Acidity Of Terminal Alkynes — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Acidity of Terminal Alkynes Terminal alkynes are weakly acidic due to the sp-hybridized carbon atom on the triple bond. The sp-hybridized carbon has a higher s-character (50%) compared to sp2 (33%) and sp3 (25%) hybridized carbons, making it more electronegative. This increased electronegativity stabilizes the negative charge on the conjugate base (the acetylide anion), making the terminal hydrogen atom more acidic than those in alkenes or alkanes. They can be deprotonated by sufficiently strong bases to form acetylide anions. When reacting with strong bases like NaNH2 in liquid ammonia, the evolution of ammonia gas (pungent smell) may be observed. If the acetylide salt formed is insoluble, a precipitate may appear. For example, silver acetylides are often white precipitates. The reaction mixture might also change color depending on the reagents and any indicators present. The reaction is thermodynamically favorable when a base is used whose conjugate acid has a higher pKa than the terminal alkyne (pKa ~25). For instance, ammonia (pKa ~38) is a weaker acid than terminal alkynes, so NaNH2 (its conjugate base) is a sufficiently strong base to deprotonate terminal alkynes. Similarly, butane (pKa ~50) is much weaker than terminal alkynes, making n-BuLi an excellent choice. The strong base (nucleophile) abstracts the acidic terminal proton from the alkyne. The electron pair from the C-H bond forms a lone pair on the terminal carbon, creating a stable sp-hybridized carbanion (acetylide anion). The conjugate acid of the base is formed. Confusing terminal alkynes with internal alkynes; only terminal alkynes have an acidic proton. Using weak bases (e.g., NaOH, NaHCO3) which are not strong enough to deprotonate terminal alkynes (pKa ~25). Failing to recognize that acetylide anions are strong nucleophiles and bases, leading to side reactions like SN2 attacks or elimination if appropriate electrophiles or acidic protons are present. Incorrectly assuming that all C-H bonds are equally acidic; emphasizing the role of sp-hybridization. Forgetting the requirement for anhydrous and often inert conditions, as acetylides are highly reactive towards water and oxygen.