Advanced Topic 864 of 1102 Organic Chemistry
Acidity of Terminal Alkynes
Terminal alkynes are weakly acidic and form acetylides with strong bases.
In-depth explanation of Acidity of Terminal Alkynes
A terminal alkyne has a hydrogen on an sp carbon, and this C-H bond is weakly acidic with a pKa of about 25 because the sp carbon has 50 percent s character and holds its electrons close to the nucleus. This acidity is much greater than that of alkenes or alkanes, though weaker than that of water, so it can be removed by a very strong base such as sodium amide, NaNH2, but not by hydroxide. The resulting acetylide ion is a good nucleophile for forming carbon-carbon bonds. Silver and copper acetylides are insoluble and precipitate from solution.
Going deeper: Acidity of Terminal Alkynes
Key ideas: Terminal alkynes have pKa about 25, more acidic than alkenes (44) and alkanes (50) because the sp carbon has 50 percent s character, holding the anion pair closer to the nucleus. NaNH2 (conjugate acid pKa 38) fully deprotonates; NaOH does not. Acetylides are good nucleophiles for SN2 with primary halides (chain extension) and add to carbonyls. Ag+ or Cu+ gives insoluble acetylides, testing for terminal alkynes.
Common mistakes and connections: Students think alkynes are as acidic as carboxylic acids or are deprotonated by hydroxide. Another error is applying acetylide SN2 to secondary or tertiary halides, which eliminate. Internal alkynes do not respond. Links to hybridization, carbanion stability and alkyne synthesis.
Visualization of Acidity of Terminal Alkynes
Terminal alkynes (pKa 25) are far more acidic than alkenes and alkanes because the sp carbon stabilizes the acetylide anion. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Terminal alkynes give a white precipitate of silver acetylide when treated with Tollens' reagent.
Frequently asked questions about Acidity of Terminal Alkynes
What is Acidity of Terminal Alkynes?
Terminal alkynes are weakly acidic and form acetylides with strong bases.
Is Acidity of Terminal Alkynes a beginner, intermediate or advanced topic?
Acidity of Terminal Alkynes is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Acidity of Terminal Alkynes?
Study these earlier topics in Organic Chemistry first: Chromatography, Elemental Analysis, Degree of Unsaturation.
What comes after Acidity of Terminal Alkynes?
The next topics in Organic Chemistry are Keto-Enol Tautomerism and Alpha Hydrogen Acidity.
Can I practise Acidity of Terminal Alkynes with quizzes?
Yes. The interactive lesson includes a self-check question on Acidity of Terminal Alkynes, and the Organic Chemistry quiz draws on every topic in this area.
Where Acidity of Terminal Alkynes fits in Organic Chemistry
Acidity of Terminal Alkynes is topic 136 of 148 in Organic Chemistry.