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 in which the hydrogens are not all equivalent is more complicated in that a mixture of every possible monochloro derivative is formed, as the chlorination of butane illustrates:
These two products arise because in one of the propagation steps a chlorine atom may abstract a hydrogen atom from either a methyl or a methylene group of butane.
The resulting free radicals react with chlorine to give the corresponding alkyl chlorides. Butyl radical gives only 1-chlorobutane; sec-butyl radical gives only 2-chlorobutane.
If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstrac- tion, the n-butyl/sec-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. sec-Butyl radical is there- fore formed in greater amounts, and n-butyl radical in lesser amounts, than expected statistically. This behavior stems from the greater stability of secondary compared with primary free radicals. The transition state for the step in which a chlorine atom abstracts a hydrogen from carbon has free-radical character at carbon.
A secondary hydrogen is abstracted faster than a primary hydrogen because the transi- tion state with secondary radical character is more stable than the one with primary rad- ical character. The same factors that stabilize a secondary radical stabilize a transition state with secondary radical character more than one with primary radical character.
As carbocations go, CH3 + is particularly unstable, and its existence as an inter- mediate in chemical reactions has never been demonstrated. Primary carbocations, although more stable than CH3 +, are still too unstable to be involved as intermediates in chemical reactions. The threshold of stability is reached with secondary carbocations. Many reactions, including the reaction of secondary alcohols with hydrogen halides, are believed to involve secondary carbocations. The evidence in support of tertiary carbo- cation intermediates is stronger yet.
In summary then, the chlorination of alkanes is not very selective. The various kinds of hydrogens present in a molecule (tertiary, secondary, and primary) differ by only a factor of 5 in the relative rate at which each reacts with a chlorine atom.
Bromine reacts with alkanes by a free-radical chain mechanism analogous to that of chlorine. There is an important difference between chlorination and bromination, howeven Bromination is highly selective for substitution of tertiary hydrogens. The spread in reactivity among primary, secondary, and tertiary hydrogens is greater than thousand.
In practice, this means that when an alkane contains primary, secondary, and tertiary hydrogens, it is usually only the tertiary hydrogen that is replaced by bromine.
This difference in selectivity between chlorination and bromination of alkanes needs to be kept in mind when one wishes to prepare an alkyl halide from an alkane:
- Because chlorination of an alkane yields every possible monochloride, it is used only when all the hydrogens in an alkane are equivalent.
- Bromination is normally used only to prepare tertiary alkyl bromides from alkanes.