Alkynes pack a punch when it comes to acidity. Their terminal hydrogen is more acidic than other hydrocarbons, thanks to the sp-hybridized carbon. This unique structure allows strong bases to easily snatch that hydrogen, forming acetylide anions.

The acidity of hydrocarbons follows a clear trend: alkynes > alkenes > alkanes. This pattern stems from orbital hybridization, with more s-character leading to greater stability in the resulting anions. Understanding these principles is key to predicting reactivity in organic reactions.

Alkyne Acidity and Formation of Acetylide Anions

Formation of acetylide anions

• Terminal alkynes contain a hydrogen atom bonded to the sp-hybridized carbon making the C-H bond more polarized and the hydrogen more acidic compared to sp2 or sp3 hybridized carbons
• Strong bases like sodium amide ($NaNH_2$) or alkali metal hydroxides ($NaOH$) can deprotonate the terminal hydrogen forming an alkali metal acetylide salt and the conjugate acid of the base ($HC\equiv C-CH_3 + NaNH_2 \rightarrow H_2NNa + ^{-}C\equiv C-CH_3$)
• The resulting acetylide anion has the negative charge localized on the sp-hybridized carbon and is stabilized by the s-character of the sp-hybridized orbital
• The formation of acetylide anions can be explained using the Bronsted-Lowry acid-base theory, where the alkyne acts as the acid and the strong base as the conjugate base

Acidity comparison of hydrocarbons

• Acidity increases in the order: alkanes < alkenes < alkynes as reflected by their pKa values with lower pKa indicating higher acidity (alkynes: 25, alkenes: 44, alkanes: 50)
• Alkynes are the most acidic due to the sp-hybridized carbon where the s-character of the sp-hybridized orbital stabilizes the acetylide anion
• Alkenes are more acidic than alkanes because of the sp2-hybridized carbon which has more s-character than the sp3-hybridized orbital in alkanes
• Alkanes are the least acidic with sp3-hybridized carbons that have the least s-character resulting in the least stable carbanions
• The presence of electron-withdrawing groups can increase the acidity of hydrocarbons by stabilizing the resulting anion

Orbital hybridization in anion stability

• Hydrocarbon anion stability is influenced by the hybridization of the carbon bearing the negative charge with stability increasing with increasing s-character of the orbital: sp3 < sp2 < sp
• Acetylide anions formed from sp-hybridized carbons are the most stable because:

1. The sp-hybridized orbital has 50% s-character
2. The high s-character stabilizes the negative charge by holding it closer to the nucleus

• Alkenyl anions formed from sp2-hybridized carbons are less stable than acetylide anions due to:

1. The sp2-hybridized orbital having 33% s-character
2. The lower s-character providing less stabilization of the negative charge

• Alkyl anions formed from sp3-hybridized carbons are the least stable since:

1. The sp3-hybridized orbital has only 25% s-character
2. The low s-character offers the least stabilization of the negative charge

Factors affecting anion stability and reactivity

• Resonance structures play a crucial role in stabilizing anions by delocalizing the negative charge
• The stability of anions influences their nucleophilicity, with more stable anions generally being less nucleophilic