PractifyUs

NEET Chemistry · Hydrocarbons

Aromatic hydrocarbons: aromaticity and electrophilic substitution

Taught in NCERT Class 11 Chemistry, Chapter 9: Hydrocarbons.

2 verified questions: Easy 1 · Medium 1 · NEET level 0

Sample questions

Sample 1Easy

Despite its unsaturation, benzene mainly undergoes

  1. A

    nucleophilic substitution reactions

  2. B

    electrophilic substitution reactions

  3. C

    free radical addition reactions

  4. D

    nucleophilic addition reactions

Show the answer
The answer is B.

The delocalised π electrons make benzene unusually stable and a source of electrons. It therefore mainly undergoes electrophilic substitution, as in nitration, halogenation and Friedel-Crafts reactions.

  • A.

    The electron-rich ring repels nucleophiles.

  • C.

    Addition, possible only under special conditions, would destroy the stable delocalised system.

  • D.

    Benzene has no polar multiple bond for a nucleophile to attack.

NCERT: Class 11 Chemistry, Unit 9 (Hydrocarbons), Aromatic Hydrocarbons: Chemical Properties

Sample 2Medium

Which of the following species is aromatic?

  1. A

    Cyclopentadienyl cation

  2. B

    Cyclopentadienyl anion

  3. C

    Cyclooctatetraene (non-planar tub form)

  4. D

    Cyclopenta-1,3-diene

Show the answer
The answer is B.

An aromatic species must be cyclic, planar, fully conjugated and contain (4n + 2) π electrons. The cyclopentadienyl anion is planar with all five carbons sp2 and has 6 π electrons (n = 1).

  • A.

    The cation has only 4 π electrons, which does not satisfy the (4n + 2) rule.

  • C.

    The tub-shaped molecule is non-planar and has 8 π electrons, so it is not aromatic.

  • D.

    Cyclopentadiene has an sp3 CH2 carbon, so its π system is not completely delocalised around the ring.

NCERT: Class 11 Chemistry, Chapter 9, 9.5

Practise Aromatic hydrocarbons: aromaticity and electrophilic substitution

Practice at the level you choose, with the answer after every question; timed tests with the answers when you submit. Every question is verified against NCERT first.

Sign in to practise

More in Hydrocarbons