Friedel–Crafts reactions. Part XXV. Acetylation and benzoylation of iodobenzene and of o-, m-, and p-iodotoluenes
The Friedel–Crafts acetylation and benzoylation of iodobenzene and the isomeric iodotoluenes have been investigated in a range of solvents and at a range of temperatures. Under specific conditions good yields of p-iodoacetophenone or p-iodobenzophenone may be obtained. The reactivities of the iodotoluenes follow the sequence p- < o- < m-. The main products formed in the acylation of o-iodotoluene are 3-iodo-4-methyl- and 4-iodo-3-methyl-acetophenones (or -benzophenones). m-Iodotoluene generally affords acetophenones or benzophenones in the order 4-iodo-2-methyl- > 2-iodo-4-methyl- > 2-iodo-6-methyl. In acylations p-iodotoluene gives very-low yields of 3-iodo-6-methylacetophenone only, or a mixture of 3-iodo-6-methyl- and 2-iodo-5-methyl-benzophenones; rearranged ketones, those formally derived from o-iodotoluene, are also formed. In addition all the iodo-substrates give by-products in major amounts: di-iodobenzenes, or di- and tri-iodotoluenes; aceto- or benzophenone, or the isomeric methylaceto- or methylbenzo-phenones. Acylations of o- and p-iodotoluenes also give isomeric chloromethylaceto-(or chloromethylbenzo-)phenones. Benzoylations carried out in ethylene chloride solution give low yields of 2-chloroethyl benzoate.