Chlorination, of alkanes

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

In this way, the operating temperature is reached without thermal overshoots 

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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... 

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... 

In summary then the chlorination of alkanes is not very selective The various kinds of hydrogens present m a molecule (tertiary secondary and primary) differ by only a factor of 5 m the relative rate at which each reacts with a chlorine atom... 

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 ... 

The situation is even worse for chlorination of alkanes that have more than one sort of hydrogen. For example, chlorination of butane gives two mono-chlorinated products in addition to dichlorobutane, trichlorobutane, and so on. Thirty percent of the monochloro product is 1-chlorobutane, and seventy percent is 2-chlorobutane. 

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. 

Chain termination. The chlorination of alkanes by rm-butyl hypochlorite is believed to follow a chain mechanism, but there is a dispute about the termination step.10 Derive the steady-state rate equation for each, making the long-chain approximation. 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbFs. " Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved... 

By an industrial investigation of a gas-phase reaction, the chlorination of alkanes, thermal management (faster temperature ramping, avoidance of overshoots) was improved and, hence, control over radical formation was exerted. As a result, a significant increase in space-time yield to about 430 g h 1 was achieved using a hybrid micro-reactor plant compared with the conventional performance of 240 g h [127, 161]. 

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 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). 

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 ... 

Nanocrystalline MgO and CaO with high surface area are able to absorb large amounts of chlorine, which undergo dissociative chemisorption. These can serve as rather selective, catalytic alkane chlorination reagents, which suggests that trapped Cl atoms are involved in the reaction.295 Liquid-phase low-temperature chlorination of alkanes is also possible in the presence of various alkenes as inductors and AIBN [azobis(isobutyronitrile)] 296... 

It is possible to achieve chlorination of alkanes using sulfuryl chloride (S02Cl2, bp 69°) in place of chlorine ... 

Table 5.32. AgSbF6-Induced Chlorination of Alkanes with Chlorine in the Dark525...

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. 

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