✔ 最佳答案
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.
參考: miraco, miraco, miraco, miraco
