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. ... 

Side-chain bromination at the benzylic position occurs when an alkylbenzene is treated with /V-bromosuccinimide (NBS). For example, propylbenzene gives (l-bromopropyl)benzene in 97% yield on reaction tvith NBS in the presence of benzoyl peroxide, (PhC02)2f as a radical initiator. Bromination occurs exclusively in the benzylic position and does not give a mixture of products. 

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... 

Smith, K., Fry, K., Butters, M. and Nay, B. Para-selective mononitration of alkylbenzenes under mild conditions by use of benzoyl nitrate in the presence of a zeolite catalyst, Tetrahedron Lett., 1989, 30, 5333-5336. 

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... 

Decomposition of alkylbenzene Ph-Me, Ph-Et, Ph- Pr Friedel-Crafts acylation acetylation, benzoylation, and so on Isomerization of paraffin open-chain C1-C7, cyclic Cs-Cn Esterification -> AcOH + MeOH, EtOH, and so on CgOH + phthalic acid, and so on Cationic polymerization Me, Et, Bu vinyl ether Oligomerization 3-pinene, 1-octene, 1-decene Others -> aldol condensation, and so on... 

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%). ... 

The activity of some metals such as indium, zinc, and iron as efficient catalysts in Friedel-Crafts acylahon is well known, even if nof clearly explained. For example, indium metal (20% mol) shows high efficiency in the acylation of various aromafic compounds wifh BC. As an example, the indium-mediated benzoylation of aromafic compounds with electron-donating groups gives the corresponding diaryl ketones in high yield (Table 3.13). Alkylbenzenes are less reactive, and with mesitylene, the yield is 77% at 150°C. Deactivated benzenes such as chloro- and bro-mobenzene are inactive. 

Table 4.3 Comparison of the activity of FeCl3 and FeY in the benzoylation of alkylbenzenes with BC...

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. ... 

Orientation of the incoming acyl group in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables. In general, however, para attack predominates for alkylbenzenes. The percentage of ortho attack increases with the electrophilicity of the acylium ion and approaches 1 1 with such less selective species as formyl and 2,4-dinitrobenzoyl ions. For simple alkanoyl and benzoyl halides, the o p ratio is usually 1 20 or higher. ... 

---