Summary

• Terminal alkynes are unusual for simple hydrocarbons in that they can be deprotonated (pK_a = 26) using an appropriate base (typically NaNH₂, pK_a = 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 alkyne.
• 3^o alkyl halides are more likely to undergo elimination.
• One or both of the terminal H atoms in ethylene (acetylene) H-CºC-H can be substituted providing access to monosubstituted (R-CºC-H) and symmetrical (R = R') or unsymmetrical (R ¹ R') disubstituted alkynes R-CºC-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.

What is the product of the reactions of CH3-CºC- with each of the following:
(a) 2-bromopropane		(d)  ethanol
(b) 1-iodooctane		(e)  ethyl tosylate
(c) (R)-2-bromohexane		(f)   bromobenzene

Related reactions:

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

Summary

• The two p bonds of an alkyne can be formed using two consecutive elimination reactions.
• The leaving groups are usually halides (esp. Br or Cl)
• Reagent : usually NaNH₂ (a strong base)
• Either geminal (1,1-) or vicinal (1,2-) dihalides can be used.
• Since 1,2-dihalides can be prepared by addition of X₂ to an alkene, an alkene can be converted into an alkyne in two steps.
• These reactions are typically E2 reactions and occur via an alkenyl halide.
• The strong base is needed in order to cause elimination of the alkenyl halide.

What is the alkyne product from the reactions of the following with NaNH2 :
(a) 2,2-dibromopropane		(c) 1,2-dibromohexane
(b) 1,1-dibromooctane		(d)  2,3-dibromohexane