Carbocation Stability and Addition

Overview

Topic: Stability of carbocations and how it affects products in electrophilic addition to alkenes.
Goal: Identify carbocations as primary, secondary, or tertiary, define carbocations, and explain relative stability and product major/minor outcomes.

Carbocation: A carbon atom with three bonds and a positive charge; an unstable reaction intermediate.
R Group: A chain or group of carbon atoms (e.g., methyl, ethyl); acts as an electron-donating (electron-pushing) group.
Primary Carbocation: Carbocation with one R group and two hydrogens attached.
Secondary Carbocation: Carbocation with two R groups and one hydrogen attached.
Tertiary Carbocation: Carbocation with three R groups attached.
Symmetrical Alkene: An alkene where substituents on both double-bond carbons are identical; gives one product in electrophilic addition.
Asymmetrical Alkene: An alkene with different substituents on the double-bond carbons; gives two possible products.

Tertiary > Secondary > Primary
Reason: R groups donate electron density (electron-pushing) and stabilize the positive charge.
Stable carbocation formation leads to a major product; less stable leads to a minor product.

Step 1: Pi bond attacks electrophile (H+), forming a carbocation intermediate.
Step 2: Two possible carbocations may form depending on which carbon gets H.
Step 3: Nucleophile (Cl-) donates a lone pair to the carbocation, forming the final product.
If alkene symmetrical: both carbocation pathways are identical → one product.
If alkene asymmetrical: two different carbocations form → major and minor products based on stability.

Two possible products: 1-chlorobutane and 2-chlorobutane.
Carbocation possibilities:
- Carbocation on C-2: secondary (two R groups) → more stable.
- Carbocation on C-1: primary (one R group) → less stable.
Major product: formed via secondary carbocation (2-chlorobutane).
Minor product: formed via primary carbocation (1-chlorobutane).

R groups stabilize carbocations by electron donation; more R groups increase stability.
Predict major product by locating the more substituted (more R groups) carbocation intermediate.
Understanding carbocation stability explains product distributions in electrophilic additions.

Concept	Characteristic
Carbocation definition	Carbon with three bonds and positive charge; unstable intermediate
R Group role	Electron-pushing (stabilizes positive charge)
Primary carbocation	One R group, less stable
Secondary carbocation	Two R groups, intermediate stability
Tertiary carbocation	Three R groups, most stable
Symmetrical alkene outcome	One product (identical carbocations)
Asymmetrical alkene outcome	Two possible products; major from more stable carbocation

Review electrophilic addition mechanisms in detail (next videos cover full mechanisms).
Practice identifying substitution of carbocation intermediates in example alkenes.
Predict major/minor products by comparing carbocation stability for given reactions.