Friedel-Crafts alkylation ortho effect

Steric effects play a major role in determining the ortho para ratio in Friedel-Crafts alkylations. The amount of ortho substitution of toluene decreases as the size of the entering alkyl group increases along the series methyl, ethyl, /-propyl. No ortho product is found when the entering group is /-butyl. ... 

Friedel-Crafts alkylation. Reaction of phenylacetaldehyde (la) with iodotri-methylsilane effects an intramolecular ortho-alkylation to give the cyclic ether 2a.5... 

The ethylation of toluene by diethylhalonium ions gives ethyltoluenes with ortho para isomer ratios between 0.60 and 0.96. The ortho para isomer ratios obtained for the alkylation of toluene in conventional Friedel-Crafts ethylations range from 1.17 to 1.84 (average 1.60). Such differences are considered to be due to the steric ortho effect caused by diethylhalonium ions, and are in accordance with the most probable displacement reaction on the bulky diethylhalonium ions by the aromatic substrate. This can be envisioned to proceed through an SN2-type transition state involving no free alkyl cations [Eq. (5.109)]. 

One exception to the amorphous structure has been reported [30]. Crystalline polybenzyl was obtained from the low temperature (- 125° C) polymerization of benzyl chloride. However, the reaction was difficult to reproduce [31,32]. Consequently this procedure is not an effective method for the synthesis of linear polybenzyls. The usual amorphous, highly branched structure is formed as a result of a lack of positional selectivity and multiple substitution of the arene rings. Similar polymeric structures are obtained upon the polymerization of other nonsubstituted benzyl halides and benzyl alcohol [29]. The highly branched structure is a consequence of the involvement of benzyl carbenium ions in the Friedel-Crafts reaction. Benzyl substituents activate the monosubstituted phenyl groups toward further benzylation reaction. However, monomers containing alkyl substituents that sterically hinder substitution at the ortho position have been polymerized to linear polybenzyls. For example, the following... 

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 ... 

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