Alkynes, those versatile carbon-carbon triple bonds, can be transformed into valuable carbonyl compounds through hydration. This process, catalyzed by mercury(II) salts or achieved through hydroboration-oxidation, opens up a world of synthetic possibilities for organic chemists.

Understanding the mechanisms and products of alkyne hydration is crucial for predicting and controlling reactions. Whether you're aiming for aldehydes from terminal alkynes or ketones from internal ones, mastering these concepts will make you a hydration sensation!

Hydration of Alkynes

Mechanism of mercury(II)-catalyzed alkyne hydration

• Mercury(II) salts ($\ce{HgSO4}$) catalyze hydration of alkynes by activating the alkyne for nucleophilic attack by water
  • Alkyne coordinates to mercury(II) ion making it more electrophilic
  • Water acts as a nucleophile and attacks the activated alkyne forming an unstable vinyl cation intermediate
• Vinyl cation intermediate rapidly rearranges to form an enol which is a tautomer of the corresponding ketone or aldehyde
  • Enol has an $\ce{OH}$ group attached to a $\ce{C=C}$ double bond
  • Keto form has a $\ce{C=O}$ double bond and no $\ce{OH}$ group
• Keto-enol tautomerism converts the less stable enol to the more stable ketone or aldehyde product
  • Involves the shift of a proton and a double bond

Products of terminal vs internal alkyne hydration

• Mercury(II)-catalyzed hydration
  • Terminal alkynes (1-butyne) yield aldehydes as the major product
  • Internal alkynes (2-butyne) yield ketones as the major product
• Hydroboration-oxidation
  • Terminal alkynes (1-hexyne) yield aldehydes as the major product
    • Proceeds through an intermediate alkylborane which is oxidized to an alcohol then subsequently to an aldehyde
  • Internal alkynes not compatible due to formation of stable dialkylboranes that resist oxidation

Synthesis applications of alkyne hydration

• Synthesize a ketone from an internal alkyne
  1. Choose internal alkyne with desired substituents on either side of triple bond (3-hexyne)
  2. Treat alkyne with mercury(II) salt and water under acidic conditions
  3. Ketone product (3-hexanone) will have same substituents as starting alkyne
• Synthesize an aldehyde from a terminal alkyne
  • Choose terminal alkyne with desired substituent on triple bond (1-octyne)
  • Method 1: Treat alkyne with mercury(II) salt and water under acidic conditions to directly form aldehyde (octanal)
  • Method 2: Perform hydroboration-oxidation on the terminal alkyne
    1. Treat alkyne with borane ($\ce{BH3}$) to form alkylborane intermediate
    2. Oxidize alkylborane with hydrogen peroxide ($\ce{H2O2}$) under basic conditions to form alcohol
    3. Oxidize alcohol using a mild oxidant (PCC) to form the aldehyde (octanal)

Regioselectivity and Addition Patterns

• Hydration of alkynes follows electrophilic addition mechanism
• Markovnikov addition: Addition of water to unsymmetrical alkynes results in the hydroxyl group attaching to the more substituted carbon
• Anti-Markovnikov addition: Can be achieved through hydroboration-oxidation, resulting in the opposite regioselectivity
• Oxymercuration: A variation of the mercury(II)-catalyzed hydration that proceeds through a mercurinium ion intermediate, also following Markovnikov's rule