Chlorination of butane

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... 

Another experiment of the competition type involves the comparison of the reactivity of different atoms in the same molecule. For example, gas-phase chlorination of butane can lead to 1- or 2-chlorobutane. The relative reactivity k /k of the primary and secondaiy hydrogens is the sort of information that helps to characterize the details of the reaction process. 

Methane, ethane, and cyclobutane share the common feature that each one can give only a single monochloro derivative. All the hydrogens of cyclobutane, for exanple, are equivalent, and substitution of any one gives the sane 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 fonned, as the chlorination of butane illustrates ... 

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. 

Table 7.2 Kinetic parameters and physical data for the chlorination of butanic acid.

Problem 5.21 (a) What is the stereochemistry of CH,CHC1CH,CH, isolated from the chlorination of butane ... 

Chlorobutanes. Chlorobutanes can be obtained by diverse procedures, such as (1) the liquid-phase or thermal chlorination of butane, (2) addition of chlorine or hydrogen chloride to butenes, (3) reaction of hydrochloric acid with butanols and butylene glycols, (4) chlorination of chlorobutenes and chlorobutadienes, and (5) hydrochlorination of tetramethylene oxide (tetrahydrofuran) and butadiene. 

The thermal chlorination of butane is similar to that of propane and can be carried out according to the procedures developed by Hass and coworkers. The product contains 1-chloro- and 2-chlorobutane as well as 1,2-dichloror- and 1,3-dichlorobutanes. In the photochemical chlorination, chlorine is introduced into a body of n-butane that is cooled to -25V... 

In addition to more highly chlorinated products, chlorination of butane yields a mixture of compounds with the formula C4H9CI. 

PROBLEM 5.24 Show that the chlorination of butane at carbon-2 will give a 50 50 mixture of enantiomers. 

Scheme 5.31 Mechanism of the L-prolinamide-catalyzed a-chlorination of butanal with NCS.

This disconnection suggests that in the synthetic direction, the OH group of oxime TM 7.2d approaches the cyano group, completing the cyclization. Presentation of both functional groups as acyclic cyano-oxime suggests interconversion to 3-cyanobutan-2-one and then to 3-chlorobutan-2-one, easily available by chlorination of butan-2-one. Acetanilide is an available commodity used in the production of para-chlorosulfonyl derivative TM 7.2c. 

The chlorination of higher-molecular-weight alkanes yields a mixture of isomeric monochlorinated products. For example, the chlorination of butane or 2-methyl-propane, which have nonequivalent hydrogen atoms, yields significant amounts of isomeric monochlorinated derivatives. 

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. 

Let s consider the chlorination of butane again. If the propagation step generates a primary radical, the product is a primary halide. Thus, the -butyl radical gives 1-chlorobutane. 

Lets consider the chlorination of butane again in greater detail. We see that there are four ways to get 2-chlorobutane and six ways to obtain 1-chlorobutane. The ratio of 2-chlorobutane to 1-chlorobutane therefore equals the relative rate of abstraction of a secondary hydrogen atom times 4 divided by the relative rate of abstraction of a primary hydrogen atom times 6. 

---