Question

Why are alkynes acidic in comparison to alkanes and alkenes?

Answer 1

Alkynes, Alkanes and alkenes

• In organic chemistry, an alkyne is a hydrocarbon that has at least one Carbon -Carbon triple bond $\left(\mathrm{C}\equiv \mathrm{C}\right)$.
• Alkanes are organic compounds having no other functional groups than single-bonded carbon and hydrogen $\left(\mathrm{C}-\mathrm{H}\right)$ atoms.
• Unsaturated hydrocarbons containing at least one carbon-to-carbon double bond $\left(\mathrm{C}=\mathrm{C}\right)$ are known as alkenes.

Acidity:

• A terminal alkyne's acidity is caused by a high amount of $s$ character in the $sp$ hybrid orbital, which forms a single covalent connection with the $s$ orbital of the hydrogen atom.
• Because of the high $s$ character of a $sp$ hybridised carbon, the overlapping region of the $\mathrm{\sigma }$ bond moves extremely close to the Carbon atom.
• The hydrogen atom becomes somewhat positive as a result of this polarisation of the bond.
• The hydrogen atom is a weak proton because of its tiny positive charge, which can be removed by a strong base.
• The $s$ character in hybridised carbon bonds is less in alkanes and alkenes, meaning fewer electronegative carbon atoms as well as a corresponding lower shift towards such atoms in the overlap region $\mathrm{\sigma }$ bond.
• Thus, acidity of proton also decreases in alkenes and alkanes. Hence, alkynes have the most acidic proton.