bridge and opens the ring, accounting for the observed Markovnikov's regioselectivity.

Thereafter, a proton transfer to the solvent leads to the formation of an organomercuric enol, which rapidly converts into a more stable keto tautomer via the keto-enol tautomerism.

Tautomers are constitutional isomers that interconvert and differ in the locations of a hydrogen atom and a double bond. The equilibrium favors the more stable isomer, in this case, the keto form.

Protonation of the carbonyl group of the keto tautomer forms an oxonium ion, which loses a mercuric ion to give the enol form of the final product.

The last step involves tautomerization of the enol into the desired ketone.

Lastly, acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes since they produce one final product. Unsymmetrical alkynes form a mixture of products that need to be separated, thus lowering the overall yield.