zum Directory-modus

Alkynes: Properties

Alkynes: Acidity

The acidity of protons attached to sp hybridized C atoms is extremely important for the understanding of alkyne chemistry; terminal alkynes are much more acidic than alkenes or alkanes.

Tab.1
Examples
Fig.
Acetylene
Fig.
Fig.
Acetylide anion
pKa ~ 25
Fig.
Ethylene
Fig.
Fig.
Vinyl anion
pKa ~ 44
Fig.
Methane
Fig.
Fig.
Methyl anion
pKa ~ 49

Only the strongest known bases deprotonate alkenes to their respective anions and alkanes do not react at all at room temperature under these conditions. Though aqueous lyes or alcoholates are too weak to deprotonate alkynes, amide anions or common organic lithium bases smoothly deprotonate alkynes.

Fig.
Fig.

R' = CH3 or n-butyl

The high stability of the acetylide anion is the basis for the strong acidity of alkyne protons. The free electrons occupy a sp hybrid orbital, while they are located in a sp2 hybrid orbital for a vinyl anion and in a sp3 hybrid orbital for an alkyl anion. The larger the s portion of the anionic hybrid orbital, the closer to the positive nucleus the negative charge is located and the better it is stabilized. Acetylide anions are strong nucleophiles with numerous applications in organic synthesis.

Page 2 of 4