Chemical Reactions of Benzene: MDCAT Chemistry notes

Electrophilic substitution

Benzene's delocalized π cloud attracts electrophiles. The ring keeps its stable aromatic system by substituting a hydrogen instead of adding across a bond. The general mechanism has three steps:

1. A catalyst generates the electrophile $\mathrm{E^+}$.
2. $\mathrm{E^+}$ attacks the ring and forms a positively charged intermediate (arenium ion, or σ-complex).
3. $\mathrm{H^+}$ is lost and aromaticity is restored.

Substitution is preferred to addition because it keeps the six delocalized π electrons, and the extra stability that comes with them. Addition would destroy this system, so it happens only under harsh conditions.

The five key substitutions

Reaction	Reagents and catalyst	Electrophile	Product
Halogenation	$\mathrm{Cl_2}$ or $\mathrm{Br_2}$ with $\mathrm{FeCl_3}$, $\mathrm{FeBr_3}$ or anhydrous $\mathrm{AlCl_3}$	$\mathrm{Cl^+}$ / $\mathrm{Br^+}$	Chlorobenzene or bromobenzene
Nitration	Conc. $\mathrm{HNO_3}$ + conc. $\mathrm{H_2SO_4}$	$\mathrm{NO_2^+}$ (nitronium)	Nitrobenzene
Sulphonation	Fuming $\mathrm{H_2SO_4}$ (oleum)	$\mathrm{SO_3}$	Benzenesulphonic acid
Friedel–Crafts alkylation	R–X + anhydrous $\mathrm{AlCl_3}$	$\mathrm{R^+}$ (carbocation)	Alkylbenzene, e.g. toluene
Friedel–Crafts acylation	RCOCl + anhydrous $\mathrm{AlCl_3}$	$\mathrm{RCO^+}$ (acylium)	Aryl ketone

How the catalyst works

$\mathrm{AlCl_3}$ and $\mathrm{FeCl_3}$ are Lewis acids. They take a halide ion and leave a positive electrophile behind:

$$\mathrm{Br_2 + FeBr_3 \to Br^+ + FeBr_4^-}$$ $$\mathrm{CH_3Cl + AlCl_3 \to CH_3^+ + AlCl_4^-}$$

In sulphonation, $\mathrm{SO_3}$ is itself electron-poor at sulphur and acts as the electrophile. It is not $\mathrm{HSO_4^-}$ or $\mathrm{H_2SO_4}$.

Worked examples

$$\mathrm{C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl}$$ $$\mathrm{C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3\ (acetophenone) + HCl}$$

Both Friedel–Crafts reactions make a new C–C bond between the ring and the side chain. Clemmensen reduction and nitration do not.

Ring versus side chain: the conditions decide

• Toluene + $\mathrm{Cl_2}$ with $\mathrm{AlCl_3}$ (or $\mathrm{FeCl_3}$), in the dark: ring substitution gives o- and p-chlorotoluene.
• Toluene + $\mathrm{Cl_2}$ in UV light or heat, without catalyst: free radical substitution in the side chain gives benzyl chloride, $\mathrm{C_6H_5CH_2Cl}$.

Addition and oxidation (harsh conditions)

• $\mathrm{H_2}$ with Ni at high temperature gives cyclohexane.
• $\mathrm{Cl_2}$ in UV light (no catalyst) gives benzene hexachloride, $\mathrm{C_6H_6Cl_6}$.
• Oxidation with air over $\mathrm{V_2O_5}$ at high temperature gives maleic anhydride. This is where $\mathrm{V_2O_5}$ appears, not in Friedel–Crafts reactions.

Common MDCAT traps

• Friedel–Crafts alkylation uses an alkyl halide. Acylation uses an acyl chloride and gives a ketone.
• The catalyst must be anhydrous $\mathrm{AlCl_3}$. $\mathrm{H_2SO_4}$, sunlight and $\mathrm{V_2O_5}$ are wrong answers.
• The electrophile in sulphonation is $\mathrm{SO_3}$.
• Chlorination of benzene with $\mathrm{FeCl_3}$ is electrophilic substitution, not addition and not a free radical reaction.
• With UV light and no catalyst, chlorine attacks toluene's side chain rather than the ring.

Quick revision

• Benzene undergoes electrophilic substitution.
• Nitration uses the $\mathrm{NO_2^+}$ electrophile from the mixed acids.
• Acetophenone is made by Friedel–Crafts acylation.
• Benzenesulphonic acid is made by sulphonation.
• Bromination uses $\mathrm{Br^+}$, generated by $\mathrm{FeBr_3}$.