Addition of HX across a
double bond
And Carbocation rearrangement
Addition of HX across a double
bond

An acid, HX, such as HCl, HBr and HI add
across a double in this fashion.
Cl
H3C
H3C
Or
CH3
+ HCl
Cl
H3C

CH3
CH3
This is called Markovnikov addition of HX
across a double bond.
Markovnikov Addition


Markovnikov’s rule states that when adding
something across a double bond, the anion
(negative ion) will bond with the more substituted
carbon.
Example:
CH3
H3C

+
CH3
Cl
HCl
H3C
CH3
CH3
Notice how there is no distinct stereochemistry.
This is because the ion can attack either side of
the carbon.
Quick Review:

Draw the products of the following two reactions:
CH3
CH3 H3C
+
H3C
HBr
CH3
H3C
Br
CH3
CH3
H3C
H3C
H3C
H3C
CH3
CH3
+
HCl
Cl
CH3
CH3
Rearrangement


The anion, in normal circumstances, will always
attack the most substituted carbon in the
molecule, not just the most substituted carbon in
the double bond.
Example:
H3C
CH3
CH3
+
HCl
H3C
CH3
Cl
CH3
Why does rearrangement occur:

An intermediate is formed in the reaction between
HCl and an Alkene. It follows by this mechanism:
CH3
H3C
H3C

+
+
H Cl
-
+
CH
H3C
H
H3C
CH3
H3C
+
C
H
In this intermediate, a carbocation is formed.
Carbocations can easily rearrange to a more
stable carbon.
H3C
CH3
Carbocation stability
A primary carbocation is a carbocation
bonded to one other carbon.
 A secondary carbocation is a carbocation
bonded to two other carbons. It is more
stable than a primary carbocation.
 A tertiary carbocation is a carbocation
bonded to three other carbons and is more
stable than a secondary carbocation.
 The general order is 1º< 2º< 3º

What type of carbocations are
these? Which are the most stable?
CH3
+
H3C
C

CH3
CH3
+
H2C
CH3
CH3
H3C
+
CH

CH3
INCORRECT
CORRECT