Why Toluene Reacts Faster Than Benzene but Nitrobenzene Is Sluggish

Benzene is drawn with three double bonds, yet it does not decolourise bromine water and prefers substitution to addition. Its ring of delocalised electrons gives it a special stability called aromaticity, and the groups attached to that ring decide how fast it reacts and where new groups go.

This lesson covers the structure and aromaticity of benzene, Huckel's rule, and electrophilic aromatic substitution with the directing effects of substituents.

**Benzene, ${\mathrm{C}}_6{\mathrm{H}}_6$, is a planar ring of six $\mathrm{s}{\mathrm{p}}^2$ carbons whose six pi electrons are delocalised over the whole ring, so it is a resonance hybrid of two Kekule structures and is far more stable than a simple triene.

Structure:**

- Each carbon is $\mathrm{s}{\mathrm{p}}^2$ hybridised, forming sigma bonds to two carbons and one hydrogen at 120°
- The unhybridised p orbitals on all six carbons overlap sideways above and below the ring
- The six pi electrons form a continuous ring-shaped cloud, shown by a circle inside a hexagon

Evidence for delocalisation:

- All six carbon-carbon bonds are 139 pm long, between a single bond (154 pm) and a double bond (134 pm)
- Benzene gives only one monosubstituted product, such as one bromobenzene, so all six hydrogens are equivalent
- It resists addition reactions typical of alkenes and undergoes substitution instead

Worked example — resonance energy. Hydrogenating cyclohexene, with one double bond, releases about 120 kJ mol${}^{-1}$. Three isolated double bonds would release

$$3\times 120=360{\text{ kJ mol}}^{-1}$$

but hydrogenating benzene releases only about 208 kJ mol${}^{-1}$. The difference, $360-208=152$ kJ mol${}^{-1}$, is the resonance energy — the extra stability from delocalisation.

An everyday example. Washing powders use detergents built on a benzene ring, made by attaching a long alkyl chain and then a sulphonate group to benzene.

The substance. Benzene does not flip between two Kekule structures — it has one real structure in which every bond is identical.

In electrophilic aromatic substitution, an electrophile attacks the pi electrons of benzene to form a positively charged intermediate, which loses a proton to restore the aromatic ring, and groups already on the ring decide both the speed and the position of attack.

Three-step mechanism:

- Generation of the electrophile, often with a catalyst
- Attack by the pi electrons, forming a resonance-stabilised carbocation called the arenium ion, which has temporarily lost aromaticity
- Loss of ${\mathrm{H}}^{+}$, restoring the aromatic ring

The key reactions:

- Nitration — the nitrating mixture of concentrated nitric and sulphuric acids gives the nitronium ion, $\mathrm{HN}{\mathrm{O}}_3+2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4\to \mathrm{N}{\mathrm{O}}_2^{+}+{\mathrm{H}}_3{\mathrm{O}}^{+}+2HS{\mathrm{O}}_4^{-}$, and benzene becomes nitrobenzene
- Halogenation — chlorine with anhydrous $\mathrm{FeC}{\mathrm{l}}_3$ or $\mathrm{AlC}{\mathrm{l}}_3$ gives $\mathrm{C}{\mathrm{l}}^{+}$, and benzene becomes chlorobenzene
- Friedel-Crafts alkylation — $\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}+\mathrm{AlC}{\mathrm{l}}_3\to \mathrm{C}{\mathrm{H}}_3^{+}+\mathrm{AlC}{\mathrm{l}}_4^{-}$, giving toluene
- Friedel-Crafts acylation — $\mathrm{C}{\mathrm{H}}_3\mathrm{COCl}$ with $\mathrm{AlC}{\mathrm{l}}_3$ gives $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{O}}^{+}$, and benzene becomes acetophenone

Directing effects of substituents:

- Ortho and para directing, activating — $-\mathrm{OH}$, $-\mathrm{N}{\mathrm{H}}_2$, $-\mathrm{OC}{\mathrm{H}}_3$ and $-\mathrm{C}{\mathrm{H}}_3$ push electrons into the ring, especially at the ortho and para positions
- Ortho and para directing, deactivating — the halogens withdraw electrons by the -I effect but donate lone pairs to the ortho and para positions by +R
- Meta directing, deactivating — $-\mathrm{N}{\mathrm{O}}_2$, $-\mathrm{CN}$, $-\mathrm{CHO}$, $-\mathrm{COOH}$ and $-\mathrm{S}{\mathrm{O}}_3\mathrm{H}$ withdraw electrons most strongly from the ortho and para positions, leaving the meta position least depleted

Worked example. Nitrating toluene gives mainly 2-nitrotoluene and 4-nitrotoluene, and it reacts faster than benzene; nitrating nitrobenzene needs harsher conditions and gives 1,3-dinitrobenzene.

An everyday example. Dye factories in Gujarat start from nitrobenzene, made by nitration, and convert it into aniline, the parent of many textile dyes.

The substance. Friedel-Crafts reactions fail on nitrobenzene — its ring is so deactivated that nitrobenzene is even used as a solvent for these reactions.