Acidity of Alkynes : Formation of Acetylide Ions

Acidity of Alkynes: Formation of Acetylide Ions

– Acidity of Alkynes is the inportant factor of activity of Alkynes.

– Terminal alkynes are much more acidic than other hydrocarbons.

– Removal of an acetylenic proton forms an acetylide ion, which plays a central role in alkyne chemistry.

– The acidity of an acetylenic hydrogen stems from the nature of the sp hybrid ≡ C-H bond.

– The following table shows how the acidity of a -C-H bond varies with its hybridization, increasing with the increasing s character of the orbitals:

sp3 < sp2 < sp

(Remember that a smaller value of pKa corresponds to a stronger acid.)

Acidity of Alkynes : Formation of Acetylide Ions

– The acetylenic proton is about 1019 times more acidic than a vinyl proton.

– Abstraction of an acetylenic proton gives a carbanion that has the lone pair of electrons in the sp hybrid orbital.

– Electrons in this orbital are close to the nucleus, and there is less charge separation than in carbanions with the lone pair in sp2 or sp3 hybrid orbitals.

– Ammonia and alcohols are included for comparison; note that acetylene can be deprotonated by the amide (NH2)ion, but not by an alkoxide ion (OR).

– Very strong bases (such as sodium amide, NaNH2) deprotonate terminal acetylenes to form carbanions called acetylide ions (or alkynide ions).

– Hydroxide ion and alkoxide ions are not strong enough bases to deprotonate alkynes.

– Internal alkynes do not have acetylenic protons, so they do not react.

Acidity of Alkynes : Formation of Acetylide Ions

– Sodium amide (Na+ –NH2) is frequently used as the base in forming acetylide salts.

– The amide ion (NH2) is the conjugate base of ammonia, a compound that is itself a base.

– Ammonia is also a very weak acid, however, with Ka = 10-35 (pKa = 35)

– One of its hydrogens can be reduced by sodium metal to give the sodium salt of the amide ion, a very strong conjugate base.

– Acetylide ions are strong nucleophiles.

– In fact, one of the best methods for synthesizing substituted alkynes is a nucleophilic attack by an acetylide ion on an unhindered alkyl halide.

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