What is the correct order of acidity of the following?

Propiolic acid > Acrylic acid > Acetic acid

The correct answer is option 3. Propiolic acid > Acrylic acid > Acetic acid.

The acidity of carboxylic acids is influenced by the stability of the conjugate base that forms after losing a proton \((H^+)\). The stability of the conjugate base can be understood through resonance stabilization and inductive effects:

Acetic acid \((CH_3COOH)\):

Acetic acid has a structure where the negative charge on the conjugate base (acetate ion, \(CH_3COO^-\)) can be delocalized through resonance with the carbonyl group \((C=O)\). This resonance stabilization disperses the negative charge over two oxygen atoms (from the carbonyl and carboxyl groups), stabilizing the acetate ion and increasing the acidity of acetic acid.

Acrylic acid \((CH_2=CHCOOH)\):

Acrylic acid has a structure where the negative charge on the conjugate base (acrylate ion, \(CH_2=CHCOO^-\)) can also be delocalized through resonance. In this case, resonance occurs between the double bond and the carbonyl group, spreading the negative charge over the two oxygen atoms. This resonance stabilization makes acrylic acid more acidic than acetic acid.

Propiolic acid \((HC≡C-COOH)\):

Propiolic acid has a structure where the conjugate base (propiolate ion, \(HC≡C-COO^-\)) does not have significant resonance stabilization due to the linear arrangement and the lack of additional electronegative atoms adjacent to the carboxyl group. However, the presence of the triple bond adjacent to the carboxyl group allows for a more stable conjugate base compared to both acetic and acrylic acids. The sp-hybridized carbon in the triple bond can stabilize the negative charge through inductive effects, making propiolic acid more acidic than both acetic and acrylic acids.

Conclusion: Thus, the order will be :Propiolic acid > Acrylic acid > Acetic acid. This order reflects the increasing stability of the conjugate bases and thus the increasing acidity of the carboxylic acids listed.