9.3 : 1-アルキンの酸性度

In comparison to alkanes and alkenes, 1-alkynes or terminal alkynes have lower pK_a values.

Recall that the lower the pK_a, the stronger the acid. Thus, terminal alkynes are more acidic than other hydrocarbons. 

Among ethane, ethylene, and acetylene, acetylene is 10²⁶ more acidic than ethane, and 10¹⁹ stronger than ethylene.

What makes terminal alkynes more acidic than alkanes or alkenes? The answer lies in the relative stability of the carbanions formed by deprotonation of the corresponding hydrocarbon.

This stability depends on the nature of the hybridized orbital on the negatively charged carbon. In ethane, the lone pair resides in an sp³ orbital, while in ethylene it occupies the sp² orbital, and an sp orbital in acetylene.

An sp³ orbital has 25% "s" character, compared to 33% in an sp² orbital and 50% in an sp orbital.

Since s orbitals are closer to the positively charged nucleus than p orbitals, electrons in a hybrid orbital with a higher "s" character will experience stronger electrostatic attraction.

Therefore, the acetylide anion will be the most stable and readily formed in the presence of a suitable base.

How does one choose a suitable base?

For a base to deprotonate an acid, the pK_a of the base’s conjugate acid must be at least 10 units greater than the acid's pK_a.

Terminal alkynes have a pK_a of 25. Here, the base chosen is such that its conjugate acid has a pK_a of at least 35.

With sodium amide as the base, terminal alkynes form sodium acetylide and ammonia as the conjugate acid. Since the pK_a of ammonia is greater than 35, the equilibrium favors the formation of sodium acetylide, making sodium amide a suitable base for the deprotonation reaction.

In contrast, sodium hydroxide forms water as its conjugate acid. Since the pK_a of water is less than 35, the equilibrium favors the reactants. Therefore, terminal alkynes cannot be deprotonated using sodium hydroxide.