Halogenation of Alkanes
Bromination and Fluorination of Alkanes
In the radical chlorination of 2-methylbutane the reactivity ratio tertiary : secondary : primary C-H bonds has been found to be 5 : 4 : 1. In bromination, the selectivity is much higher. Since bromine has a lower reactivity, bromination requires a higher reaction temperature in order to run as fast as chlorination. In bromination at 98 °C, secondary C-H bonds react 250 times faster, while tertiary C-H bonds are attacked even 6300 times faster than primary C-H bonds. Even at 25 °C, secondary C-H bonds still react 82 times faster, while tertiary C-H bonds react 1600 times faster than primary C-H bonds.
|Halogen||Primary C-H||Secondary C-H||Tertiary C-H|
|F (25 °C)||1||1.2||1.4|
|Cl (25 °C)||1||4||5|
|Br (98 °C)||1||250||6300|
Fluorine and iodine are not as effective in the halogenation of alkanes. Fluorination is extremely exothermic and can, therefore, hardly be controlled. In addition, the selectivity regarding various types of C-H bonds is considerably restricted (see table 1). In contrast, iodination is very endothermic. Therefore, it virtually never occurs.
The Hammond postulate (see also Early and late transition states) explains the high selectivity of bromination. According to this postulate, the structure of the transition state of endothermic bromination resembles the product. That is, the intermediately formed alkyl radical. In contrast, in exothermic chlorination the transition state's structure resembles that of the starting products. The transition states of hydrogen abstraction from an alkane by either bromine or chlorine therefore differ considerably from each other. Why is it that structural differences of transition states control the selectivity regarding various types of C-H bonds in the radical halogenation of alkanes?
In the chlorination of 2-methylpropane, for example, in the two possible transition states, the C-H and H-Cl bonds are approximately equally strong. The radical character is more or less evenly distributed among the carbon and chlorine atom. However, in the transition states of the bromination of 2-methylpropane, the C-H bond is largely cleaved, while the H-Br bond has now been largely formed. As a result, the radical character is mainly located at the carbon atom. Therefore, alkyl radical stability plays a larger role in determining the selectivity regarding various types of C-H bonds in bromination than it does in chlorination.
Thus, in the radical bromination of 2-methylpropane, the tertiary alkyl radical is much more effectively generated by the bromine radical's attack than the primary alykl radical.
- These observations, regarding the chlorination and bromination of 2-methylpropane, reflect the reactivity-selectivity-principle. According to this, the lower the selectivity of a reagent is, the more reactive it is as well.
However, chlorination may be a relevant reaction, when the alkane contains only one type of C-H bond, such as in cyclohexane (only secondary). In such cases, the selectivity is irrelevant.