MU - Chemistry 210
Hydrogen Abstraction by Halogen Radicals
An Illustration of the Hammond Postulate



Fluorination, Chlorination and Bromination of Methane.
Transition State Structures for H-Abstraction.

The transitions state structures for the H-abstraction from methane by fluorine, chlorine and bromine have been determined with quantum-mechanical methods and the structures can be viewed with CHIME.

Transition States For Hydrogen Abstraction
fluorine radical abstracts from methane
chlorine radical abstracts from methane
bromine radical abstracts from methane


 

F

Cl

Br

C-H in TS

1.153

1.438

1.600

C-H in CH4

1.085

1.085

1.085

delta(C-H)

0.068

0.353

0.515

H-X in TS

1.350

1.431

1.527

H-X in HX

0.934

1.280

1.436

delta(H-X)

0.416

0.151

0.091

<(H'-C-H') in TS

112.1

116.4

117.6

delta(<(H'-C-H'))

2.6

6.9

8.1

<(H-C-H') in TS

106.7

101.2

99.2

delta(<(H-C-H'))

-2.8

-8.3

-10.3


The later the transition state, the more radical character there is in the hydrocarbon fragment. The more radical character, the more differentiation is possible between primary, secondary and tertiary carbons.

Enjoy!

Worksheet