Examine the Assertion (A)

The assertion says:

$\alpha$-halocarboxylic acid on reaction with dilute $\mathrm{NH_3}$ gives good yield of $\alpha$-amino carboxylic acid, whereas the yield of amines is very low when prepared from alkyl halides.

This is correct.

• For an alkyl halide: $R-X + NH_3 \rightarrow R-NH_2$ but the amine formed is more nucleophilic than $NH_3$, so it further reacts with alkyl halide, giving secondary, tertiary amines and even quaternary ammonium salts. Hence, the yield of only primary amine is low.

• For an $\alpha$-halocarboxylic acid: $X-CH(R)-COOH + NH_3 \rightarrow H_2N-CH(R)-COOH$ the product is an amino acid. In aqueous medium it exists mainly as a zwitter ion: $^+H_3N-CH(R)-COO^-$ In this form, the amino group is protonated and is not available for further nucleophilic substitution, so further alkylation does not occur significantly. Hence good yield is obtained.

Examine the Reason (R)

The reason says:

Amino acids exist in zwitter ion form in aqueous medium.

This is also correct.

Amino acids contain both acidic $(-COOH)$ and basic $(-NH_2)$ groups, so in aqueous medium they predominantly exist as: $^+H_3N-CHR-COO^-$

Check whether (R) explains (A)

Yes, it does.

The key reason why $\alpha$-halocarboxylic acids give good yield of $\alpha$-amino acids is that the amino acid product exists as a zwitter ion, so the amino group is not free to undergo further reaction with the halo compound. This prevents multiple substitution, unlike the case of alkyl halides.

Conclusion

• Assertion (A): Correct
• Reason (R): Correct
• Reason correctly explains Assertion

Therefore, the correct option is: $\boxed{\text{D}}$