Question:

The correct stability order of the following carbocations is : (I)H2C⨁-CH=CH-CH3, (II)⨁CH2-CH=CH-BMe2, (III)H2C⨁-CH=CH-NMe2, (IV)H2C⨁-CH=CH-OMe

Updated On: Apr 27, 2025
  • II>I>III>IV
  • III>I>II>IV
  • III>IV>I>II
  • IV>III>II>I
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The Correct Option is C

Solution and Explanation

Stability Order of the Given Carbocations:

The stability of a carbocation depends on: 

  • Resonance effects: Delocalization of the positive charge increases stability.
  • Inductive effects: Electron-donating groups stabilize the carbocation, while electron-withdrawing groups destabilize it.

Given species:

  1. H2C⁺–CH=CH–CH3: Allylic carbocation, resonance stabilized.
  2. ⁺CH2–CH=CH–BMe2: Allylic, but boron is an electron-withdrawing group → destabilizes.
  3. H2C⁺–CH=CH–NMe2: Allylic, NMe2 is an electron-donating group via resonance → highly stabilized.
  4. H2C⁺–CH=CH–OMe: Allylic, OMe is also electron-donating → stabilizes, but not as strongly as NMe2.

Stability order: III > IV > I > II

Correct option: (C)

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Concepts Used:

Organic Chemistry – Some Basic Principles and Techniques - Reaction Mechanism

SN1 Reaction Mechanism:

SN1 reaction mechanism takes place by following three steps –

  • Formation of carbocation 
  • Attack of nucleophile 
  • Deprotonation 

SN2 Reaction Mechanism:

The SN2 reaction mechanism involves the nucleophilic substitution reaction of the leaving group (which generally consists of halide groups or other electron-withdrawing groups) with a nucleophile in a given organic compound.

Electrophilic Aromatic Substitution Reaction Mechanism:

The mechanism of an electrophilic aromatic substitution reaction contains three main components which are:

  1. A new sigma bond from C=C is formed during the reaction in the arene nucleophile.
  2. Due to the breaking of the C-H sigma bond, a proton is removed.
  3. The C=C bond is reformed and it restores the aromaticity of the compound.

Electrophilic Substitution Reaction Mechanism:

The electrophilic substitution reaction mechanism is composed of three steps, which will be discussed more below.

  • Electrophile Generation
  • Carbocation Formation
  • Proton Removal