Reactivity of Benzene: MDCAT Chemistry notes

Structure of benzene

Benzene, $\mathrm{C_6H_6}$, is a flat regular hexagon. Every carbon is sp$^2$ hybridized, with bond angles of 120$^\circ$. Each carbon keeps one unhybridized p orbital perpendicular to the ring, and the six p orbitals overlap sideways to give a delocalized $\pi$ cloud above and below the ring. All six C–C bonds are identical (about 1.39 Å), intermediate between a C–C single bond (1.54 Å) and a C=C double bond (1.34 Å).

Kekulé drew benzene with three alternating double bonds. The real molecule is a resonance hybrid of the two Kekulé structures. The extra stability due to delocalization is the resonance energy (about 150 kJ mol$^{-1}$), measured from heats of hydrogenation.

Aromaticity and the Hückel rule

A compound is aromatic if it is cyclic, planar, fully conjugated and has $(4n+2)$ $\pi$ electrons (Hückel rule).

So benzene, naphthalene, anthracene and phenol are all aromatic. A substituent such as $-\mathrm{OH}$ or $-\mathrm{CH_3}$ does not remove aromaticity.

Why benzene is less reactive than alkenes

In an alkene the $\pi$ electrons are localized between two carbons and easily attacked. In benzene they are spread over six carbons, so they are less available, and any addition would destroy the aromatic sextet and lose the resonance energy. Benzene is therefore unusually stable. It does not decolourize bromine water or cold dilute $\mathrm{KMnO_4}$, unlike an alkene. High reactivity is not a characteristic of benzene.

What benzene does react by

Electrophilic substitution (normal behaviour)

• Halogenation: $\mathrm{Cl_2}$ or $\mathrm{Br_2}$ with $\mathrm{FeCl_3/FeBr_3}$
• Nitration: conc. $\mathrm{HNO_3}$ + conc. $\mathrm{H_2SO_4}$ gives nitrobenzene
• Sulphonation: fuming $\mathrm{H_2SO_4}$ gives benzenesulphonic acid
• Friedel–Crafts alkylation and acylation with $\mathrm{AlCl_3}$

Addition (only under drastic conditions)

• Hydrogenation with Ni at high temperature and pressure gives cyclohexane (3 $\mathrm{H_2}$ added).
• With chlorine in bright sunlight, three molecules of $\mathrm{Cl_2}$ add to give benzene hexachloride, $\mathrm{C_6H_6Cl_6}$. Taking up three molecules is evidence that benzene contains the equivalent of three double bonds.

Oxidation

The benzene ring resists common oxidizing agents such as $\mathrm{KMnO_4}$ and $\mathrm{K_2Cr_2O_7}$. Only under very drastic catalytic conditions ($\mathrm{V_2O_5}$, about 450$^\circ$C) is it broken to maleic anhydride. In alkylbenzenes the side chain is oxidized instead: toluene and ethylbenzene give benzoic acid, and xylenes give phthalic acids. So among benzene, toluene, ethylbenzene and xylene, only benzene is not oxidized by ordinary oxidizing agents.

Common MDCAT traps

• "High reactivity" is the odd one out; aromaticity, stability and obeying Hückel's rule all describe benzene.
• Low reactivity is due to delocalization of $\pi$ electrons, not to saturation or a small electronegativity difference.
• Addition of 3 $\mathrm{Cl_2}$ points to three double bonds, not to planarity or non-polarity.
• Phenol, naphthalene and anthracene are all aromatic; "none of these" is the answer when asked which is not.
• Toluene is oxidized (at its $-\mathrm{CH_3}$), benzene is not.

Quick revision

• Benzene: planar, sp$^2$, 120$^\circ$, six equal C–C bonds.
• Hückel rule: $(4n+2)$ $\pi$ electrons.
• Typical reaction: electrophilic substitution.
• Benzene + 3$\mathrm{Cl_2}$ (sunlight) → $\mathrm{C_6H_6Cl_6}$.
• Side chains of alkylbenzenes oxidize to $-\mathrm{COOH}$.