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NEET CHEMISTRYHydrocarbonsMedium

Question

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).

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Hydrocarbons. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYHydrocarbonsdecreasingreactivitytowardselectrophilicreagent

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