4.18Writing the structural formula for ethyl chloride reveals that there are two nonequivalent sets ofhydrogen atoms, in either of which a hydrogen is capable of being replaced by chlorine.The two dichlorides are 1,1-dichloroethane and 1,2-dichloroethane.4.19Propane has six primary hydrogens and two secondary. In the chlorination of propane, the relativeproportions of hydrogen atom removal are given by the product of the statistical distribution and therelative rate per hydrogen. Given that a secondary hydrogen is abstracted 3.9 times faster than a pri-mary one, we write the expression for the amount of chlorination at the primary relative to that at thesecondary position as:55Thus, the percentage of propyl chloride formed is 0.77y1.77, or 43%, and that of isopropyl chlorideis 57%. (The amounts actually observed are propyl 45%, isopropyl 55%.)4.20(b)In contrast with free-radical chlorination, alkane bromination is a highly selective process.The major organic product will be the alkyl bromide formed by substitution of a tertiaryhydrogen with a bromine.
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