IB Chemistry Topic R3.4.12 the Relative Stability of Carbocations Revision Notes (HL)

IB Chemistry Topic R3.4.12 the Relative Stability of Carbocations Revision Notes (HL) | RevisionDojo

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Carbocations: Why Stability is Key

In electrophilic addition reactions, the first step often involves the formation of a carbocation intermediate.

Definition

Carbocation

A carbocation is a carbon atom that carries a positive charge, formed when a bond is broken heterolytically, leaving the carbon electron-deficient.

But not all carbocations are equally stable: this stability directly influences the reaction pathway and product distribution.

Types of Carbocations and Their Stability

The stability of a carbocation depends on the number of alkyl groups attached to the positively charged carbon.
Alkyl groups donate electron density through the positive inductive effect, reducing the electron deficiency and stabilizing the carbocation.
1. Tertiary Carbocation (Most Stable): Three alkyl groups attached to the positively charged carbon.
2. Secondary Carbocation: Two alkyl groups attached to the positively charged carbon.
3. Primary Carbocation (Least Stable): One alkyl group attached to the positively charged carbon.

The stability trend can be summarized as: Tertiary > Secondary > Primary.

Example

Let’s compare the stability of different carbocations:

$CH₃⁺$ (Methyl carbocation): No alkyl groups, highly unstable.
$CH₃CH₂⁺$ (Primary carbocation): One alkyl group, slightly more stable.
$(CH₃)₂CH⁺$ (Secondary carbocation): Two alkyl groups, moderately stable.
$(CH₃)₃C⁺$ (Tertiary carbocation): Three alkyl groups, most stable due to maximum electron donation.

Tip

To determine carbocation stability, count the alkyl groups directly bonded to the positively charged carbon.
More alkyl groups mean greater stability.

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IB Chemistry R3.4.12 the Relative Stability of Carbocations Notes

Carbocations: Why Stability is Key

Types of Carbocations and Their Stability

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Carbocations: Why Stability is Key

Types of Carbocations and Their Stability

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.4.12 the Relative Stability of Carbocations Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Carbocations: Why Stability is Key

Types of Carbocations and Their Stability

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Carbocations: Why Stability is Key

Types of Carbocations and Their Stability

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