Benzoyl chloride alkylbenzenes benzoylation

Steric fectors clearly enter into determining the ortho para ratio. The hindered 2,4,6-trimethylbenzoyl group is introduced with a 50 1 preference for the para position. Similarly, in the benzoylation of alkylbenzenes by benzoyl chloride-aluminum chloride, the amount of ortho product decreases (10.3%, 6.0%, 3.1%, 0.6%, respectively) as the branching of the alkyl group is increased along the series methyl, ethyl, i-propyl, t-butyl. ... 

The acylation of alkylbenzenes with benzoyl chloride and a catalytic amount of per-fluorobutanesulfonic acid affords the corresponding alkyl-substituted benzophe-nones with unusually high amounts of ortho isomeric products (up to 30%) 38... 

Previously, the most widely used method for preparation of 4-alkyl-benzoyl chlorides on a laboratory scale has been from the benzoic acids obtained by oxidation of aromatic ketones, usually 4-alkylacetophe-nones. 14 The latter are usually prepared by acylating alkylbenzenes. Although this sequence gives high yields, it is lengthy (three completely separate steps) and the scale is restricted in the second step because of the large volumes required. The submitters state that they were unable to repeat the reported alkylation of toluic acid.15 Methods that lead to formation of ortho and para isomeric intermediates are inconvenient since they require that the isomers be separated.16"19... 

The mono- and poly-alkylated benzenes are treated using modifications of the above procedure. Monoalkylbenzenes are added to a preformed complex of acyl halides and aluminum chloride in carbon tetrachloride (Perrier modification). In this manner, the manipulation is easier, no tars are encountered, and the yields are improved (85-90%). The procedure shows no advantage, however, in the acylation of alkoxy- or chloro-aromatic compounds. The addition of benzoyl chloride to p-alkylbenzenes in the presence of aluminum chloride in cold carbon disulfide is a good procedure for making p-alkylbenzophenones (67-87%). The condensation of homologs of benzene with oxalyl chloride under similar conditions yields p,p -di alkylbenzophenones (30-55%). Polyalkylbenzenes have been acylated with acetic anhydride and aluminum chloride (2.1 1 molar ratio) in carbon disulfide in 54-80% yields. Ferric chloride catalyst has been used under similar conditions. Acetylation of p-cymene with acetyl chloride and aluminum chloride in carbon disulfide yields 2-methyl-5-isopropylaceto-phenone (55%). ... 

One other feature of the data in Table 10.10 is worthy of further comment. Notice that alkyl (acetyl, propionyl) substituted acylium ions exhibit a smaller o p ratio than the various aroyl systems. If steric factors were dominating the position selectivity, one would expect the opposite result. A possible explanation for this feature of the data could be that the aryl compounds are reacting via free acylium ions, whereas the alkyl systems may involve more bulky acid chloride-catalyst complexes. Steric factors clearly enter into determining the o p ratio. The hindered 2,4,6-trimethylbenzoyl group is introduced with a 50 1 preference for the para position. Similarly, in the benzoylation of alkylbenzenes by benzoyl chloride-aluminum chloride, the amount of ortho product decreases (10.3%, 6.0%, 3.1%, 0.6%, respectively) as the branching of the alkyl group is increased along the series methyl, ethyl, 2-propyl, t-butyl. ... 

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