Radical chlorinations isopentane

P oUem 7.2 Isopentane is allowed to undergo free-radical chlorination, and the reaction mixture is separated by careful fractional distillation, (a) How many frac tions of formula CsHnCl would you expect to collect (b) Draw structural formulas, stereochemical where pertinent, for the compounds making up each fraction. Specify each enantiomer as R or S. (c) Which if any, of the fractions, as collected, would show optical activity (d) Account in detail—just as was done in the preceding section— for the optical activity or inactivity of each fraction. 

The discrepancy from the experimental values is due to the fact that H atoms bound to different types of C atoms are replaced by chlorine at different rates. The substitution of Cfcrt— H takes place via a tertiary radical. The substitution of Csec—H takes place via the somewhat less stable secondary radical, and the substitution of Cprjm—H takes place via even less stable primary radicals (for the stability of radicals, see Table 1.2). According to Hammond s postulate, the rate of formation of these radicals should decrease as the radical s stability decreases. Hydrogen atoms bound to Ctert should thus be substituted more rapidly than H atoms bound to Csec, and these should in turn be substituted by Cl more rapidly than H atoms bound to Cprjm. As the analysis of the regioselectivity of the monochlorination of isopentane carried out by means of Table 1.4 shows, the relative chlorination rates of C —H, C —H, and C. —H are 4.4 33 1, in agreement with this expectation. 

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