halogenation

In addition reaction of halogens to alkene C2H4, which is faster? Why ?
a) C2H4 and F2
b) C2H4 and Cl2
c) C2H4 and Br2
d) C2H4 and I2


The answer is : a) C2H4 and F2

CH2=CH2­ + X-X → X-CH2-CH2-X

Firstly, amount the four C-X bonds, C-F bond is the strongest.
Bond enthalpy (kJ/mol): C-F (467) > C-Cl (346) > C-Br (290) > C-I (228)

Secondly, F-F bond is unexpected weak because of the great repulsion between lone pair electrons in the two small F atoms. Hence, F-F can be broken more readily than expected, and thus the activation energy is not so high.
Bond enthalpy (kJ/mol): Cl-Cl (243.4) > Br-Br (192.9) > F-F (158) > I-I (151.2)

2011-04-19 15:17:26 補充：
Reference of data: http://www.webchem.net/data/BondEnthalpies.htm

2011-04-19 21:40:27 補充：
Rate of reaction: a > b > c > d

2011-04-28 21:01:42 補充：
The transition state is [X....CH2-CH2...X] (there is also dotted line between X and X). If C-X bond is strong, the transition state converts to the product more readily, and the product converts back to the transition state less readily.

since this is an electrophilic reaction, shouldn't electrophilicity of the halogen molecules also be mentioned?