When a benzene ring undergoes a second electrophilic substitution reaction, the substituent already present on the ring influences two key factors: the rate of reaction (by activating or deactivating the ring) and the position of the incoming group (directing effect).
🔑 Key Principle
Electron-donating groups increase the electron density of the benzene ring, activating it towards electrophilic attack and directing substitution to the 2- and 4-positions. Electron-withdrawing groups decrease electron density, deactivating the ring and directing substitution to the 3-position.
Ring Activation vs Deactivation
Substituents alter the electron density of the benzene ring through inductive effects and resonance overlap:
A substituent that donates electron density into the benzene \( \pi \)-system, increasing its nucleophilic character and accelerating the rate of electrophilic substitution.
A substituent that withdraws electron density from the benzene ring, decreasing its nucleophilic character and decelerating the rate of electrophilic substitution.
- Activating Groups (Electron-Donating): Groups like \( -\text{OH} \) (phenol), \( -\text{NH}_2 \) (phenylamine), and alkyl groups (such as \( -\text{CH}_3 \) in methylbenzene) donate electron density into the delocalised system. Phenol, for instance, has a lone pair of electrons on the oxygen atom that overlaps directly with the \( \pi \)-ring system, significantly activating it. As a result, phenol reacts rapidly with bromine water at room temperature without requiring a catalyst.
- Deactivating Groups (Electron-Withdrawing): Groups like \( -\text{NO}_2 \) (nitrobenzene), \( -\text{COOH} \), and \( -\text{CHO} \) withdraw electron density. The nitrogen in the nitro group has a positive formal charge, withdrawing electrons through both induction and resonance. This makes the ring much less nucleophilic, meaning further substitution reactions require higher temperatures and concentrated fuming acid reactants.
Substituent Directing Effects
Substituents direct incoming groups to specific positions on the ring (numbered relative to the existing substituent at carbon-1):
A substituent that directs incoming electrophilic groups to carbon-2 (ortho) and carbon-4 (para) relative to itself.
A substituent that directs incoming electrophilic groups to carbon-3 (meta) relative to itself.
Classification of Substituents
| Substituent Group | Effect on Ring Reactivity | Directing Orientation | Mechanism of Influence |
|---|---|---|---|
| \( -\text{NH}_2 \), \( -\text{NHR} \) | Strongly Activating | 2- and 4-directing | Resonance donation from nitrogen lone pair. |
| \( -\text{OH} \) | Strongly Activating | 2- and 4-directing | Resonance donation from oxygen lone pair. |
| Alkyl groups (\( -\text{CH}_3 \)) | Weakly Activating | 2- and 4-directing | Inductive donation (positive inductive effect). |
| Halogens (\( -\text{Cl} \), \( -\text{Br} \)) | Weakly Deactivating | 2- and 4-directing | Inductive withdrawal, but resonance donation directs. |
| \( -\text{NO}_2 \) | Strongly Deactivating | 3-directing | Inductive and resonance withdrawal of electrons. |
| \( -\text{COOH} \), \( -\text{COOR} \) | Deactivating | 3-directing | Electron withdrawal via polar carbonyl bond. |
Although halogens are weakly deactivating because of their electronegativity, their lone pairs allow them to act as 2- and 4-directors. This is the only exception where a deactivating group is not a 3-director, so be careful not to make this common mistake in exams.
The two steps required are:
1. Nitration to introduce the nitro group.
2. Oxidation of the methyl group to form the carboxylic acid using alkaline \( \text{KMnO}_4 \).
Explain why performing step 1 before step 2 produces the desired product, whereas reversing the steps does not.
Solution:
1. Nitrating first (Step 1 then Step 2):
The methyl group (\( -\text{CH}_3 \)) in methylbenzene is an activating 2- and 4-director. Nitration of methylbenzene will direct the incoming nitro group to the 4-position (along with the 2-position, which can be separated). Subsequent oxidation of the methyl group to a carboxylic acid yields 4-nitrobenzoic acid.
2. Oxidising first (Step 2 then Step 1):
Oxidation of methylbenzene yields benzoic acid. The carboxylic acid group (\( -\text{COOH} \)) is a deactivating 3-director. If benzoic acid is then nitrated, the nitro group will be directed to the 3-position, producing 3-nitrobenzoic acid instead of the desired 4-nitrobenzoic acid product.