Reactions of alkynes: A summary

Table of Contents

• Alkynes

• Terminal alkynes vs internal alkynes

• Addition reactions of alkynes

  • Reduction (Addition of H2)

  • Halogenation (Addition of X2)

  • Hydrohalogenation (addition of -H and -X)

  • Hydroboration-oxidation (Addition of -H and -OH)

  • Mercury(II)-catalyzed hydration (Addition of -H an …

  • Ozonolysis

• Substitution reactions of alkynes

Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. This triple bond consists of one sigma bond and two pi bonds.

Terminal alkynes vs internal alkynes

Terminal alkynes contain a triple bond at the end of the carbon chain. Typically, one hydrogen is bonded to one of the carbons of the triple bond.

General Formula: RC≡CH

Disubstituted or internal alkynes have a triple bond between two carbon atoms within the carbon chain.

General Formula: RC≡CR'

Addition reactions of alkynes

Reduction (Addition of H₂)

Alkynes can be reduced to alkenes and alkanes. The complete reduction of alkyne yields an alkane.

The partial reduction of alkyne yields an alkene.

Halogenation (Addition of X₂)

Alkynes react with halogens like Cl₂ or Br₂. The addition of one equivalent of a halogen produces a dihalide and generally proceeds via anti addition.

The dihalide can react with another equivalent of halogen to produce a tetrahalide.

Hydrohalogenation (addition of -H and -X)

• Regiochemistry: Markovnikov

Alkynes react with one equivalent of hydrogen halide (HX, where X is a halogen like Cl or Br) to form vinyl halides.

Vinyl halides can further react with HX to form geminal dihalides.

Hydroboration-oxidation (Addition of -H and -OH)

• Regiochemistry: anti-Markovnikov

Like alkenes, alkynes undergo hydroboration-oxidation, which occurs with anti-Markovnikov regiochemistry as the -OH adds to the less substituted carbon. The initial enol product is unstable and quickly rearranges to form a more stable aldehyde (for terminal alkynes) or ketone (for internal alkynes)

Mercury(II)-catalyzed hydration (Addition of -H and -OH)

• Regiochemistry: Markovnikov

Unlike alkenes, alkynes react slowly with aqueous acid alone. Mercury(II) sulfate (HgSO₄) is used as a catalyst to speed up the reaction. This reaction occurs with Markovnikov regiochemistry as the -OH group adds to the more substituted carbon. The initial enol product is unstable and quickly rearranges to form a more stable ketone.

Ozonolysis

Alkynes can be cleaved when treated with oxidizing agents such as ozone or KMnO₄ followed by H₂O.

Internal alkynes produce two carboxylic acids

Terminal alkynes produce one carboxylic and CO₂

Substitution reactions of alkynes

Terminal alkynes are unique due to their relatively high acidity compared to alkenes and alkanes.

When terminal alkynes are treated with a strong base such as NaNH₂, the terminal hydrogen is removed, forming an acetylide ion.

Acetylide ions are highly nucleophilic and function as nucleophiles in S_N2 reactions, primarily with primary or methyl halides.

These reactions are called alkylation reactions and provide a versatile way to form C-C bonds and extend the length of a carbon chain.