Reactions of Alkynes

Nomenclature and Structure of Alkynes

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. Their names are derived from the corresponding alkanes by replacing the '-ane' ending with '-yne.' The position of the triple bond is indicated by the lowest possible number.

• 1-Butyne: Triple bond starts at carbon 1.

• 2-Butyne: Triple bond starts at carbon 2.

• 2-Hexyne: Triple bond starts at carbon 2 in a six-carbon chain.

• 5-Methyl-3-heptyne: A seven-carbon chain with a methyl group at carbon 5 and a triple bond starting at carbon 3.

Example: 2-hexyne is written as CH3CH2C≡CCH2CH3.

Preparation of Alkynes

Alkynes can be synthesized by the elimination of dihalides (vicinal or geminal) using a strong base such as sodium amide (NaNH2).

• Vicinal dihalide: Halogens on adjacent carbons.

• Geminal dihalide: Both halogens on the same carbon.

• Reaction:

Example: 1,2-dibromopropane treated with excess NaNH2 and water yields propyne.

Alkyne Reactions with Hydrogen Halides (HX)

Alkynes react with hydrogen halides (HBr, HCl, etc.) in either one or two equivalents, leading to different products depending on the amount of reagent and the type of alkyne (terminal or internal).

• Terminal alkyne + 1 eq. HBr: Forms a vinyl halide (Markovnikov addition).

• Terminal alkyne + excess HBr: Forms a geminal dihalide.

• Internal alkyne + 1 eq. HBr: Forms a mixture of isomeric vinyl halides.

• Internal alkyne + excess HBr: Forms a mixture of geminal dihalides.

Example:

Alkyne Reactions with Halogens (X2)

Alkynes react with halogens (Br2, Cl2) to form dihaloalkenes or tetrahaloalkanes, depending on the amount of halogen used.

• 1 equivalent X2: Forms a trans-dihaloalkene.

• Excess X2: Forms a tetrahaloalkane.

Example:

Hydration of Alkynes: Keto-Enol Tautomerism

Alkynes undergo hydration in the presence of acid and mercuric sulfate (HgSO4) to yield enols, which rapidly tautomerize to ketones or aldehydes.

• Terminal alkynes: Yield methyl ketones (Markovnikov addition).

• Internal alkynes: Yield mixtures of ketones.

• Keto-enol tautomerism: The enol intermediate rearranges to the more stable keto form.

General equation:

Hydroboration-Oxidation of Alkynes

Hydroboration-oxidation provides anti-Markovnikov hydration of alkynes, leading to aldehydes from terminal alkynes and ketones from internal alkynes.

• Reagents: Disiamylborane or 9-BBN, followed by H2O2/NaOH.

• Terminal alkynes: Yield aldehydes.

• Internal alkynes: Yield ketones.

Example:

Ozonolysis of Alkynes

Ozonolysis cleaves the triple bond of alkynes, producing carboxylic acids. For terminal alkynes, one product is always carbon dioxide (CO2).

• Reagents: O3 followed by H2O.

• Terminal alkynes: Yield carboxylic acid and CO2.

• Internal alkynes: Yield two carboxylic acids.

Example:

Hydrogenation of Alkynes

Alkynes can be reduced to alkanes or alkenes depending on the catalyst and conditions used.

• Complete hydrogenation (Pd/C): Converts alkyne to alkane.

• Partial hydrogenation (Lindlar catalyst): Converts alkyne to cis-alkene (syn addition).

• Partial hydrogenation (Na/NH3): Converts alkyne to trans-alkene (anti addition).

Example:

Acidity of Terminal Alkynes and Alkylation

Terminal alkynes are more acidic than alkenes and alkanes due to the high s-character of the sp-hybridized carbon. The terminal hydrogen can be abstracted by a strong base (e.g., NaNH2), forming an acetylide anion, which can then undergo alkylation with primary alkyl halides.

• Acidity order: Alkynes > Alkenes > Alkanes

• Reaction:

• Alkylation:

Example: Propyne can be synthesized by alkylation of acetylene with ethyl bromide.

Summary Table: Major Reactions of Alkynes

Practice and Application

• Predict the product of an alkyne reaction given the reagents and conditions.

• Draw mechanisms for the addition and elimination reactions involving alkynes.

• Apply knowledge of regioselectivity (Markovnikov vs. anti-Markovnikov) and stereochemistry (cis vs. trans) in alkyne reactions.

Additional info: Some images and schemes in the original notes are reaction maps and practice questions, which reinforce the above concepts. For full mastery, students should practice drawing mechanisms and predicting products for each reaction type.