- Halogenations of Anthracenes and Dibenz[a,c]anthracene with N-Bromosuccinimide and N-Chlorosuccinimide
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Halogenation of dibenz[a,c]anthracene (1) by NBS in CCl4 affords the products of 9- and 10-monobromination in the ratio of 9:1. The reaction is accelerated by iodine, and HBr effects rearrangement of 9-bromo product to the sterically less crowded 10-bromo isomer. The mechanism is proposed to involve reversible addition of Br2, followed by elimination of HBr. Reaction of NCS with 1 in CCl4 requires addition of HCl and affords exclusively 9-chlorination. The different reactivities of NBS and NCS are ascribed to the relative amounts of free halogen produced (due to differences in N-X bond strengths involving Br and Cl), and the different sizes of the halogens. Under similar conditions, NCS chlorinates 9-bromoanthracene (2a) to afford 9,10-dichloroanthracene and 9-bromo-10-chloroanthracene in the ratio of 65:35. This reaction ostensibly occurs by addition of Cl2 to 2a, followed by preferential loss of HBr rather than HCl. 9-Methylanthracene (3) affords exclusively 9-(bromomethyl)anthracene with NBS in the absence of iodine, but mainly (67%) 9-bromo-10-methylanthracene in the presence of iodine. Chlorination of 3 with NCS in the presence of HCl also affords mostly (65%) nuclear halogenation. Nuclear bromination of anthracene, 9-methylanthracene, and dibenz[a,c]anthracene by NBS in the absence of added HBr is accelerated by iodine. This effect is probably due to an increase in the amount of bromine produced from NBS in the presence of iodine.
- Duan, Shaoming,Turk, Jeff,Speigle, Joseph,Corbin, Jean,Masnovi, John,Baker, Ronald J.
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p. 3005 - 3009
(2007/10/03)
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- The Reaction of Iodine Monochloride with Polycyclic Aromatic Compounds: Polar and Electron Transfer Pathways
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Several polycyclic aromatic hydrocarbons were treated with iodine monochloride.Although iodination is the predominant reaction of benzenoid arenes, chlorination is the sole reaction with anthracene, phenanthrene and naphthalene compounds (except for 5), whose oxidation half-wave potentials are less than 1.58 V vs Ag/Ag(+) (0.1 M).Arenes with higher potentials are unreactive.Naphthalene (9) and its derivatives with mild electron-withdrawing substituents are chlorinated; the exception (5) yields the 1-iodo product (21).The reaction is first order in substrate and second order in ICl.An electron transfer pathway involving radical cation intermediates is assumed.Ion-pair collapse of the radical cation-ICl intermediates is proposed for the chlorinations and radical-pair collapse for the iodination.
- Turner, Dean E.,O'Malley, Robert F.,Sardella, Dennis J.,Barinelli, Lucio S.,Kaul, Pushkar
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p. 7335 - 7340
(2007/10/02)
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