Transcript Slide 1

Mechanism of Electrophilic Addition of bromine
to an alkene
Addition of hydrogen halides
Mechanism….
HBr is the electrophile (H d+)
The facts…
•
•
•
•
HF slowest (bond energy 568 KJ/mol)
HCl (432)
HBr (366)
HI fastest (298)
Remember: weaker bond energy – faster
reaction and more reactive.
But with unsymmetrical alkenes, the hydrogen adds to the carbon atom
with more hydrogen atoms (Markovnikoff’s rule)
2-bromopropane
Why is this so?
Lets look at the stability of the carbocation intermediate
• Primary
• Secondary
• Tertiary
Primary < Secondary < Tertiary
CH3 CH2- etc. adds electron density to a
carbocation (by electron pushing effects),
therefore helping to stabilize it by reducing
its positive charge. The more CHx around
it, the more stable it is (remember carbon
is slightly more electronegative than
hydrogen)
CH3__C+
Electron pushing effect
Hydration of alkenes
In industry ethene reacts with steam at high temperature
and pressure in the presence of phosphoric (V) acid:
CH2=CH2(g) + H2O(g)
CH3CH2OH(g)
Mechanism of hydration
O
HO
O
P
OH
OH
HO
P
O
OH
H2PO4-
+
H+
H
H2 C CH2
HC CH2+
H
H+
H H
..
O
H
HC C H
H O+
H
CH3CH2OH + H+