Alkylation of acetylide ions template

Propyne (pKa about 25) is deprotonated by amide ion; the acetylide attacks bromoethane by SN2 to give pent-2-yne. With a secondary or tertiary halide the strongly basic acetylide removes a β-H instead (E2).

Mechanism step by step

NaNH₂ deprotonates a terminal alkyne; the acetylide makes a new C–C bond by SN2 on a primary alkyl halide

1. Deprotonation – Amide (NH₃ pKa about 38) removes the sp C–H (pKa about 25): complete
2. SN2 on a 1° halide – Backside attack on CH₃CH₂Br: the new C–C bond
3. 2° halide: E2 instead – With 2-bromopropane the acetylide acts as a base: propene, and the alkyne back

Key points

• Terminal alkynes (pKa about 25) are far more acidic than alkenes (about 44) or alkanes (about 50): the sp carbon holds the lone pair close (50 % s character).
• NaNH₂ (or BuLi) is strong enough; hydroxide and alkoxides (pKa of water and ethanol about 16) are not.
• Only methyl and primary halides (and tosylates) work: secondary and tertiary halides give E2 alkenes because acetylide is a strong base.
• Ethyne can be alkylated twice (NaNH₂, R–X; then NaNH₂, R′–X) to build internal alkynes; reduction then gives cis or trans alkenes.

Drawn with 44 atoms, 11 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 9 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 10 Textbook