Free Radical Chlorination of Alkanes

Spin density surface for chlorine atom+methane transition state shows location of unpaired electron. 

Chlorine gas reacts directly and highly exothermically with alkanes, giving rise to alkyl chlorides and hydrogen chloride, e.g., for addition to methane. 

The process is believed to initiate with formation of chlorine atom (either thermally or photochemically), which then abstracts a hydrogen from methane. 

The resulting methyl radical abstracts a chlorine atom from CI2, leading to product and generation of another chlorine atom. 

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

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As a result, free-radical chlorination of alkanes is a nonselective process. Except when only one type of replaceable hydrogen is present (methane, ethane, neopentane, unsubstituted cycloalkanes), all possible monochlorinated isomers are usually formed. Although alkyl chlorides are somewhat less reactive than alkanes, di- and polychlorinations always occur. The presence of a chlorine atom on a carbon atom tends to hinder further substitution at that carbon. The one exception is ethane that yields more 1,1-dichloroethane than 1,2-dichloroethane. The reason for this is that chlorination of an alkyl chloride occurs extremely slowly on the carbon atom adjacent to the one bearing the chlorine atom (vicinal effect).115... 

Fletcher, B. Suleman, N. K. Tanko, J. M. Free Radical Chlorination of Alkanes in Supercritical Carbon Dioxide The Chlorine Atom Cage Effect as a Probe for Enhanced Cage Effects in Supercritical Fluid Solvents. J. Am. Chem. Soc. 1998, 120, 11839-11844. 

Free radical chlorination of alkanes has been reviewed42. The main effort in the study of these reactions has been devoted to the role of solvent43, the effect of -substituents44 and comparison between gas- and liquid-phase processes45. 

Substitution ofH to Cl Free-Radical Chlorination of Alkanes or Lower Chlorinated Alkanes... 

The free radical chlorination of alkanes represents a classic procedure for the functionalization of alkanes... 

B Fletcher, NK Suleman, JM Tanko. Free radical chlorination of alkanes in supercritical carbon dioxide the chlorine atom cage effect as a probe for enhanced cage effects in supercritical fluid solvents. J Am Chem Soc 120 11839, 1998. 

Table 2-1 Selectivity of Chlorine and Bromine in Free-Radical Halogenation of Alkanes ...

Chlorination of Alkanes. The most direct and economical method for the manufacture of chloromethanes is the thermal free-radical chlorination of methane.176 177 Whereas in the 1940s and 1950s photochlorination was practiced in some plants, thermal chlorination is the principal industrial process today. The product chloromethanes are important solvents and intermediates. Commercial operations perform thermal chlorination at about 400-450°C. Vapor-phase photochemical chlorination of methane may be accomplished at 50-60°C. Fast and effective removal of heat associated with thermally induced free-radical substitution is a crucial point. Inadequate heat control may lead to explosion attributed to the uncontrollable pyrolysis liberating free carbon and much heat ... 

There are only two possible products from free-radical chlorination of the starting alkane (CH3)3CCH2C(CH3)3 ---- — (CH3)3CCH2C(CH3)2 + (CH3)3CCHC(CH3)3... 

The reported values for PCA-1, and in particular PCA-70, were higher than their respective true values. It is not clear why results for the PCA-70 mixture, whose GC profile and composition are similar to those of the PCA-60 standard, were less accurate then the results for the PCA-1 sample, whose GC profile and composition were quite different to the external standard. One possible explanation could be the amount of additives/stabilizers used by the manufactures, which are not measurable using ECNI or ECD detection. This makes the preparation of standard solutions from commercial products problematic for quantitation of PCAs and suggests that only pure PCA commercial formulations or synthetic mixtures prepared by free-radical chlorination of pure n-alkanes should be used for the preparation of external standards. 

Elemental chlorine can be used in free-radical halogenation reactions, too, but these reactions are less easily controlled, because the Cl- radical is more reactive than the Br- radical and hence less selective. The reagents t-BuOCl and SO2CI2 are used as alternative chlorinating agents. The F- radical is so reactive, and the reaction F-F + C-H —> H-F + C-F is so exothermic, that free-radical fluorinations result in violent and uncontrollable exotherms (explosions). At the other extreme, free-radical iodinations of alkanes do not work well at all, as the H- abstraction step is too endothermic. 

There are only two possible products from free-radical chlorination of the starting alkane ... 

Hie free radical chlorination of three alkanes (cyclohexane, 2,3-dimethylbutane, and neopentane) was examined both in SC-CO2, and, for comparison purposes, in conventional organic solv ts. All of these experiments were carried out at 40 °C, and at alkane concentrations < 0.03 M 20). At such low alkane concentrations, little monochloride arises from reaction of Cl with alkane comprising die cage walls, ku (Scheme 1). 

A classic example of how reactivity is related to selectivity is concerned with the free radical halogenation of alkanes by CI2 and Br2. In this free radical chain reaction, the step that sets the position of the halogen in the alkane is a hydrogen atom abstraction step. The carbon based radical created in this first propagation step then abstracts a halogen atom from the CI2 or Br2, giving the alkyl halide (see below). In free radical halogenation by either CI2 or Br2, tertiary alkyl halides are created preferentially to secondary, which in turn are formed preferentially to primary alkyl halides. This reflects the fact that the order of radical stability decreases from tertiary to secondary to primary. Yet, the extent of the selectivity for tertiary over secondary over primary is quite different for chlorination and bromination. 

As in the free-radical halogenation of alkanes, chlorination of alkylbenzenes is less selective than bromination. Given the relative rates per hydrogen for hydrogen atom abstraction from 1-phenylbutane by chlorine for the elementary step shown, calculate the percentage of 1-chloro-1-phenylbutane in the C10H13CI product. 

Substitution reactions involve the replacement of one atom, or a group of atoms, by another. For example, the free-radical substitution of alkanes by chlorine in sunlight... 

In both the following exercises assume that all the methylene groups in the alkane are equally reactive as sites of free radical chlorination... 

Hydrocaibons Free radical chlorination or bromfriatlon of alkanes gives a complex... 

Chlorination of Alkanes. Free-radical chlorination is the most commonly used method for the chlorination of a saturated hydrocarbon.31 106-108 111 112 Both thermal and photochemical processes may be carried out in the liquid or vapor phase. The liquid-phase photochemical procedure is preferred for polychlorination gas-phase photochemical reactions can yield either mono- or polychlorinated product. 

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