Chlorination of Other Alkanes

Recall from Section 15.3 that the chlorination of CH3CH2CH3 affords a 1 1 mixture of CH3CH2CH2CI (formed by removal of a 1° hydrogen) and (CH3)2CHC1 (formed by removal of a 2° hydrogen). 

CH3CH2CH3 has six 1° hydrogen atoms and only two 2° hydrogens, so the expected product ratio of CH3CH2CH2CI to (CH3)2CHC1 (assuming all hydrogens are equally reactive) is 3 1. Because the observed ratio is 1 1, however, the 2° C-H bonds must be more reactive that is, it must be easier to homolytically cleave a 2° C-H bond than a 1° C-H bond. Recall from Section 15.2 that 2° C-H bonds are weaker than 1° C-H bonds. Thus, 

When alkanes react with CI2, a mixture of products results, with more product formed by cleavage of the weaker C-H bond than you would expect on statistical grounds. 

Problem 15.11 Which C-H bond in each compound is most readily broken during radical halogenation  

Energy diagram for the propagation steps in the chlorination of ethane 

CH3CH2CH3 affords a 1 1 mixture of By reirovaroraP OT en) and (CH3)2CHC1 (formed by removal of 

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The same mechanism al.so applies qualitatively to chlorination of other alkanes. The only difference is in the nature of the C-H bonds available in the alkane to be broken. Tliey are generally less strong than those in methane, following a DH° order of Cll.i > I" > 2 > 3 . (Note I = primary, 2° = secondary, and 3 = tertiary. Thc.se arc commonly used symbols.) The weakest (3 ) are the most readily broken tlms. alkanes with different types of C-H bonds display a built-in xflerimiy of 3" - 2" > I" in their reactions with chlorine. This. section describes this. selectivity quantitatively, illustrating how both reactivity differences and. siaiisiic il factors combine to produce the observed ratios of products in. several rcprescnlativc sy.siems. 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

In other technologies340,341 a mixture of n-alkyl chlorides formed by the chlorination of n-alkanes is used directly in alkylation with aluminum chloride. 

Chlorination of an alkane is not usually suitable for the laboratory preparation of an alkyl chloride any one product is necessarily formed in low yield, and is-difficult to separate from its isomers, whose boiling points are seldom far from its own. Bromination, on the other hand, often gives a nearly pure alkyl bromide in high yield. As we shall see, it is possible to predict just which isomer will predominate if this product is the one desired, direct bromination could be a feasible synthetic route. 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

Methane ethane and cyclobutane share the common feature that each one can give only a single monochloro derivative All the hydrogens of cyclobutane for example are equivalent and substitution of any one gives the same product as substitution of any other Chlorination of alkanes m which the hydrogens are not all equivalent is more com plicated m that a mixture of every possible monochloro derivative is formed as the chlo rmation of butane illustrates... 

Bunce, N. J., K. U. Ingold, J. P. Landers, J. Lusztyk, and J. C. Scaiano, Kinetic Study of the Photochlorination of 2,3-Dimethyl-butane and Other Alkanes in Solution in the Presence of Benzene. First Measurements of the Absolute Rate Constants for Hydrogen Abstraction by the Free Chlorine Atom and the Chlorine Atom-Benzene tr -Complex. Identification of These Two Species as the Only Hydrogen Abstractors in These Systems, . /. Am. Chem. Soc., 107, 5464-5472 (1985). 

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hope), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions. 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbF. "2 Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved with benzenescleninyl chloride PhSe(0)Cl and AIC13 or AIBr3. n However, while some substituted alkenes give high yields of chloro substitution products, others (such as styrene) undergo addition of Cl2 to the double bond (5-26).113 Electrophilic fluorination has already been mentioned (p. 690). 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

In methane, all four hydrogen atoms are identical, and it does not matter which hydrogen is replaced. In the higher alkanes, replacement of different hydrogen atoms may lead to different products. In the chlorination of propane, for example, two monochlorinated (just one chlorine atom) products are possible. One has the chlorine atom on a primary carbon atom, and the other has the chlorine atom on the secondary carbon atom. 

Of course, the overall AH for the 1 reaction of an alkane with chlorine, must also take into account the AH of this sea Step whfeh is-3 49 +243= lO kJ mol-1, making chterirtetoon mudi more exothermic than brommation. u naf(On continues the trend, and methane-fluorine mixtures are. explosive. For iodine, on the other hand, the first step becomes so endothermic, even for formation of atertiary redical, thatthe second step (AH= -234 +151=-83 kj mol-1) is not exothermic enough to make reaction favourable overall. Radical lodinatlons therefore do not take place. 

Chlorinated alkenes are similar to chlorinated alkanes. The two chlorinated solvents most frequently found in groundwater are trichloroethene and tetrachloroethene. Although not used as a solvent, 1,2-dichloroethene may be found due to the breakdown of other alkenes, Vinyl chloride may occur as a breakdown product of other chlorinated alkenes, but is most likely to be found in water as a consequence of the leaching from polyvinyl chloride (PVC) water pipes, which contain high residuals of vinyl chloride. This chemical is usually best controlled through product specifications. 

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... 

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