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Thermal initiation compounds

Formation of Product Mixtures Chlorination of alkanes generally results in the formation of isomeric chlorinated products, depending on which hydrogen is abstracted by Cl• in the first propagation step. The percent composition of the product mixture is determined by (a) the probability factor (the number of hydrogens which, on substitution by chlorine, lead to a given product) and (b) the inherent reactivity of each type of hydrogen. For example, let’s examine the chlorination of 2-methylbutane. The first column of the table below shows the percent composition of the product mixture that would be expected if only the probability factor is taken into account. The second column displays the percent composition actually found by experimentation. CH3 CH CH3 CH2 CH3 CH2Cl CH CH3 CH2 CH3 CH3 CCl CH3 CH2 CH3 CH3 CH CH3 CHCl CH3 CH3 CH CH3 CH2 CH2Cl Cl2 hν Proportion expected by probability 1/12 = 8% 2/12 = 17% 3/12 = 25% Proportion found by experiment 30% 22% 33% 15% 6/12 = 50% 3 Clearly the relative reactivities of the hydrogens in this molecule are not equivalent. It is possible to calculate the relative reactivities of these hydrogens towards chlorination using the data in the table. Forty-five percent of the products actually obtained resulted from the chlorination of a primary carbon. Since there are 9 hydrogens attached to a primary carbon, each primary hydrogen is responsible for 5% (45%/9) of the product. Thirty-three percent of the products resulted from chlorination of a secondary carbon. There are two hydrogens attached to a secondary carbon; therefore, each secondary hydrogen is responsible for 16.5% (33%/2) of the product. One tertiary carbon with only one hydrogen attached to it is present in the molecule, and is responsible for 22% of the product. We will calculate the reactivities of the secondary and tertiary hydrogens relative to that of the primary hydrogens, which we will take to be 1. Thus, the secondary hydrogens are (16.5)/5 = 3.3 times as reactive toward chlorination as primary hydrogens, while the tertiary hydrogens are (22)/5 = 4.4 times as reactive. R3CH > R2CH2 > RCH3 4.4 3.3 1 In this experiment, you will study the chlorination of 1-chlorobutane. Four different dichlorinated products can be formed, their percentages being determined by the probability factor and the reactivity of each type of hydrogen, which in this case is a function of the influence of the chlorine atom as well as whether the hydrogen is primary or secondary. From the relative amounts of the four dichlorinated products, you will determine the relative reactivities of the different types of hydrogens. CH3 CH2 CH2 CH2Cl Initiator CH2 CH2 CH2 CH2Cl CH3 CH2 CH CH2Cl CH3 CH2 CH2 CHCl CH3 CH CH2 CH2Cl Cl Cl Cl Cl SO2Cl2 1,4-dichlorobutane b.p. 162° 1,3-dichlorobutane b.p. 134 1,2-dichlorobutane b.p. 124° 1,1-dichlorobutane b.p. 114°