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.
Concept: The mechanism of akynide synthesis.4m
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.3m