Overview
This lecture covers the unique acidity of terminal alkynes, compares it to other hydrocarbons, and explains how to deprotonate them using a strong base.
Acidity of Alkynes
- Terminal alkynes have a hydrogen attached to an sp-hybridized carbon at the end of the molecule.
- Terminal alkynes have a pKa around 25–26, indicating moderate acidity.
- Alkynes are significantly more acidic than alkanes (pKa ≈ 50) and alkenes (pKa ≈ 44).
- Despite higher acidity, terminal alkynes are much less acidic than water (pKa ≈ 15.4).
Deprotonation of Terminal Alkynes
- Sodium hydroxide (NaOH), a common strong base, cannot deprotonate terminal alkynes.
- Deprotonation with NaOH fails because water (the conjugate acid) is more acidic than the terminal alkyne.
- A much stronger base, sodium amide (NaNH₂), is required to deprotonate terminal alkynes.
- Sodium amide's conjugate acid (ammonia, pKa ≈ 37–38) is much weaker than the alkyne, making the reaction go to completion.
Acetylide Ion Formation and Relevance
- Deprotonation with NaNH₂ produces an acetylide ion (the conjugate base of a terminal alkyne).
- Acetylide ions are excellent nucleophiles and are useful in SN2 reactions and organic synthesis.
- The acetylide ion may be shown with or without its sodium counterion (Na⁺).
Key Terms & Definitions
- Terminal Alkyne — An alkyne with a carbon-carbon triple bond at the end of the molecule.
- pKa — A measure of acid strength; lower values mean stronger acids.
- Deprotonation — Removal of a hydrogen ion (proton) from a molecule.
- Sodium Amide (NaNH₂) — A strong base used to deprotonate terminal alkynes.
- Acetylide Ion — The conjugate base formed when a terminal alkyne loses a proton.
- Nucleophile — An electron-rich species that donates electrons in a chemical reaction.
Action Items / Next Steps
- Review mechanisms for deprotonating terminal alkynes using NaNH₂.
- Prepare for upcoming lessons on chemical reactions involving alkynes.