Overview

The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pK_a — the lower the pK_a, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pK_a values and are, therefore, more acidic. For example, the pK_a values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.

Ethane (pKa = 51)

Ethene (pKa = 44)

Ethyne (pKa = 25)

With a pK_a difference of 26 units, acetylene is 10²⁶ times more acidic than ethane. Similarly, a difference of 19 pK_a units makes it 10¹⁹ times stronger than ethene.

Hybridization effect

The pronounced increase in the acidity of terminal alkynes relative to the other hydrocarbons can be explained by considering the stability of the corresponding carbanions formed by deprotonation. Note that, in the nomenclature of organic compounds, the suffix "-ide" indicates that the molecule is a negatively charged ion.

ethanide (an alkyl anion) sp3 lone pair, 25% “s” character

ethenide (a vinylic anion) sp2 lone pair, 33% “s” character

acetylide (an acetylenic anion) sp lone pair, 50% “s” character

The stability of the carbanion depends on the nature of the hybridized orbital occupied by the lone pair of electrons. As shown above, in ethane the lone pair resides in an sp³ orbital, while in ethene it occupies the sp² orbital and an sp orbital in the case of acetylene. The sp³, sp², and sp orbitals have 25%, 33%, and 50% "s" character, respectively. Since "s" orbitals are closer to the positively charged nucleus, a hybrid orbital with a higher "s" character will effectively stabilize the negative charge. Thus, the acetylide ions will be the most stable and readily formed in the presence of a suitable base.

Choosing a suitable base

In general, for a base to deprotonate an acid, the pK_a of the base's conjugate acid must be at least 10 pK_a units greater than that of the acid.

Terminal alkynes have a pK_a of 25. Therefore, an appropriate base would be one where the conjugate acid has a pK_a that is at least 35. Recall that for an acid-base reaction, the equilibrium favors the formation of weaker acids and bases from stronger acids and bases.

stronger acids + stronger bases weaker acids + weaker bases

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 25, the equilibrium favors the formation of sodium acetylide, making sodium amide a strong enough base for the deprotonation reaction.

Apart from sodium amide, sodium hydride, butyllithium, and lithium diisopropylamide (LDA) are other commonly used bases to form acetylide ions.

Sodium hydride

Butyllithium (n-BuLi)

Lithium diisopropylamide (LDA)

In the presence of sodium hydroxide as the base, terminal alkynes form sodium acetylide and water as the conjugate acid. However, since the pK_a of water is less than 25, the equilibrium favors the reactants. Therefore, sodium hydroxide is not a suitable base to form acetylide ions.

Synthesis of organometallic reagents

The relative acidity of terminal alkynes finds application in the synthesis of organometallic compounds when treated with Grignard or organolithium reagents. These are examples of transmetalation reactions involving the transfer of a metal atom from one carbon to another, thereby forming new metal–carbon bonds. However, they can also be interpreted as acid–base reactions that favor the formation of weaker acids and bases.

Procedure

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.