Terminal Alkynes and Alkynides

Now it’s time to introduce some of the most important nucleophiles in all of organic chemistry, alkynides

Recall that most hydrocarbons are terrible acids, having pKas from 40-50. However, terminal alkynes (alkynes with a hydrogen) are uniquely acidic due to the hybridization effect, having a pKa of 25.

This means they can be easily deprotonated using a strong base (typically NaH or NaNH2). 

The resulting conjugate base is known as a sodium alkynide, and these are often used to lengthen carbon chains to through SN2 reactions.

  • For now, this will be the only way to increase the number of carbons in your product.

Beware: You can only use this reaction with primary alkyl halides. Secondary alkyl halides will favor E2!

Example: Predict the final product of the following reaction sequence.