Markownikoff’s Rule

“When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon

of the alkene bearing the larger number of hydrogens and smaller number of carbons"

Markownikoff’s Rule

Addition of HBr to propene CH3CH=CH2:

• H–Br can add in two different ways

• Two products are possible:

CH3CH2–CH2Br

H–Br Br–H

Contains more H atoms

CH3CH=CH2CH3CH=CH2

CH3CHBr–CH3

Minor product Major product

Addition reactions of unsymmetrical alkenes

Primary carbocationOne C joined to C+

Secondary carbocationTwo C joined to C+

Minor product Major product

It’s all controlled by carbocations

Primary carbocation

• One carbon chains attached to C+

Secondary carbocation:

• Two carbon chains attached to C+

Tertiary carbocation:

• Three carbon chains attached to C+

Most stable

Leaststable

Types of carbocation

Electron-donating ability of alkyl groups.

• Each alkyl group pushes electrons towards C+ charge of carbocation.

• Positive charge is spread over alkyl groups

The more alkyl groups attached to the C+

• The more the positive charge is spread out

• The greater the stability of the carbocation

Primary

Secondary

Tertiary

Reasons for carbocation stability

Most stable

Leaststable