Chlorination exothermic nature

Due to the highly exothermic nature of the process, the replacement of primary, secondary and tertiary hydrogens upon reaction with electrophilic fluorine atoms is not as selective as for other radicals. For example, early work by Tedder [30,34], showed that the order of selectivity follows the usual pattern, i. e. tert > sec > prim, but the relative selectivity of fluorine atoms is less than chlorine atoms (Table 2). 

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). 

In chlorinations either a substitution or an addition process can occur with the ultimate reaction pathway(s) determined by a combination of factors, which include the reaction conditions, the positions and natures of any substituents present, and the catalyst used. Uncatalyzed chlorination of benzothiadiazole is an exothermic reaction that gives rise to a mixture of isomeric tetrachloro addition products. These are converted in basic medium into 4,7-dichloro-2,1,3-benzothiadiazole (70RCR923). When an iron(III) catalyst is present 4- and 7-chloro substitution becomes the dominant process. Chlorination of a number of 4-substituted 2,1,3-benzothiadiazoles (43) using an oxidative process gave a combination of chlorinated and oxidized products. The 4-hydroxy, 4-amino-, 4-methyl-amino, and 4-acetoxy derivatives of 43 all formed the chloroquinones (44) (40-61% yields). With the 4-aIkoxy substrates both 44 and some 5,7-dichlorinated product were obtained (88CHE96). 

The low-temperature process, as its name implies, operates at a relatively low temperature where the exothermic heat of the direct chlorination reaction is removed by cooling water. The natural circulation is driven by the gas lift effect of the gaseous feeds before solution and the density differences of a cooler leg that has a relatively higher liquid density than the reactor leg. 

Chlorination of 2,1,3-benzothiadiazoles can take place either by addition or substitution, depending on the reaction conditions, the nature of the substituents in the benzene ring, and the catalyst employed. The uncatalyzed reaction of (1) with chlorine is exothermic and produces an isomeric mixture of tetrachloro addition products, which form 4,7-dichloro-2,l,3-benzothiadiazole on treatment with base <70RCR923>. In the presence of an iron catalyst, chlorine substitution in the 4,7-positions predominates. 

Recall the Hammond postulate (Section 6.6), which describes the nature of the transition state in each case. For the chlorination process, the rate-determining step is exothermic, and therefore, the energy and structure of the transition state more closely resemble reactants than products (Figure 11.13). 

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