Alkanes radical chlorination

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

Methane is the most difficult alkane to chlorinate. The reaction is initiated by chlorine free radicals obtained via the application of heat (thermal) or light (hv). Thermal chlorination (more widely used industrially) occurs at approximately 350-370°C and atmospheric pressure. A typical product distribution for a CH4/CI2 feed ratio of 1.7 is mono- (58.7%), di-(29.3%) tri- (9.7%) and tetra- (2.3%) chloromethanes. 

Problem 5.2 Radical chlorination of alkanes is not generally useful because mixtures of products often result when more than one kind of C-H bond is present in the substrate. Draw and name all monochloro substitution products CgM 13CI you might obtain by reaction of 2-methylpentane with C)2. 

Despite the limitations of radical chlorination of alkanes, the reaction is still useful for synthesizing certain halogenated compounds. For which of the following compounds does radical chlorination give a single moiiochloro product ... 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

Chain reaction (Section 5.3) A reaction that., once initiated, sustains itself in an endlessly repeating cycle of propagation steps. The radical chlorination of alkanes is an example of a chain reaction that is initiated by irradiation with light and then continues in a series of propagation steps. 

Initiator (Section 5.3) A substance with an easily broken bond that is used to initiate a radical chain reaction. For example, radical chlorination of alkanes is initiated when light energy breaks the weak Cl-Cl bond to form Cl-radicals. 

An industrial investigation studied the radical chlorination of alkanes in micro heatexchangers toanalyse thermal effects onradical production [29,30]. Itwas knownfrompriorstudiesinareactorconsistingoftwoconventionaltubes.onefor... 

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

However, it is instructive to consider radical chlorination of alkanes just a little further, to appreciate the mechanistic concepts. If we carry out light-induced chlorination of propane, then we obtain... 

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. 

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. 

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... 

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 ... 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

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. 

Only two alkanes have the molecular formula C4H10 butane and isobutane (2-methylpropane)— both of which give two monochlorides on free-radical chlorination. However, dehydrochlorination of one of the monochlorides derived from butane yields a mixture of alkenes. 

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... 

Fig. 1.23. Thermochemical analysis of that propagation step of radical chlorination (left) and bromination (right) of alkanes that determines the regioselectivity of the overall reaction. The AW values were determined experimentally the values for the activation enthalpies (AW ) are estimates.

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. 

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. 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 hint at the answer. The data in Table 5.3 indicate that a tertiary C—H bond 1390 kJ/mol (93 kcal moil>l is weaker than a... 

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. 

Problem 5.2 Radical chlorination of alkanes is not generally useful because mixtures of products... 

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