Major Products from Addition to Unsymmetrical Alkenes HL Only
Quick Notes:
- Unsymmetrical alkenes react with hydrogen halides (e.g. HBr) or water (in acid) to give one major product.
- This is explained by the relative stability of carbocation intermediates.
- More substituted carbocations are more stable:
- Tertiary > Secondary > Primary
- The major product forms via the most stable carbocation.
- This follows Markovnikov’s Rule: The H adds to the carbon with more H atoms already, while the halide or OH adds to the carbon with fewer H atoms.
Full Notes:
This page builds on R3.4.11 make sure you understand that page fully before looking at this one!
Major and Minor Products in Unsymmetrical Alkenes
When adding to an unsymmetrical alkene, two possible products can form.
The two possible products won’t be formed in equal amounts. The product formed most is called the major product and the one formed the least is the minor product.
We can predict the major product based on the carbocation intermediate formed in the reaction.
The major product forms from the most stable carbocation.
Alkyl (carbon) groups bonded to the positively charged carbon in the intermediate stabilise the positive charge by giving electron density to the positively charged carbon. This is called the positive inductive effect.
The more alkyl groups there are bonded to the positively charged carbon, the more stable the carbocation is and the more likely it is to form.
Example: Propene + HBr
Major Product: 2-Bromopropane (Secondary Carbocation)
Minor Product: 1-Bromopropane (Primary Carbocation)
Summary
- Unsymmetrical alkenes can form major and minor products during electrophilic addition.
- The major product forms via the more stable carbocation intermediate.
- Carbocation stability increases in the order: tertiary > secondary > primary.
- Markovnikov’s Rule: Hydrogen attaches to the carbon with more hydrogens already bonded.