Chemical nature of Alkynes:
An alkyne molecule (except ethyne) has three parts
R - C≡C H
Alkyl part Alkyne Acidic H
Part
(i) Alkyl part : The alkyl part being inert in alkyne & thus does not show substitution reaction.
(ii) Alkyne part : It consists of one σ & 2 Π bonds. Due to more strain than alkene, Π bonds are highly reactive and less stable. The polarization of Π bonds in alkynes leads to addition reaction. Unlike alkene, alkyne reacts with two molecules of additive to form saturated molecule.
(iii) Acidic hydrogen part : H atom attached to sp hybridized carbon or triply bonded carbon atom is acidic in nature. The acidic character is due to the fact than an increase in s character of carbon atom give rise to higher electonegativity to it and thus H atom attached on sp hybridized carbon acquires polarity to show acidic nature.
Type of bond ≡C-H =C-H -C-H
s character 50% or 1/2 33.3% or 1/3 25% or 1/4
Thus R-C≡C-H+δ + Base → R-C≡C- + H-Base
The hybridized carbon atom being more electronegative, is best able to accommodate the electron pair in the anion left after the removal of proton.
Relative acidic nature : HOH≈HOR > HC≡CR > HNH2 > H2C=CH2 > CH3-CH3
Relative basic nature : OH-≈OR- < C≡CR < NH2 < CH=CH2 < CH2-CH3