Amino acids contain amino (–NH₂) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins. They may contain other functional groups also. All α-amino acids have trivial names, which usually reflect the property of that compound or its source. Glycine is so named since it has sweet taste (in Greek glykos means sweet) and tyrosine was first obtained from cheese (in Greek, tyros means cheese.) Amino acids are generally represented by a three letter symbol, sometimes one letter symbol is also used. Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesized in the body, are known as nonessential amino acids. On the other hand, those which cannot be synthesized in the body and must be obtained through diet, are known as essential amino acids.

The acid showing salt-like character in aqueous solution is 

α-Aminoacetic acid 

The correct answer is option 4. \(\alpha \)-Aminoacetic acid.

Let us delve into why \(\alpha \)-aminoacetic acid (glycine) exhibits salt-like character in aqueous solution, compared to the other acids listed.

\(\alpha \)-Aminoacetic Acid (Glycine):

Zwitterionic Nature:

Glycine has a unique structure among amino acids where it contains both an amino group \((–NH_2)\) and a carboxyl group \((–COOH)\) attached to the same carbon atom (the \(\alpha \)-carbon).

In aqueous solution, due to the acidic and basic properties of the amino and carboxyl groups, respectively, glycine can exist as a zwitterion (a molecule with both positive and negative charges).

The amino group can be protonated to form \(NH_3^+\), and the carboxyl group can lose a proton to form \(COO^−\). This results in the zwitterionic structure

Salt-Like Character:

The zwitterionic form of glycine resembles the structure of salts where there are both positively and negatively charged groups within the same molecule. This gives glycine unique properties in aqueous solution:

It can form ionic interactions with water molecules and other ions.

It has enhanced solubility in water compared to non-zwitterionic molecules of similar size due to the ion-dipole interactions.

It behaves more like a salt in terms of its aqueous chemistry, despite being an amino acid.

Comparison with Other Acids:

Acetic Acid \((CH_3COOH)\): Acetic acid primarily exists in its molecular form \((CH_3COOH)\) in aqueous solution, with the carboxyl group protonated. It does not exhibit zwitterionic characteristics or salt-like behavior.

Benzoic Acid \((C_6H_5COOH)\): Similar to acetic acid, benzoic acid exists primarily as molecules \((C_6H_5COOH)\) with the carboxyl group protonated in aqueous solution.

Formic Acid \((HCOOH)\): Formic acid also exists primarily in its molecular form \((HCOOH)\) with the carboxyl group protonated in aqueous solution.

Conclusion: Glycine, or \(\alpha \)-aminoacetic acid, exhibits salt-like character in aqueous solution due to its ability to form a zwitterionic structure where it possesses both positive and negative charges within the molecule. This zwitterionic nature enables glycine to interact with water and ions in a manner similar to salts, distinguishing it from other acids that primarily exist in molecular form in aqueous solutions.