Question

Place these molecules in order of ease of nitration, with the most reactive first:

$C_{6} H_{6} C_{6} H_{5} {CH}_{3} C_{6} H_{5} COOH C_{6} H_{5} OH$ .

Accepted Answer

The reactivity of the benzene ring towards nitration is dependent on how electron rich it is.

Electron with-drawing groups like $- COOH$ deactivates the ring there by decreases the electron density of the benzene ring. Hence, benzoic acid, $C_{6} H_{5} COOH$ is the least reactive towards nitration among the given set of compounds.

Toluene, $C_{6} H_{5} {CH}_{3}$ and Phenol, $C_{6} H_{5} OH$ are more reactive than benzene, $C_{6} H_{6}$ since their functional groups add electron density to the ring. Toluene's methyl group encourages accumulation of electron density through the inductive effect, and the hydroxyl group in phenol stabilises the ring by resonance effect, there by increase in the electron density much in benzene ring.

These different patterns of donation increases the electron density of the ring which makes them more reactive towards electrophilic substitution reactions like nitration, more than benzene. Therefore, the order of the given compounds towards nitration is,

$C_{6} H_{5} OH > C_{6} H_{5} {CH}_{3} > C_{6} H_{6} > C_{6} H_{5} COOH$

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