Bromine reaction with alkanes

Chlorination of alkanes is less exothermic than fluonnation and bromination less exothermic than chlorination Iodine is unique among the halogens m that its reaction with alkanes is endothermic and alkyl iodides are never prepared by lodmation of alkanes... 

Chlorine or bromine reacts with alkanes in the presence of light (hv) or high temperatures to give alkyl halides. Usually, this method gives mixtures of halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to convert inert alkanes to reactive alkyl halides. The simplest example is the reaction of methane with CI2 to yield a mixture of chlorinated methane derivatives. 

Bromine is an electrophile which will undergo an addition reaction with alkenes in the dark. The lack of a reaction in the dark with Br2 indicates that the molecule is not an alkene. In the presence of light, bromine will undergo a substitution reaction with alkanes. Therefore, the molecule is most likely an alkane and the only alkane with the molecular formula C3H6 is cyclopropane. 

In ultraviolet light, chlorine or bromine react with alkanes to form chloroalkanes or bromoalkanes (the general term is halogenoalkanes). The reaction with halogens is called halt enation and, since it takes place in the presence of light, it is termed a photochemical reaction. 

Bromine reacts direcdy with alkanes but this reaction has Httle value because mixtures are obtained. However, photochemical bromination of alkyl bromides can be quite selective (23). 

In contrast with alkane chlorination, alkane bromination is usually much more selective. In its reaction with 2-methylpropane, for example, bromine abstracts the tertiary hydrogen with greater than 99% selectivity, as opposed to the 35 65 mixture observed in the corresponding chlorination. 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Reactions with halogens give boron halides. While reaction with chlorine can be explosive with diborane, it is slow with bromine. Diborane reacts with alkanes forming alkylboranes. Reactions with aromatics give arylboranes. 

The dihydrido complex [RhH2(ri5-C5Me5)(PMe3)] forms C—H insertion products when irradiated in the presence of alkanes (ethane, propane).227,228 Reaction with CHBr3 leads to bromoalkylrhodium complexes, which on treatment with bromine give ethyl bromide or 1-bromopropane in 70-85% yield. The less stable iridium complex formed with neopentane could not be converted directly to neopentyl bromide.229 It gave, however, a mercury derivative that yielded the bromide after treatment with bromine. 

The alkenes are distinguished from the alkanes by their solubility in concentrated sulphuric acid and their characteristic reactions with dilute potassium permanganate solution and with bromine. Characterisation may be based upon the determination of their physical and/or spectral properties. Characterisation by way of solid adducts with nitrosyl chloride has been quite widely used in the terpene field the preparation of adducts with 2,4-dinitrobenzenesulphenyl chloride is described below (see also Section 8.1.1, p. 1128). 

The bromine solution is red the product that has the bromine atoms attached to carbon is colorless. Thus a reaction has taken place when there is a loss of color from the bromine solution and a colorless solution remains. Since alkanes have only single C—C bonds present, no reaction with bromine is observed the red color of the reagent would persist when added. Aromatic compounds resist addition reactions because of their aromaticity the possession of a closed loop (sextet) of electrons. These compounds react with bromine in the presence of a catalyst such as iron filings or aluminum chloride. 

Heat or light is usually needed to initiate this halogenation. Reactions of alkanes with chlorine and bromine proceed at moderate rates and are easily controlled. Reactions with fluorine are often too fast to control, however. Iodine reacts very slowly or not at all. We will discuss the halogenation of alkanes in Chapter 4. 

---