1. What happens on decarboxylation?

A diaminobenzoic acid has the form $C_6H_3(NH_2)_2COOH$ When it undergoes decarboxylation, the $-COOH$ group is removed and replaced effectively by hydrogen on the ring.

So each diaminobenzoic acid gives a phenylenediamine isomer: $C_6H_3(NH_2)_2COOH \xrightarrow{\text{decarboxylation}} C_6H_4(NH_2)_2$

The possible products are therefore the three isomers of phenylenediamine:

• $1,2$-diaminobenzene (o-phenylenediamine)
• $1,3$-diaminobenzene (m-phenylenediamine)
• $1,4$-diaminobenzene (p-phenylenediamine)

2. Count the six possible diaminobenzoic acids

Fix $COOH$ at position 1. Then choose 2 positions out of 2,3,4,5,6 for the two $NH_2$ groups, taking benzene symmetry into account.

The six distinct diaminobenzoic acids are:

• $2,3$-diaminobenzoic acid
• $2,4$-diaminobenzoic acid
• $2,5$-diaminobenzoic acid
• $2,6$-diaminobenzoic acid
• $3,4$-diaminobenzoic acid
• $3,5$-diaminobenzoic acid

Now remove position 1 (the carboxyl carbon attachment) and see the relative positions of the two $NH_2$ groups in the resulting benzene.

3. Map each acid to the phenylenediamine formed

After decarboxylation, the relative positions of the two amino groups determine whether the product is ortho, meta, or para.

• $2,3$-diaminobenzoic acid $\to$ adjacent groups $\Rightarrow o$-phenylenediamine
• $2,4$-diaminobenzoic acid $\to$ meta arrangement $\Rightarrow m$-phenylenediamine
• $2,5$-diaminobenzoic acid $\to$ para arrangement $\Rightarrow p$-phenylenediamine
• $2,6$-diaminobenzoic acid $\to$ meta arrangement $\Rightarrow m$-phenylenediamine
• $3,4$-diaminobenzoic acid $\to$ ortho arrangement $\Rightarrow o$-phenylenediamine
• $3,5$-diaminobenzoic acid $\to$ meta arrangement $\Rightarrow m$-phenylenediamine

So the counts are:

• Three acids give $m$-phenylenediamine
• Two acids give $o$-phenylenediamine
• One acid gives $p$-phenylenediamine

Thus product C (formed from only one acid) is $p$-phenylenediamine.

4. Melting point of product C

Known melting points of phenylenediamines are approximately:

• $o$-phenylenediamine: about $104^\circ \mathrm{C}$
• $m$-phenylenediamine: about $63^\circ \mathrm{C}$
• $p$-phenylenediamine: about $140$–$142^\circ \mathrm{C}$

Therefore, product C has melting point: $142^\circ \mathrm{C}$

So the correct option is D.

5. Comparison with stored answer

Stored correct answer: D

Derived answer: D

They agree.