Benzene is a highly stable molecule because of its delocalised \( \pi \)-electron cloud. However, this cloud represents an area of high electron density, which naturally attracts electron-deficient species called electrophiles. To maintain its thermodynamic stability, benzene undergoes electrophilic substitution instead of addition, preserving the delocalised system.
🔑 Key Principle
During electrophilic substitution, the stable delocalised ring of \( \pi \)-electrons is temporarily disrupted as it donates a pair of electrons to form a bond with the electrophile, forming a cyclohexadienyl cation intermediate. Re-aromatisation immediately follows through the elimination of a proton (\( \text{H}^+ \)).
General Substitution Mechanism
Every electrophilic substitution reaction of benzene follows a two-stage pathway:
- Electrophilic Attack: Two electrons from the delocalised \( \pi \)-system are donated to the electrophile (\( \text{E}^+ \)), forming a \( \text{C-E} \) single bond. This disrupts the delocalised ring, creating a positive intermediate with five \( sp^2 \) carbons and one \( sp^3 \) carbon.
- Proton Elimination: The unstable intermediate loses a hydrogen atom as a proton (\( \text{H}^+ \)). The pair of electrons from the \( \text{C-H} \) bond is returned to the \( \pi \)-system, restoring the delocalised ring.
1. Nitration of Benzene
Nitration is the substitution of a hydrogen atom on the benzene ring for a nitro group (\( -\text{NO}_2 \)).
An electrophilic substitution reaction that introduces a nitro (\( -\text{NO}_2 \)) group onto an aromatic ring system.
- Reagents: Concentrated nitric acid (\( \text{HNO}_3 \)) and concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)) catalyst.
- Conditions: Heat under reflux at \( 50\text{-}55^\circ\text{C} \).
Step A: Generation of the Electrophile
Nitric acid is not a strong enough electrophile to attack benzene. Concentrated sulfuric acid acts as a stronger acid, protonating the nitric acid to generate the highly reactive nitronium ion (\( \text{NO}_2^+ \)):
Step B: Regeneration of the Catalyst
The \( \text{H}^+ \) ion eliminated from the ring reacts with the bisulfate ion (\( \text{HSO}_4^- \)) to regenerate the sulfuric acid catalyst:
Keep the temperature strictly between \( 50\text{-}55^\circ\text{C} \) during nitration to prevent multiple substitutions. Temperatures exceeding \( 60^\circ\text{C} \) will cause further nitration to form 1,3-dinitrobenzene or 1,3,5-trinitrobenzene.
Industrial Applications of Nitrated Aromatics
- Explosives: Methylbenzene (toluene) is nitrated under high temperatures to form 2,4,6-trinitrotoluene (TNT). The rapid decomposition of nitro groups makes it highly explosive.
- Dyes and Pharmaceuticals: Nitrobenzene can be reduced using a mixture of tin (Sn) and concentrated hydrochloric acid (\( \text{HCl} \)) under reflux to produce phenylamine. Phenylamine is a key feedstock for manufacturing azo dyes, polymers, and analgesics.
2. Friedel-Crafts Acylation of Benzene
Friedel-Crafts acylation introduces an acyl group (\( \text{R-C}(=\text{O})- \)) onto the benzene ring. This forms an aromatic ketone, which is useful in synthesis because the carbonyl carbon can undergo further functional group modifications.
An electrophilic substitution reaction that introduces an acyl group (\( \text{RCO-} \)) onto an aromatic ring, using an acyl chloride and an anhydrous aluminium chloride (\( \text{AlCl}_3 \)) catalyst.
- Reagents: Acyl chloride (e.g. ethanoyl chloride) and an anhydrous aluminium chloride (\( \text{AlCl}_3 \)) catalyst.
- Conditions: Reflux, strictly anhydrous conditions.
Step A: Generation of the Electrophile
The aluminium chloride catalyst acts as a halogen carrier (a Lewis acid). It accepts a lone pair of electrons from the chlorine atom of the acyl chloride, weakening the \( \text{C-Cl} \) bond to form a highly reactive acylium ion (\( \text{R-C}^+=\text{O} \)) electrophile:
Step B: Regeneration of the Catalyst
The \( \text{H}^+ \) ion eliminated from the ring reacts with the tetrachloroaluminate ion (\( \text{AlCl}_4^- \)) to regenerate the catalyst and produce toxic hydrogen chloride gas fumes:
Friedel-Crafts reactions must be performed under anhydrous conditions. Aluminium chloride reacts violently with water to form aluminium hydroxide and hydrogen chloride gas, which deactivates the catalyst:
\[ \text{AlCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + 3\text{HCl} \]Solution:
1. Overall Equation:
\[ \text{C}_6\text{H}_6 + \text{CH}_3\text{CH}_2\text{COCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COCH}_2\text{CH}_3 + \text{HCl} \]The organic product is 1-phenylpropan-1-one.
2. Electrophile Generation:
\[ \text{CH}_3\text{CH}_2\text{COCl} + \text{AlCl}_3 \rightarrow \text{CH}_3\text{CH}_2\text{C}^+=\text{O} + \text{AlCl}_4^- \]3. Mechanism:
- A curly arrow starts from the delocalised ring of benzene and points directly to the positively charged carbon atom of \( \text{CH}_3\text{CH}_2\text{C}^+=\text{O} \).
- The intermediate contains a positive charge inside a horseshoe-shaped ring that is open towards the carbon bonded to both \( -\text{H} \) and \( -\text{COCH}_2\text{CH}_3 \).
- A curly arrow starts from the \( \text{C-H} \) bond and points to the disrupted ring to restore the delocalised system, releasing \( \text{H}^+ \).