Chapter 9 : Alkynes

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 (i.e. a carbon atom bearing a negative charge).

This carbanion can be used as a C centered nucleophile. These are important systems because the reaction of a carbanion with a C centered electrophile (such as alkyl halides) allows for the formation of new C-C bonds and hence larger more complex molecules.

In order to appreciate what makes the terminal alkyne more acidic thanmost other hydrocarbons, we should look at the stability of the conjugatebase (i.e. the carboanion).

For each type of carbanion shown, the nature of the hybrid orbital containingthe electron pair is important. Increased s character (sp= 50%, sp² = 33% and sp³ = 25%) impliesthat the alkyne sp orbital is closer to the nucleus and so thereis greater electrostatic stabilisation of the electron pair. Thereforethe conjugate base of the alkyne is the most stable and the most readilyformed.
However the terminal alkyne C-H bond is not strongly acidic and a strongbase, such as the amide ion, NH₂^-, is required toform the carbanion.

QUESTION : Could you use a base such as NaOH or NaOEt for this reaction ? ANSWER

Related reactions

• Preparation of aldehyde and ketone enolates
• Preparation of ester enolates

© Dr. Ian Hunt, Department of Chemistry