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NEET CHEMISTRYMedium

The order of decreasing reactivity towards an electrophilic reagent for the following compounds is: (i) Benzene (ii) Toluene (iii) Chlorobenzene (iv) Phenol

A

(i) > (ii) > (iii) > (iv)

B

(ii) > (iv) > (i) > (iii)

C

(iv) > (iii) > (ii) > (i)

D

(iv) > (ii) > (i) > (iii)

Step-by-Step Solution

  1. Reactivity Principle: Reactivity towards electrophilic substitution depends on the electron density in the benzene ring. Electron-donating groups (activating groups) increase electron density and reactivity, while electron-withdrawing groups (deactivating groups) decrease it.
  2. Analyze Substituents:
  • Phenol (iv): The -OH group has lone pairs that participate in resonance (+R effect), strongly increasing electron density at ortho and para positions. It is a strong activating group [NCERT 11th, Ch 13, Sec 13.5.6; Source 30, 97, 152].
  • Toluene (ii): The methyl group (-CH3) increases electron density via hyperconjugation and inductive effect (+I). It is an activating group, but weaker than -OH [NCERT 11th, Ch 13, Sec 13.5.6; Source 88, 142].
  • Benzene (i): The standard reference with no substituent.
  • Chlorobenzene (iii): The chlorine atom exhibits a strong inductive effect (-I) which withdraws electrons, and a weaker resonance effect (+R). The net effect is electron withdrawal (-I > +R), making it a deactivating group [NCERT 12th, Ch 10, Sec 10.7; Source 32, 153].
  1. Order: Activators > Benzene > Deactivators.
  • Strong Activator (Phenol) > Weak Activator (Toluene) > Benzene > Deactivator (Chlorobenzene).
  • Order: (iv) > (ii) > (i) > (iii).
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