Question

Two statements, one Assertion (A) and the other Reason (R) are given. Choose the correct option.
Assertion: 2-aminoethanoic acid and p-aminobenzene sulphonic acid can exist as Zwitter ions while p-aminobenzoic acid cannot.
Reason: When the acid group is a relatively strong proton donor and the \(-NH_2\) group is sufficiently basic it can accept a \(H^+\) ion from the acid group to form the dipolar ion.

• A. Both A and R are correct and R is the correct explanation of A.
• B. A is correct but R is wrong.
• C. A is wrong but R is correct.
• D. Both A and R are correct but R is not the correct explanation of A.

Hint:

Zwitter ion formation requires both a good proton donor group and a sufficiently basic proton acceptor group in the same molecule.

Answer 1

The Correct Option is A

Step 1: Understanding zwitter ion formation.
A zwitter ion is a dipolar ion having both positive and negative charges in the same molecule.
It is formed when an acidic group donates a proton and a basic group accepts that proton.
\[ \mathrm{-COOH \rightarrow -COO^- + H^+} \] \[ \mathrm{-NH_2 + H^+ \rightarrow -NH_3^+} \]

Step 2: Case of 2-aminoethanoic acid.
2-aminoethanoic acid is glycine:
\[ \mathrm{NH_2CH_2COOH} \] The carboxyl group donates \(H^+\), and the amino group accepts it.
So, it exists as:
\[ \mathrm{^{+}NH_3CH_2COO^-} \] Hence, glycine exists as a zwitter ion.

Step 3: Case of p-aminobenzene sulphonic acid.
In p-aminobenzene sulphonic acid, the sulphonic acid group \(-SO_3H\) is a strong acidic group.
It can donate a proton to the amino group.
Thus, the molecule exists mainly as a dipolar ion:
\[ \mathrm{^{+}NH_3-C_6H_4-SO_3^-} \] So, p-aminobenzene sulphonic acid can also exist as a zwitter ion.

Step 4: Case of p-aminobenzoic acid.
In p-aminobenzoic acid, the acidic group is \(-COOH\), which is weaker than \(-SO_3H\).
Also, the \(-NH_2\) group attached to benzene is less basic because its lone pair is delocalised into the aromatic ring.
Therefore, proton transfer is not strong enough to make it exist predominantly as a zwitter ion.

Step 5: Checking the assertion.
The assertion says that 2-aminoethanoic acid and p-aminobenzene sulphonic acid can exist as zwitter ions, while p-aminobenzoic acid cannot.
This statement is correct.

Step 6: Checking the reason.
The reason says that zwitter ion formation occurs when the acid group is a relatively strong proton donor and the \(-NH_2\) group is sufficiently basic to accept \(H^+\).
This is also correct.

Step 7: Final conclusion.
The reason correctly explains why some amino acids or amino sulphonic acids form zwitter ions while p-aminobenzoic acid does not.
Thus, both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.
\[ \boxed{\text{Option (A)}} \]