The hydration of alkynes, or the addition of water to alkynes, is a valuable reaction in organic chemistry. Similar to alkenes, this process generally requires a strong acid catalyst, such as sulfuric acid (H2SO4), and is often facilitated by mercuric sulfate (HgSO4). However, unlike alkenes that yield alcohol products upon hydration, alkynes typically produce ketones due to a subsequent rearrangement called keto-enol tautomerization.
Tautomerism: Keto-Enol Equilibrium
Tautomers are constitutional isomers that rapidly interconvert, typically involving the shift of a labile hydrogen atom and the relocation of a double bond. This rapid interconversion is termed tautomerism, or tautomeric equilibrium, particularly when referring to the equilibrium between a ketone and an enol (keto-enol tautomerism). An enol possesses both a carbon-carbon double bond and a hydroxyl (-OH) group. It's important to recognize that tautomerism extends beyond keto-enol systems.
The keto and enol forms are in equilibrium, but with few exceptions, the keto tautomer is more thermodynamically stable and thus favored.
Acid-Catalyzed Hydration of Alkynes
Mechanism
The acid-catalyzed hydration of alkynes involves several steps:
Electrophilic Attack: The reaction begins with the electrophilic attack of a proton (H+) from the strong acid catalyst on the alkyne's π bond. This protonation occurs preferentially at the carbon that will form the more stable carbocation, following Markovnikov's rule. In the case of propyne, the secondary carbocation is favored over the primary carbocation.
Read also: What to Feed a Chinese Water Dragon
Nucleophilic Attack: Water (H2O) acts as a nucleophile and attacks the carbocation intermediate, forming a C-O bond and a protonated enol.
Deprotonation: Another water molecule or the conjugate base of the acid catalyst removes a proton from the protonated enol, generating an organomercury enol.
Tautomerization: The enol intermediate is unstable and undergoes keto-enol tautomerization to form a more stable ketone. This involves the transfer of a proton from the hydroxyl group to the carbon-carbon double bond.
Markovnikov's Rule
For terminal alkynes, the addition of water follows Markovnikov's rule, resulting in the formation of a methyl ketone as the final product. However, for internal alkynes, the addition of water is not always regioselective, and a mixture of ketone products can be obtained, especially if the alkyne is asymmetrical. Hydration of symmetrical internal alkynes produces a single ketone product.
Mercury-Catalyzed Hydration of Alkynes
The mercury-catalyzed hydration of alkynes offers an alternative approach:
Read also: Hydrating on Keto
Electrophilic Attack by Mercury: The reaction starts with an electrophilic attack of the mercuric ion (Hg2+) on the alkyne, forming a mercurinium ion. Depending on the reference, this step can be illustrated with the Hg+2 ion alone, mercury acetate (Hg(OAc)2), mercury sulfate (HgSO4), or several other variations.
Nucleophilic Attack by Water: Water attacks the mercurinium ion from the more substituted side, leading to a protonated intermediate.
Deprotonation and Enol Formation: Through a series of mechanistic steps, the intermediate transforms into an enol.
Tautomerization: The enol undergoes keto-enol tautomerization, yielding the final ketone product.
Hydroboration-Oxidation of Alkynes
Hydroboration-oxidation of alkynes provides a complementary method for alkyne hydration, offering anti-Markovnikov addition.
Read also: Cucumber Water for Weight Loss
Hydroboration: A dialkyl borane reagent (R2BH), such as disiamylborane (Sia2BH) or 9-borabicyclo[3.3.1]nonane (9-BBN), is used to add borane (BH3) across the triple bond. The borane adds to the less substituted carbon of the alkyne, and the hydrogen atom adds to the more substituted carbon.
Oxidation: Oxidative work-up replaces the borane with a hydroxy group (-OH), creating an enol intermediate.
Tautomerization: The enol tautomerizes to form either an aldehyde or a ketone, depending on the alkyne. Terminal alkynes produce aldehyde products, while internal alkynes produce ketone products.
Regioselectivity
The hydroboration-oxidation of alkynes is anti-Markovnikov regioselective, meaning that the oxygen is placed on the less alkyl-substituted carbon. However, for internal alkynes, the regioselectivity of these reactions may be less effective.
Electrophilic Addition Reactions of Alkynes
Electrophilic addition reactions to alkynes are similar to those of alkenes but generally slower. The triple bonds of alkynes, due to their high electron density, are attacked by electrophiles. However, alkynes are less reactive than alkenes due to the compact C-C electron cloud.
Mechanism
π-Complex Formation: The reaction begins with the formation of a π-complex between the alkyne and the electrophile.
Vinyl Cation Formation: The electrophile bonds to one of the alkyne carbon atoms, forming a vinyl cation intermediate. Vinyl cations are less stable than alkyl carbocations.
Nucleophilic Attack: A nucleophile attacks the vinyl cation, forming the addition product.
Regioselectivity
As with electrophilic addition to unsymmetrical alkenes, the Markovnikov rule is followed, adding the electrophile to the less substituted carbon.
Factors Affecting the Rate of Electrophilic Addition to Alkynes
The reactions of alkynes with electrophilic reagents are generally slower than the corresponding reactions of alkenes. This is due to several factors:
Tightly Bound π-Electrons: The sp-hybridized carbons of alkynes exert a strong attraction for their π-electrons, which are consequently bound more tightly than the π-electrons of a double bond.
Stability of Vinyl Cations: Vinyl cations are less stable than alkyl carbocations, resulting in a higher activation energy for their formation.