JEE Chemistry 2025 Question with Solution

Given: The starting material is 3-nitrotoluene and the reaction sequence is bromination, reduction, diazotization, and replacement of diazonium group by hydrogen.

Find: The molar mass of major product (A).

Step 1: On bromination with Br$_2$/Fe, substitution occurs at the position directed by the methyl group, giving major product 4-bromo-3-nitrotoluene.

Step 2: On treatment with Sn/HCl, the nitro group is reduced to an amino group, so the compound becomes 4-bromo-3-aminotoluene.

Step 3: With NaNO$_2$, HCl, 273 K, the amino group is converted into the diazonium salt.

Step 4: With H$_3$PO$_2$, H$_2$O, the diazonium group is replaced by hydrogen. Therefore, the final product is 4-bromotoluene with molecular formula C$_7$H$_7$Br.

Molar mass calculation:

$$M = 7(12) + 7(1) + 1(80)$$ $$= 84 + 7 + 80 = 171$$

Therefore, the molar mass of product (A) is $171 \, \text{g mol}^{-1}$.

Given: A substituted toluene undergoes four successive reactions.

Find: The molar mass of the major product.

Track the functional group changes through the sequence:

1. Bromination: introduce Br on the ring to form 4-bromo-3-nitrotoluene as the major product.
2. Reduction: convert -NO$_2$ into -NH$_2$.
3. Diazotization: convert -NH$_2$ into diazonium salt.
4. Hypophosphorous acid reduction: replace the diazonium group by H.

So the substituents finally present on the benzene ring are CH$_3$ and Br, which gives 4-bromotoluene.

Hence, formula of (A) is C$_7$H$_7$Br.

Now calculate molar mass:

$$\begin{aligned} M &= 7 \times 12 + 7 \times 1 + 80 \\ &= 84 + 7 + 80 \\ &= 171 \end{aligned}$$

Thus, the required molar mass is 171.

Given: The sequence converts -NO$_2$ to -NH$_2$, then to diazonium salt, and finally replaces that group by H.

Find: The molar mass of the final product.

The shortcut is to notice that the last three steps together effectively remove the nitro-derived substituent from the ring and replace it by hydrogen. Therefore, after the first bromination step, only the introduced Br and the original CH$_3$ remain in the final product.

So the final product is bromotoluene, specifically 4-bromotoluene, with formula C$_7$H$_7$Br.

$$M = 7(12) + 7(1) + 80 = 171$$

Therefore, the answer is 171.