Why is chlorination less selective than bromination?
1 Answer
Chlorination is less selective than bromination because chlorination has smaller differences in activation energy for attack at 1°, 2°, and 3° positions.
Consider the halogenation of propane at the 1° and 2° positions.
CH₃CH₂CH₃ + X₂ → CH₃CH₂CH₂X and (CH₃)₂CHX
Formation of the different halopropanes occurs during the chain propagation steps.
CH₃CH₂CH₂-H + ·X → CH₃CH₂CH₂· + H-X
(CH₃)₂CH-H + ·X → (CH₃)₂CH· + H-X
The rates depend on the activation energies. The approximate values of
- CH₃CH₂CH₂-H + ·Cl → CH₃CH₂CH₂· + H-Cl;
#E_"a"# = 17 kJ/mol - (CH₃)₂CH-H + ·Cl → (CH₃)₂CH· + H-Cl;
#E_"a"# = 13 kJ/mol - CH₃CH₂CH₂-H + ·Br → CH₃CH₂CH₂· + H-Br;
#E_"a"# = 67 kJ/mol - (CH₃)₂CH-H + ·Br → (CH₃)₂CH· + H-Br;
#E_"a"# = 54 kJ/mol
The Arrhenius equation predicts the rate constant for a reaction.
If the
At 300 K,
For the chlorination reaction,
So
This assumes that the
For the bromination reaction,
So
This is again a bit high, because the
But we see that as the difference in activation energies increases, the selectivity increases.