Butane radical chlorination

Addition to double bonds is not the only kind of reaction that converts an achiral molecule to a chiral one Other possibilities include substitution reactions such as the formation of 2 chlorobutane by free radical chlorination of butane Here again the prod uct IS chiral but racemic... 

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. 

Assuming the relative rate of secondary to primary hydrogen atom abstraction to be the same in the chlorination of propane as it is in that of butane, calculate the relative amounts of propyl chloride and isopropyl chloride obtained in the free-radical chlorination of propane. 

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. 

Let us return to the reaction we used as our example in Sec. 7.4, free-radical chlorination of sec-butyl chloride, but this time focus our attention on one of the other products, one in which a second chiral center is generated 2,3-dichloro-butane. This compound, we have seen (Sec. 4.18), exists as three stereoisomers, ineso and a pair of enantiomers. 

In the radical chlorination of 2,2-dimethylhexane, chlorine substitution occurs much more rapidly at C5 than it does at a typical secondary carbon (e.g., C2 in butane). Consider the mechanism of radical polymerization and then suggest an explanation for the enhanced rate of substitution at C5 in 2,2-dimethylhexane. 

When butane undergoes radical chlorination, two constitutional isomers are obtained ... 

For example, the radical bromination of butane gives a 98% yield of 2-bromobutane, compared with a 71% yield of 2-chlorobutane obtained when butane is chlorinated (Section 13.4). 

Now that we understand the origin of the products, we can ask why the reaction of butane with chlorine yields 1-chlorobutane and 2-chlorobutane in the ratio 28 72 ( 1 3). Butane has six primary hydrogen atoms and four secondary hydrogen atoms, so there are six ways to form the butyl radical and four ways to form the rec-butyl radical. If the primary and secondary hydrogen atoms of butane reacted at the same rate, the ratio of 1-chlorobutane to 2-chlorobutane would be 6 4, but it isn t. 

If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstraction, the n-butyl/5ec-butyl radical ratio and, therefore, the 1-chloro/2-chlorobutane ratio, would be given by the relative numbers of hydrogens in the two equivalent methyl groups of CH3CH2CH2CH3 (six) compared with those in the two equivalent methylene groups (four). The product distribution expected on a statistical basis would be 60% 1-chloro-butane and 40% 2-chlorobutane. The experimentally observed product distribution, however, is 28% 1-chlorobutane and 72% 2-chlorobutane. 5ec-Butyl radical is therefore formed in greater anounts, and n-butyl radical in lesser anounts, than expected statistically. 

All this was later put on a sound basis as a result of more precise measurements of rate constants and of activation energies. However, it did not require precise measurements to predict which chlorinated hydrocarbons would decompose by a radical chain mechanism and which by the unimolecular mechanism. Clearly, if the chlorinated hydrocarbon, or the product from the pyrolysis of the chlorinated hydrocarbon reacted with chlorine atoms to break the chain then the chain mechanism would not exist. Such chlorinated hydrocarbons would decompose by the unimolecular mechanism. Mono-chlorinated derivatives of propane, butane, cyclohexane, etc. would afford propylene, butenes, cyclohexene, etc. All these olefins are inhibitors of chlorine radical chain reactions because of the attack of chlorine atoms at their allylic positions to give the corresponding stabilized allylic radicals which do not carry the chain. 

Abstraction of the secondaiy hydrogen atom is more exothermic than abstraction of the primary hydrogen atom, for the related reasons that (1) secondary C-H bonds are weaker than primary ones and (2) secondary radicals are more stable than primaiy ones. So, we get more 2-chloropropane than l-chloropropane. But in this case, that isn t the only factor involved remember that there are six primaiy hydrogen atoms and only two secondary ones, so the relative reactivity of the primaiy and secondaiy positions is even more different than the simple ratio of products from the reaction suggests. This statistical factor is more evident in the second example we gave above, the chlorination of iso butane. Now the choice is between formation of a tertiaiy radical and formation of a primary one. 

The ratio of 2,2-dimethyl butane to 2-methyl pentane produced by these reactions will be k5a/k5h. In general, chlorine atom is less selective in hydrogen abstraction reactions than are hydrocarbon free radicals and hence fc2a/ 2b > > k5a/k5h. Consequently, one would expect that the first increment of HCl would decrease the ratio of 2,2-dimethyl butane to 2-methyl pentane in the C6 alkylation product. 

The chlorine radical can react with butane by abstraction of a hydrogen atom ... 

In the first experiment we will chlorinate 1-chlorobutane because it is easier to handle in the laboratory than gaseous butane and we will use sulfuryl chloride as our source of chlorine radicals because it is easier to handle than gaseous chlorine. Instead of using light to initiate the reaction we will use a chemical initiator, 2,2 -azobis-(2-methylpropionitrile). This azo compound (R—N=N—R) decomposes at moderate temperatures (80-100°C) to give two relatively stable radicals and nitrogen gas ... 

Table III also shows that hydrogen and the chlorinated butanes are reduced substantially when ethyl chloride is irradiated in the presence of benzene. The other products are essentially unaffected by this additive. In the radiolysis of certain alkanes (4), benzene, added in small amounts, does not interfere with the fast ion-molecule reactions of primary ionic fragments or with free radical processes, but it will efficiently condense unreactive or long-lived ions in the system. It is reasonable to assume that this is also true for alkyl halide systems and that the reduction in product yields compared with the pure compound upon adding benzene may be attributed to the interception of unreactive ions. Since the rate constants for reactions of the expected primary ions with ethyl chloride are very large (see Table II), the concentration of benzene used in our experiments should not interfere with the initial fast ion-molecule reactions. For ethyl chloride ion-molecule reactions, C4Hi0C1+ is the only unreactive ion of appreciable abundance which is expected in this system at the elevated pressures used in the radiolysis experiments. Thus, the reduced product yields in the presence of benzene additive can be identified tentatively with the removal of this stable ion and the elimination of its resultant neutralization products.

The expected (statistical) distribution of products is 60% 1-chlorobutane and 40% 2-chlorobutane because six of butane s 10 hydrogens can be substituted to form 1-chlorobutane, whereas only four can be substituted to form 2-chlorobutane. This assumes, however, that all of the C—H bonds in butane are equally easy to break. Then, the relative amounts of the two products would depend only on the probability of a chlorine radical colliding with a primary hydrogen, compared with its colliding with a secondary hydrogen. When we carry out the reaction in the laboratory and analyze the product, however, we find that it is 29% 1-chlorobutane and 71% 2-chlorobutane. Therefore, probability alone does not explain the regioselectivity of the reaction. Because more 2-chlorobutane is obtained than expected and the rate-determining step of... 

When a chlorine radical reacts with butane, it can abstract a hydrogen atom from an internal carbon, thereby forming a secondary alkyl radical, or it can abstract a hydrogen atom from a terminal carbon, thereby forming a primary alkyl radical. Because it is easier to form the more stable secondary alkyl radical, 2-chlorobutane is formed faster than 1-chlorobutane. 

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