Chem 3/24

Terminal Alkynes

• A terminal alkyne is characterized by having a hydrogen on one end.

Oxymercuration of Alkynes

• In oxymercuration, water is added to the terminal alkyne in a Markovnikov manner.

• This involves the addition of an -OH group to the more substituted carbon atom, while a hydrogen is added to the less substituted carbon.

• The product after oxymercuration is called an enol.

• Enols are unstable, and with the introduction of acid or base, they can readily convert to a ketone.

• This transformation can occur even under neutral conditions but is faster with acid or base.

Reaction Steps

1. Oxymercuration:

   • Treat terminal alkyne with mercuric acetate and water.

   • The product is an enol.

2. Conversion to Ketone:

   • The enol can convert to a ketone via simple acid/base catalysis.

Characteristics of Enol to Ketone Transformation

• The enol structure is stabilized as it satisfies the octet rule.

• The transformation can involve the formation of an oxonium ion, which is a more stable format than a typical carbocation.

Hydroboration-Oxidation of Alkynes

• Hydroboration also works with alkynes but includes a modification for terminal alkynes.

• The first step involves the addition of BH3 following by treatment with hydrogen peroxide (H2O2) and base, which yields an enol.

• The enol can be converted to an aldehyde after undergoing basic conditions.

Products from Unsymmetrical Internal Alkynes

• With unsymmetrical internal alkynes, one can yield two different products that are indistinguishable in terms of their chemistry due to both carbons bearing similar substitution levels.

• Typically leads to a mixture of products.

Hydrogenation of Alkynes

• Hydrogenation process involves the addition of H2 across the triple bond of alkynes, reducing them to alkenes.

• This process occurs via syn addition when using a catalyst like Lindlar's catalyst, which facilitates the conversion while preserving stereochemistry.

• Lindlar's catalyst is a specially poisoned form of palladium, designed to allow the reaction to proceed selectively to cis-alkenes.

Acetylide Ions

• Acetylide ions can serve as nucleophiles and may engage in nucleophilic substitutions, particularly in opening epoxides.

• Useful in synthesis for forming new carbon-carbon bonds.

• Can also be utilized to produce desired structures in synthesis problems.