Electrophilic aromatic substitution. Part 27. Chemical selectivities disguised by mass diffusion. Part 6. The kinetics of nitration in aqueous sulphuric acid of durene (1,2,4,5-tetramethylbenzene), nitrodurene, and nitroprehnitene (nitro-1,2,3,4-tetramethylbenzene). A comparison of the rates of nitration of methylnitrobenzenes in aqueous sulphuric acid with the corresponding rates from mixing–disguised nitrations with nitronium hexafluorophosphate in nitromethane based on a theoretical mixing–reaction model