Radical Chain Chlorination Using Sulfuryl Chloride

The preferential formation of D instead of C when S( )2d is employed proves that a steric effect prevails here, upstaging the electronically based stability sequence D C. Being a ster-ically demanding radical, S02C1 can abstract only an H atom from adamantane that is acces- 

Side Note 1.3. Pyramidalized Bridgehead Radicals—and/or Less Substituted Planar Radicals as Intermediates of Radical Chlorinations 

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A more energy-efficient variation of photohalogenation, which has been used since the 1940s to produce chlorinated solvents, is the Kharasch process (45). Ultraviolet radiation is used to photocleave ben2oyl peroxide (see Peroxides and peroxide compounds). The radical products react with sulfuryl chloride (from SO2 and CI2) to Hberate atomic chlorine and initiate a radical chain process in which hydrocarbons become halogenated. Thus, for Ar = aryl,... 

The first step in preparing the very useful elastomer Hypalon involves treating a mixture of long-chain alkanes, H(CH2) H, where n =50-200, with sulfuryl chloride (S02C 2) in the presence of substances that can initiate radical-chain chlorination, as described in Section 4-5B. The product molecules contain many C-CI bonds and a few C-S02-Cl bonds, the latter of which are subsequently used in a curing step to improve the physical properties. How can the chain mechanism for chlorination with S02CI2 be modified to account for the formation of C-S02-Cl bonds ... 

Chlorination of the side chain can also be achieved using sulfuryl chloride in the dark, but in the presence of a radical initiator such as benzoyl peroxide, and using f-butyl hypochlorite, MegC-OCI, from which free radicals are generated on gentle warming. 

Scheme 11.4 illustrates some representative halogenation reactions. The reaction in Entry 1 was conducted by slow addition of bromine to excess 2-methylpentane at 60°C, with irradiation from a tungsten light bulb. The reaction in Entry 2 is a typical benzylic bromination, carried out at 125°C with irradiation from a sun lamp. Entries 3 and 4 are examples of NBS bromination using benzoyl peroxide as the initiator. Entry 3 is interesting in that none of the allylic isomer 2-bromo-3-heptene is found. Entries 5 and 6 are examples of chlorination by f-butyl hypochlorite in which the f-butoxy radical is the chain carrier. Note that in Entry 6, both the primary and secondary allylic products are formed. The reaction in Entry 7 uses sulfuryl chloride as the halogenation reagent. Note that in contrast to chlorination with CI2 (see p. 1021), the reaction shows selectivity for the benzylic position. 

Sulfuryl chloride (SO2CI2, see below for structure) is a liquid reagent that may be used for chlorinations of alkanes as a substitute for gaseous elemental chlorine. Propose a mechanism for chlorination of CH4 using sulfuryl chloride. (Hint Follow the usual model for a radical chain process, substituting SO2CI2 for CI2 where appropriate.)... 

Alkanes can be simultaneously chlorinated and chlorosulfonated. This commercially useful reaction has been applied to polyethylene (201—203). Aromatics can be chlorinated on the ring, and in the presence of a free-radical initiator alkylaromatic compounds can be chlorinated selectively in the side chain Ring chlorination can be selective. A patent shows chlorination of 2,5-di- to 2,4,5-trichlorophenoxyacetic acid free of the toxic tetrachlorodibenzodioxins (204). With alkenes, depending on conditions, the chlorination can be additive or substitutive. The addition of sulfuryl chloride can occur to form a 2-chloroalkanesulfonyl chloride (205). 

Chlorine atoms may be generated from molecular chlorine under mild conditions using a catalytic amount of an initiator, In-ln. Thus, homolysis of a molecule of initiator occurs upon irradiation or gentle heat to give free radicals, In (Eq. 9.3). These free radicals may then react with molecular chlorine to produce In-Cl and a chlorine atom (Eq. 9.4) to initiate the free-radical chain reaction. For safety and convenience, sulfuryl chloride, SO2CI2, rather than molecular chlorine is used in this experiment as the source of chlorine radicals. 

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