Mononitration of Substituted Benzene: Concept Explanation

The given reactant is a disubstituted benzene ring with two substituents: a methoxy group (-OCH₃) and a nitro group (-NO₂). We need to determine the position where the new nitro group will attach during mononitration.

Step 1: Identify the Directing Effects of Existing Substituents

Substituents on benzene influence electrophilic substitution (like nitration) through two effects:

• Activating/Deactivating: Methoxy (-OCH₃) is a strong activating group (electron-donating). Nitro (-NO₂) is a strong deactivating group (electron-withdrawing).
• Orientation (Directing): Methoxy is an ortho/para-director. Nitro is a meta-director.

Since the methoxy group is a much stronger director than the nitro group, it will dominate the orientation of the incoming electrophile (NO₂⁺).

Step 2: Analyze the Relative Positions

The two substituents are meta to each other. The methoxy group will direct the new nitro group to its ortho and para positions. We must check which of these positions is sterically and electronically favored.

The possible positions for attack relative to the -OCH₃ group are labeled below (the nitro group is fixed at the meta position):

Ortho to -OCH₃: This position is also meta to the existing -NO₂ group. A meta-director (like -NO₂) weakly deactivates the meta position.

Para to -OCH₃: This position is ortho to the existing -NO₂ group. This is highly unfavorable due to the strong electron-withdrawing effect and significant steric hindrance from the nitro group.

Therefore, the ortho position to -OCH₃ (which is meta to -NO₂) is the major product. It is sterically less crowded and electronically more favorable than the para position.

Step 3: Final Answer

The major product is the one where the new nitro group is attached ortho to the methoxy group and meta to the existing nitro group. This corresponds to the structure in the second option.

Related Topics & Formulae

Electrophilic Aromatic Substitution

The general mechanism for nitration involves the attack of the electrophile NO₂⁺ (nitronium ion) on the aromatic ring.

$\mathrm{Ar}-H+{\mathrm{NO}}^2{+}^{+}\to \mathrm{Ar}-\mathrm{NO}2+H^{+}$

Directing Effects Theory

Activating (Ortho/Para Directors): -OH, -OR, -NH₂, -alkyl groups. They donate electrons to the ring, making it more reactive.

Deactivating (Meta Directors): -NO₂, -CN, -COOH, -COR, -SO₃H. They withdraw electrons from the ring, making it less reactive.

Exception: Halogens (-Cl, -Br) are deactivating but ortho/para directors due to their resonance effect.

Rules for Disubstituted Benzenes

1. If the directing effects of two groups reinforce each other (e.g., both are o/p directors), the product is predictable.
2. If the directing effects oppose each other (as in this case), the stronger activating group dominates. The order of activation strength is: -NH₂ > -OH > -OR > -alkyl.
3. Steric hindrance must always be considered, especially with bulky groups.