Friedel-Crafts alkylation ortho/para selectivity

Toluene-benzene reactivity ratios under a number of Friedel-Crafts conditions are recorded in Table 9.9. As would be expected on the basis of the low substrate selectivity, position selectivity is also modest. The amount of ortho product is often comparable to the para product. Steric effects play a major role in determining the o p ratio in Friedel-Crafts alkylations. The amount of ortho substitution of toluene... 

A study of alkylations with a group of substituted benzyl halides and a range of Friedel-Crafts catalysts has provided insight into the trends in selectivity and orientation that accompany changes in both the alkyl group and the catalysts. There is a marked increase in substrate selectivity on going from / -nitrobenzyl chloride to /i-methoxybenzyl chloride. For example, with titanium tetrachloride as the catalyst, Aitoi Abenz increases from 2.5 to 97. This increase in substrate selectivity is accompanied by an increasing preference for para substitution. With /i-nitrobenzyl chloride, the ortho para ratio is 2 1 (the... 

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