Bromination of alkanes

This difference in selectivity between chlorination and bromination of alkanes needs to be kept in mind when one wishes to prepare an alkyl halide from an alkane... 

Of the reactions that involve carbon radicals, the most familiar- are the chlorination and bromination of alkanes (Sections 4.14 through 4.18) ... 

Bromine will also halogenate alkanes, but in this case we find that bromine is considerably less reactive than chlorine. As a result, the reaction becomes much more selective, and the product ratios are more distinctive. In fact, bromination of alkanes is so selective that it is a feasible laboratory process to make alkyl bromides from alkanes. 

Chlorination (and bromination) of alkanes such as methane, CH., has a radical-chain mechanism, as follows INITIATION STEP... 

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

Subsequently, selective ionic chlorination of methane to methyl chloride was achieved in the gas phase over solid superacid catalysts.526 For example, chlorination of methane in excess chlorine over Nafion-H and SbF5-graphite gave methyl chloride with 88% and 98% selectivity, respectively (185°C and 180°C, 18% and 7% conversion).527 Similarly, electrophilic bromination of alkanes has also been carried out528 (Scheme 5.53). 

Bromnation of alkanes. The combination of bromine and mercuric oxide is effective for free-radical bromination of alkanes ... 

Electrophilic bromination of alkanes in solution was also carried out with Br2 in the presence of silver hexafluoroantimonate. Scheme 6.23, again. 

Chlorination or bromination of alkanes is an example of a substitution reaction. A hydrogen on an alkane is replaced by a halogen hydrogen halide is the by-product. The reaction is initiated by light or heat. 

Bromination of alkanes is less interesting both preparatively and technically. Hydrogen is most difficult to replace by Br from methyl groups, and easiest from methine groups, so that, for instance, when 2,3-dimethylbutane and bromine are irradiated at room temperature 2,3-dibromo-2,3-dimethylbutane is easily obtained.335 A paper by Kharasch et al,336 should be consulted for the chain mechanism of thermal or photochemically initiated bromination. 

Bromination of alkanes follows the same mechanism as chlorination, mechanism for the monobromination of ethane... 

Bromination of alkanes is mainly used to prepare tertiary alkyl bromides. 

Ag SbF catalyzed electrophilic bromination of alkanes has also been carried out as shown in Scheme 16. 

The N-haloimide halogenations are also controlled partially by the fact that Cl or the iV-succinimidyl radical are much more reactive than Br in hydrogen abstracting reactions and, towards a hydrocarbon of low reactivity such as neopentane, a bromine atom chain would be quite ineffective. With mixtures of NBS and CI2, halogenation occurs to form the alkyl bromides but with the selectivity expected for chlorine atom attack Apparently ClBr is formed and reacts with the alkyl radical to form RBr and a chlorine atom. A similar situation exists for the bromination of alkanes using a mixture... 

Chlorination and Bromination of Alkanes (Section 8.4) Chlorination and bromination of alkanes are regioselective in the order 3° H > 2° H > 1° H. Bromination has a higher regioselectivity than chlorination. The mechanism involves a radical chain process. 

---