Markownikoff’s Rule
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"
Addition reactions of unsymmetrical alkenes
Addition of HBr to propene CH3CH=CH2:
•
H–Br can add in two different ways
•
Two products are possible:
CH3CH=CH2
H–Br
Contains more
H atoms
CH3CH=CH2
Br–H
CH3CH2–CH2Br
CH3CHBr–CH3
Minor product
Major product
It’s all controlled by carbocations
H
Primary carbocation
One C joined to C+
C
H
H
H
C
C
H
CH3
C
H d+
Br d-
CH3
Secondary carbocation
Two C joined to C+
H
H
H
C
H
C
H
H
Br
H
CH3
Br
H
H
H
C
C
C
Br
H
H
Minor product
H
H
H
H
H
C
C
C
H
Br
H
Major product
H
Types of carbocation
Primary carbocation
•
Least
stable
One carbon chains attached to C+
Secondary carbocation:
•
Two carbon chains attached to C+
Tertiary carbocation:
•
Three carbon chains attached to C+
Most
stable
Reasons for carbocation stability
Least
stable
Electron-donating ability of alkyl groups.
•
•
Each alkyl group pushes electrons towards
C+ charge of carbocation.
Primary
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
Secondary
Most
stable
Tertiary