97% sodium hydroxide 8.2 g was added and dissolved in a mixture of methanol 60 ml and water 20 ml. 2-nitro-4-chloro-2'-hydroxy-3'-t-butyl-5'-methylazobenzene 11.6 g was then added to the resultant solution at 50° to 60° C. over 30 minutes while stirring, and thereafter 2,3-dichloro-1,4-naphthoquinone 0.3 g and 9-fluorenone 0.4 g were added to the solution. Glucose 8 g was then added to the resultant mixture at 40° to 50° C. over two hours, and the mixture was stirred for one hour at the same temperature. As this result, almost all of the azobenzene disappeared to produce 2-(2-hydroxy-3-t-butyl-5-methylphenyl)-5-chlorobenzotriazole-N-oxide, thus Process (b) being completed.
