Chlorination, alkane regioselectivity

The radical chain mechanism of the sulfochlorination is very similar to that of the chlorination. Accordingly, in normal cases the regioselectivities of the sulfochlorination and the chlorination are equal. For example, (-1) substituents decrease the reactivities of the adjacent C-H bond. This influence can even be observed at the y position. Thus, the consecutive second sulfochlorination affords no geminal or vicinal disulfochlorides in the product. Where there are differences between the regioselectivities of sulfochlorination and chlorination (as in the case of isoalkanes), it is because under the conditions of sulfochlorination, chlorination also takes place to a considerable extent. Figure 6 shows the main components of a sulfochlorination mixture. Today the kinetics and the regioselectivity of the sulfochlorination of /z-alkanes are so well known that the kinetic modeling of the concentration-conversion curves is possible for all partners of the reaction [12]. 

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.

Furthermore, di- and polychlorination are much less prevalent. Dicarboxylic acids are predominantly chlorinated in the middle of the chain, and adamantane and bicyclo[2.2.2]octane at the bridgeheads by this procedure. The reasons for the high co - 1 specificity are not clearly understood. Alkyl bromides can be regioselectively chlorinated one carbon away from the bromine (to give vic-bromochlorides) by treatment with PCls. Alkyl chlorides can be converted to vic-dichlorides by treatment with Mods. " Enhanced selectivity at a terminal position of w-alkanes has been achieved by absorbing the substrate onto a pentasil zeolite. In another regioselective chlorination, alkanesulfonamides... 

The reaction of bromine with an alkane occurs in the order 3° > 2° > 1° hydrogen. Chlorination of alkanes is also regioselective, but much less so than bromination. For example, treatment of propane with chlorine gives a mixture of approximately 57% 2-chloropropane and 43% 1-chloropropane. 

Abstraction of hydrogen by chlorine is exothermic, which, according to Hammond s postulate, means that the transition state for H absfracfion by Cl is reached early in the course of the reaction [Figure 8.3(a)]. Therefore, fhe strucfure of the transition state for this step resembles the reactants, namely the alkane and a chlorine atom, not the product radicals. As a result, there is relatively little radical character on carbon in this transition state, and regioselectively in radical chlorination is only slightly influenced by the relative stabilities of radical intermediates. Products are determined more by whether a chlorine atom happens to collide with a 1°, 2°, or 3° H. 

Conversely, the structure of the transition state of an endothermic reaction step occurs later and looks more like the products of that step than like the reactants, and changes in products have a large effect on the rate. Hammond s postulate accounts for the fact that bromination of an alkane is more regioselective than chlorination. 

Chlorination and Bromination of Alkanes (Section 8.4) Chlorination and bromination of alkanes are regioselective in the order 3° H > 2° H > 1° H. Bromination has a higher regioselectivity than chlorination. The mechanism involves a radical chain process. 

The chlorination of alkyl side chains is also regioselective but not to the same high degree as bromination. Recall that we observed this same pattern in the regioselec-tivities of bromination and chlorination of alkanes (Section 8.4A). 

If one of several possible isomers predominates, a reaction is said to be regioselective. The data for the chlorination of butane and 2-methylpropane indicate that the chlorination of alkanes is not very regioselective. In fact, there doesn t appear to be a simple explanation for the product distribution in these chlorination reactions. For example, in the chlorination of butane, the major product, 2-chlorobutane, arises when a chlorine atom replaces a secondary hydrogen atom rather than a primary hydrogen atom. However, in the case of 2-methylpropane, the major product, l-chloro-2-methylpropane, arises when a chlorine atom replaces a primary hydrogen atom rather than a tertiary hydrogen atom. 

In contrast to chlorination, the bromination of alkanes is highly regioselective. For example, in the photochemical bromination of 2-methylpropane, more than 99% of the product results from substitution of bromine for the tertiary hydrogen atom. 

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