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Electrophilic Substitution in Benzene Derivatives
Concepts tested here
- Activating and deactivating groups
All Questions
2007 AIPMT-PRE 1 question
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The order of decreasing reactivity towards an electrophilic reagent, for the following would be
(i) benzene
(ii) toluene
(iii) chlorobenzene
(iv) phenolElectrophiles have a high affinity for electrons; they attack the site where the electron density is highest.
Electron releasing substituents increase the electron density of the benzene ring and so increase the reactivity, while electron withdrawing substituents decrease it.
$-OH$ is a strongly activating group ($+M$ effect of the lone pair on oxygen).
$-CH_3$ is a weakly activating group ($+I$ effect and hyperconjugation).
$-Cl$ is a deactivating group, because its $-I$ effect is stronger than its $+M$ effect.
The electron releasing tendency decreases in the order: $-OH \gt -CH_3 \gt -H \gt -Cl$
Therefore the order of reactivity towards an electrophile is: $C_6H_5OH \gt C_6H_5CH_3 \gt C_6H_6 \gt C_6H_5Cl$
That is, (iv) > (ii) > (i) > (iii).