Sunday, January 27, 2008

IIT JEE Revision - Acidity of Alkynes

Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.

They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.

Acetylides react with alkyl halides to give higher alkynes.

Explanation for the acidic character of alkynes

The acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes.

In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by the nucleus and are quite electronegative.

Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.

In the case of nonterminal alkynes, no hydrogen atom is attached directly to the triple bonded carbon atom (triple bond to carbon and one single bond to another carbon) and hence hydrogen atoms are not released easily.

Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.

acidic charater
HC≡HC > CH2=CH2 > CH3-CH3

The relative acidity of acetylene is more than that of ammonia but less than that of water.

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