Friedel-Crafts acylation steric effects

Q The Friedel-Crafts acylation of alkylated benzenes by [PhCO + ][SbF6-] (reaction 3.13) shows the following percentage distributions of isomers. As the number of methyl substituents on the ct-carbon atom is increased, the proportion of ortho isomer decreases. Is this likely to be an electronic or a steric effect ... 

Both groups are weakly activating, but the position ortho to the methyl group is less sterically hindered than the position ortho to the isopropyl group. The Friedel-Crafts acylation reaction is very sensitive to steric effects. 

Answer We see that we need to install an acyl group in the ortho position. If we just perform a Friedel-Crafts acylation, we would expect the para product to be the major product (because of steric effects). So, we must perform a sulfonation reaction to block the para position. Our answer is ... 

The experiments in Sections 15.2 and 15.3 illustrate the Friedel-Crafts alkylation and acylation of aromatic hydrocarbons, respectively. A complication of Friedel-Crafts reactions is apparent in the alkylation experiment, wherein rearrangements of the carbo-cations generated from the alkyl halide provide mixtures of substitution products. The acylation reaction of Section 15.3 provides an example of how a combination of electronic and steric effects can affect the orientation of electrophilic attack on an aromatic ring. 

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