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homolytically by irradiation with ultraviolet light

Experiment 8: Chlorination of 1-Chlorobutane Alkanes contain only nonpolar carbon-hydrogen and carbon-carbon single bonds, which makes them unreactive toward most acidic and basic reagents. They can, however, undergo free radical halogenation. The halogenation of alkanes is a chain reaction which, for chlorination, proceeds through the sequence of steps shown below: 1) Cl2 ® 2 Cl· initiation step 2) Cl· + R—H ® H—Cl + R· propagation step 3) R· + Cl2 ® R—Cl + Cl· propagation step 4) R· + Cl· ® R—Cl termination step 5) R· + R· ® R—R termination step Since chlorine radicals are used up in step 2 and regenerated in step 3, while alkyl radicals are used up in step 3 and regenerated in step 2, these two reactions form a closed cycle that carries out the net reaction: R—H + Cl2 ® R—Cl + H—Cl Only when either [R—H] or [Cl2] becomes small (comparable to [Cl•]) do the rates of reactions 4 and 5 become fast enough to remove R• and Cl• from the reaction and interrupt the chain. Thus, a single chlorine atom can convert many thousands of molecules of R—H to R—Cl. Chlorine radicals may be produced either photochemically or thermally. The bond between the two atoms of the chlorine molecule (58 kcal/mol) can be cleaved homolytically by irradiation with ultraviolet light: Cl2 2 Cl· Thermal initiation uses compounds that contain bonds weak enough to undergo homolytic cleavage on heating (bond energies of about 35 kcal/mol or lower). In this experiment, we will use a mixture of 2,2′-azobis-(2-methylpropionitrile) and sulfuryl chloride (Cl–SO2–Cl) to produce chlorine radicals. On heating, each molecule of 2,2′- azobis-(2-methyl-propionitrile) forms two cyanopropyl radicals and a nitrogen molecule: (CH3)2C(CN)–N=N–C(CH3)2CN ® 2 [(CH3)2C(CN)]· + N2 Reaction of cyanopropyl radicals with SO2Cl2 generates chlorine atoms, completing the sequence of initiation steps: [(CH3)2C(CN)]· + SO2Cl2 ® (CH3)2C(CN)–Cl + SO2Cl· SO2Cl· ® SO2 + Cl· 2 The propagation steps are: Cl· + R—H ® H—Cl + R· R· + Cl—SO2—Cl ® R—Cl + SO2 + Cl· Note that hydrogen chloride and sulfur dioxide, two hazardous gases, are generated by these reactions. For this reason, you will use a gas trap to neutralize these compounds as they are produced. 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