Halogenations of anthracenes and dibenz[a,c]anthracene with N-bromosuccinimide and N-chlorosuccinimide

Halogenation of dibenz[a,c]anthracene (1) by NBS in CCl4 affords the produc ts of 9- and 10-monobromination in the ratio of 9:1. The reaction is accele rated by iodine, and HBr effects rearrangement of 9-bromo product to the st erically less crowded 10-bromo isomer. The mechanism is proposed to involve reversible addition of Br-2, followed by elimination of HBr. Reaction of N CS with 1 in CCl4 requires addition of HCl and affords exclusively 9-chlori nation. The different reactivities of NBS and NCS are ascribed to the relat ive amounts of free halogen produced (due to differences in N-X bond streng ths involving Br and Cl), and the different sizes of the halogens. Under si milar conditions, NCS chlorinates 9-bromoanthracene (2a) to afford 9,10-dic hloroanthracene and 9-bromo-10-chloroanthracene in the ratio of 65:35. This reaction ostensibly occurs by addition of Cl-2 to 2a, followed by preferen tial loss of HBr rather than HCl. 9-Methylanthracene (3) affords exclusivel y 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 haloge nation. 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 produce d from NBS in the presence of iodine.