Chlorination radical chain

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

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 ... 

From bond energies (Table 4-6) we know that the weakest C—H bonds of propene are to the allylic hydrogens, H2C=CHCH2—H. Therefore, in the first step of radical-chain chlorination of propene, an allylic hydrogen is removed by a chlorine atom (Equation 14-1). The allylic C-H bonds are weaker than the alkenic C-H bonds because of the extra stabilization of the radical obtained on hydrogen abstraction (Equation 14-1). Two equivalent valence-bond structures (1a and 1b) can be written for the 2-propenyl radical the electron delocalization enhances the stability of the radical (see Section 6-5C) ... 

We have mentioned how chlorine molecules dissociate to chlorine atoms on absorption of near-ultraviolet light and thereby cause radical-chain chlorination of saturated hydrocarbons (Section 4-4D). Photochemical chlorination is an example of a photochemical reaction that can have a high quantum yield— that is, many molecules of chlorination product can be generated per quantum of light absorbed. The quantum yield of a reaction is said to be unity when 1 mole of reactant is converted to product(s) per einstein1 of light absorbed. The symbol for quantum yield is usually 4>. 

Skell, P. S. Baxter, H. N. Multiple Substitutions in Radical-Chain Chlorinations. A New Cage Effect. J. Am. Chem. Soc. 1985, 107, 2823-2824. 

Radical Chain Chlorination Using Sulfiiryl Chloride... 

This method is practiced in primary alcohols up to octadecanol1055. Highly pure products are obtained with yields of70-85%. Side reactions, namely radical-chain chlorinations and formation of esters, are minimized by conducting the procedure in the dark and by addition of MgCl2 H20. A similar catalytic effect is reported for TV-formylpyrrolidine hydrochloride1056. 

Compared with chlorination, hydrogen abstraction reactions of alkoxy radicals are relatively insensitive to solvent effects [160, 222, 223]. The results of the AIBN-initiated radical chain chlorination of 2,3-dimethylbutane with tert-butyl hypochlorite indicate a solvent effect on tert-butoxy radical reactions of much smaller magnitude, but greater selectivity in aromatic solvents [222, 223], The reduced solvent effect for this hydrogen abstraction reaction has been attributed to steric effects. Due to the bulky... 

The majw work to date on synthetic applications of remote functionalization has involved free radical chlorination. The earliest studies involved the direct attachment of aryliodine dichloride units to the steroid substrates, then intramolecular free radical chain chlorination in benzene or chlorobenzene solution (Scheme 14). Yields were only in the 50% region, but fairly good selecdvities were observed compound (6) afforded chiefly the 9-chloro derivative, while compound (7) produced the 14-chloro steroid. The yields and selectivities were considerably improved when it was realized that aromatic solvents promote intermolecular random processes by forming complexes with C1-, and when the radical relay method was developed. 

Predict which you would expect to be more selective radical chain chlorination or bromination of an alkane. Explain. 

Consider the possible free radical chain chlorination of benzene ... 

The most extensive studies we have done in geometrically directed functionalizations have involved the template directed chlorination of unactivated C-H bonds. These have been reviewed in detail [6], so only a few examples will be mentioned. The first system examined [7] involved attachment of an iodoaryl group to a steroid substrate, conversion of the iodine atom to an ICI2 group, and free radical chain chlorination. After initiation of the chain a radical was produced carrying a single chlorine on the iodine atom this chlorine then removed a hydrogen atom from the attached substrate. 

Figure 3-6 Potential-energy diagram of the reaction of methane with a chlorine atom. Partial bonds in the transition state are depicted by dotted lines. This process, propagation step 1 in the radical chain chlorination of methane, is slightly endothermic.

The kinetic data for the radical chain chlorination of several cycloalkanes (see the table below) illustrate that the C-H bonds of cyclopropane and, to a lesser extent, cyclobutane are somewhat abnormal, (a) What do these data tell you about the strength of the cyclopropane C-H bond and the stability of the cyclopropyl radical (b) Suggest a reason for the stability characteristics of the cyclopropyl radical. (Hint Consider bond-angle strain in the radical relative to cyclopropane itself.)... 

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