Alkynes Explained — Structure, Synthesis, Reductions, and Hydration Reactions | Chapter 12 of Klein Organic Chemistry as a Second Language

Alkynes introduce a new level of reactivity and synthetic flexibility into organic chemistry. In Chapter 12 of Organic Chemistry as a Second Language: First Semester Topics by David Klein, students explore how carbon–carbon triple bonds influence molecular structure, acidity, and reaction pathways.

This chapter builds directly on alkene chemistry while introducing unique reactions that allow alkynes to be transformed into alkanes, alkenes, carbonyl compounds, and carboxylic acids.

🎥 Watch the video above for a complete walkthrough of alkyne structure, synthesis, reductions, hydration mechanisms, and oxidation reactions.

Structure and Hybridization of Alkynes

Alkynes are hydrocarbons that contain at least one carbon–carbon triple bond. Chapter 12 begins by emphasizing sp hybridization, which gives alkynes their characteristic linear geometry and 180° bond angles.

The chapter distinguishes between:

• Terminal alkynes: Triple bond at the end of a carbon chain
• Internal alkynes: Triple bond located within the carbon chain

This distinction is crucial because terminal alkynes display unique acidity and reactivity.

Acidity of Terminal Alkynes

Unlike alkenes and alkanes, terminal alkynes contain an acidic proton. The high s-character of the sp-hybridized carbon stabilizes the resulting negative charge.

Strong bases such as NaNH₂ or NaH can deprotonate terminal alkynes to form alkynide ions, which are powerful nucleophiles.

Alkynide Ions and SN2 Alkylation

Once formed, alkynide ions can participate in SN2 reactions with primary alkyl halides, allowing chemists to extend carbon chains.

This reaction is a key synthetic strategy and reinforces the importance of substrate selection and steric considerations.

Synthesis of Alkynes by Elimination

Chapter 12 introduces alkyne synthesis through double elimination reactions. Vicinal or geminal dihalides undergo two successive eliminations when treated with excess strong base.

After elimination, protonation restores the neutral alkyne.

Reduction Reactions of Alkynes

One of the most powerful aspects of alkyne chemistry is the ability to selectively reduce triple bonds:

• H₂ / Pt: Full reduction to alkanes
• Lindlar’s catalyst: Partial reduction to cis-alkenes via syn addition
• Na / NH₃: Partial reduction to trans-alkenes via anti addition

These reactions allow precise control over alkene stereochemistry.

Hydration of Alkynes

The chapter presents two distinct hydration pathways:

• Acid-catalyzed hydration (HgSO₄ / H₂SO₄): Markovnikov addition forming ketones
• Hydroboration–oxidation: Anti-Markovnikov addition forming aldehydes from terminal alkynes

Both reactions proceed through enol intermediates that rapidly convert to more stable carbonyl compounds.

Keto–Enol Tautomerization

A key conceptual highlight of Chapter 12 is tautomerization. Keto–enol tautomers differ by proton placement and double-bond location, unlike resonance structures.

Klein emphasizes how acidic or basic conditions facilitate proton transfer, driving the equilibrium toward the more stable keto form.

Ozonolysis of Alkynes

Ozonolysis cleaves alkynes oxidatively:

• Internal alkynes yield two carboxylic acids
• Terminal alkynes yield a carboxylic acid and carbon dioxide

This reaction is useful for both synthesis and structural analysis.

Why Chapter 12 Is So Important

Alkynes serve as versatile intermediates in organic synthesis. Their unique acidity, reactivity, and transformation pathways make them powerful tools for building complex molecules.

By mastering the reactions in this chapter, students gain flexibility in synthesis planning and deepen their understanding of structure–reactivity relationships.

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