The Reactivity-Selectivity Principle in Halogenation of Alkanes

1. The halogenation of alkanes with three and more carbon atoms may give two products, of unequal stability. For example,
        CH3CH2CH3 + Br2 → 3% CH3CH2CH2Br + 97% CH3CH(Br)CH3

2. The factors that determining the products in halogenation are:
    (a) Reactivity of R-H system

• The differences in reactivity of alkanes are attributed to the differences in C-H bond dissociation energies (energy required to break a bond homolytically). 
• The bond dissociation energies for

           Methyl CH3CH2-H: 435 kJ mol^-1
              Ethyl CH3CH2-H (1^o carbon): 410 kJ mol^-1
           Isopropyl (CH3)2CH-H (2^o carbon): 397 kJ mol^-1
           Tert-butyl (CH3)3C-H (3^o carbon): 380 kJ mol^-1

• The weaker bonds more easier to be broken than stronger bonds. Thus, the relative order of reactivity for alkanes: methyl < primary < secondary < tertiary
  The more stable the reactant, the less reactive it will be. In terms of rate, this means that the more stable the reactant, the slower it will react.     
• The differences in bond dissociation energies can be attributed to the differences in the stability of alkyl radicals. This leads to the relative stabilities of alkyl radicals: 

    

• Alkyl radicals have different stabilities. The more stable the radical, the easier it will form because the stability of the radical is reflected in the stability of the transition state leading to its formation. Consequently, it is easier to remove a hydrogen atom from a secondary carbon to form a secondary radical than it is to remove a hydrogen atom from a primary carbon to form a primary radical.

(b) Reactivity of halogen radical

• The order of reactivity of halogens: F2 > Cl2 > Br2 > I2

   

• The fluorine radical is the most reactive of the halogen radicals, and it reacts violently with alkanes. In contrast, the iodine radical is the least reactive of the halogen radicals. in fact, it is so unreactive that it is unable to abstract a hydrogen atom from an alkane.
• The bromination is a much a slower reaction than chlorination. It has been found experimentally that the activation energy for abstraction of a hydrogen atom by a bromine radical to be about 4.5 times greater than that for abstraction of a hydrogen atom by a chlorine radical.
• Bromine radical is less reactive and more selective. In contrast, chlorine radical is more reactive and less selective in its reaction.
• The relative rates of radical formation when a bromine radical abstracts a hydrogen atom are different from the relative rates of radical formation when a chlorine radical abstracts a hydrogen atom. 

   

• The relative rates of radical formation by a bromine radical shows that a bromine radical is 1600 times faster to abstract a hydrogen atom from a tertiary carbon than abstract a hydrogen atom from a primary carbon. Consequently, a bromine radical is 82 times faster to abstract a hydrogen atom from a secondary carbon than abstract a hydrogen atom from a primary carbon. For example, the bromination of butane gives 98% of 2-bromobutane, compared with the 71% of 2-chlorobutane for chlorination of butane. Thus, bromination is more highly regioselective than chlorination.