Chapter 9 : Alkynes

Summary

• Terminal alkynes are unusual for simple hydrocarbons in that they can be deprotonated (pK_a = 26) using an appropriate base (typically NaNH₂, pKa = 36) to generate a carbanion.
• The acetylide carbanion is a good C nucleophile and can undergo nucleophilic substitution reactions (usually S_N2) with 1^o or 2^o alkyl halides (Cl, Br or I) which have electrophilic C to produce an internal alkyne.
• 3^o alkyl halides are more likely to undergo elimination.
• One or both of the terminal H atoms in ethylene (acetylene) H-C≡C-H can be susbtituted providing access to monosubstituted (R-C≡C-H)and symmetrical (R = R') or unsymmetrical (R not equal to R') disubstituted alkynes R-C≡C-R'
• Note that since the product is also an alkyne, it can also undergo the other reactions is this chapter.

MECHANISM FOR ALKYLATION OF ALKYNES
Step 1: An acid / base reaction. The amide ion acts as a base removing the acidic terminal H to generate the acetylide ion, a carbon nucleophile.
Step 2: A nucleophilic substitution reaction. The carbanion reacts with the electrophilic carbon in the alkyl halide with loss of the leaving group, forming a new C-C bond.

Related reactions:

• Acidity of teminal alkynes
• Other substitution reactions of alkyl halides
• Alkylation of enolates
• Alkylation of ester enolates

© Dr. Ian Hunt, Department of Chemistry