The Chlorination and Bromination of Alkanes

Alkanes react with chlorine (CI2) or bromine (Br2 to form alkyl chlorides or alkyl bromides. These halogenation reactions take place only at high temperatures or in the presence of light. (Irradiation with light is symbolized by hv.) 

Halogenation and combustion (burning) are the oidy reactions that alkanes undergo (without the assistance of a metal catalyst). In a combustion reaction, alkanes react with oxygen at high temperatures to form carbon dioxide and water. 

When a bond breaks so that both of its electrons stay with one of the atoms, the process is called heterolytic bond cleavage or heterolysis. 

An arrowhead with two barbs signifies the movement of two electrons. 

An arrowhead with one barb— sometimes called a fishhook— signifies the movement of one electron. 

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

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. 

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. 

As in the radical halogenation of alkanes (Section 3-8), the exothermic nature of aromatic halogenation decreases down the periodic table. Huorination is so exothermic that direct reaction of fluorine with benzene is explosive. Chlorination, on the other hand, is controllable but requires the presence of an activating catalyst, such as aluminum chloride or ferric chloride. The mechanism of this reaction is identical with that of bromination. Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. Much like the radical halogenation of alkanes, electrophilic chlorination and bromination of benzene (and substituted benzenes. Chapter 16) introduces functionality that can be utilized in further reactions, in particular C-C bond formations through organometallic reagents (see Problem 54, Section 13-9, and Real Life 13-1). 

Conversion of alkanes means substitution of a hydrogen atom for a heteroatom. This is achieved at the anode by transfer of two electrons from a C—H bond to the electrode and reaction of the intermediate car-bocation with a nucleophile. Chemically in most cases either a chlorine or bromine... 

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. 

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. 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

Bromine reacts with alkanes by a free-radical chain mechanism analogous to that of chlorine. There is an important difference between chlorination and bromination, however. Bromination is highly selective for substitution of tertiary hydrogens. The spread in reactivity among primary, secondary, and tertiary hydrogens is greater than 103. 

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). 

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. 

This same order of reactivity holds for the reaction of the halogens with other alkanes and, indeed, with most other organic compounds. The spread of reactivities is so great that only chlorination and bromination proceed at such rates as to be generally useful. 

It will be worthwhile to examine the mechanism of chlorination of methane in some detail. The same mechanism holds for bromination as well as chlorination, and for other alkanes as well as methane it even holds for many compounds which, while not alkanes, contain alkane-like portions in their molecules. Closely... 

Under the influence of ultraviolet light, or at 250-400°, chlorine or bromine converts alkanes into chloroalkanes (alkyl chlorides) or bromoalkanes (alkyl bromides) an equivalent amount of hydrogen chloride or hydrogen bromide is formed at the same time. When diluted with an inert gas, and in an apparatus designed to carry away the heat produced, fluorine has recently been found to give analogous results. As with methane, iodination does not take place at all. 

By comparing the H° values for the sum of the two propagating steps for the monohalogenation of methane, we can understand why alkanes undergo chlorination and bromination but not iodination and why fluorination is too violent a reaction to be useful. 

Radical-catalyzed chlorination and bromination occur readily on alkanes and substituted alkanes, both in the gas phase and in solution. Both thermal and photochemical generation of the halogen atoms are employed for chlorinations, sulfuryl chloride in the presence of an initiator such as dibenzoyl peroxide is also used (Scheme 4.17). 

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... 

The term paraffin, however, was probably not an appropriate one. We all know that alkanes react vigorously with oxygen when an appropriate mixture is ignited. This combustion occurs, for example, in the cylinders of automobiles, in furnaces, and, more gently, with paraffin candles. When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine. We shall study these reactions in Chapter 10. 

Bromine is generally less reactive toward alkanes than chlorine, and bromine is more selective in the site of attack when it does react. 

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