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9.3: Acidez de los 1-alquinos
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Acidez de los 1-alquinos
 
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9.3: Acidez de los 1-alquinos

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

Figure1left
Ethane
(pK= 51)
Figure1middle

Ethene
(pK= 44)

Figure1right

 

Ethyne
(pK= 25)

With a pKa difference of 26 units, acetylene is 1026 times more acidic than ethane. Similarly, a difference of 19 pKa 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.

Figure2top
ethanide (an alkyl anion)
sp3 lone pair, 25% “s” character
Figure2middle
ethenide (a vinylic anion)
sp2 lone pair, 33% “s” character
Figure2bottom
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 pKa of the base's conjugate acid must be at least 10 pKa units greater than that of the acid.

Figure3

Terminal alkynes have a pKa of 25. Therefore, an appropriate base would be one where the conjugate acid has a pKa 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 arrowweaker acids + weaker bases

With sodium amide as the base, terminal alkynes form sodium acetylide and ammonia as the conjugate acid. Since the pKa 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.

Figure4

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

Figure5left

Sodium hydride

Figure5middle
Butyllithium (n-BuLi)
Figure5right
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 pKa of water is less than 25, the equilibrium favors the reactants. Therefore, sodium hydroxide is not a suitable base to form acetylide ions.

Figure6

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.

Figure7top

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