Why is the para position preferred more than ortho in the electrophilic substitution reactions of haloarenes?
Is it really? Always?
In an older article (J. Chem. Educ., 1988, 65, 367), the authors described an experiment that demonstrates the outcome of the alkylation of chlorobenzene (1) with 2-chloropropane (2) under kinetic control.
They first added 25 mL of 1 and 2 mL of nitromethane to 1.2 g anhydrous aluminium chloride, and stirred until the aluminium chloride was fully dissolved. To the cooled mixture (3-5 °C), a solution of 3 mL 2 in 20 mL of 1 was dropwise added within 10 min, while keeping the temperature of the reaction mixture below 10 °C. The reaction was stirred for another hour, allowed to come to room temperature.
After workup, gas chromatographic analysis showed that the o/m/p ratio for the resulting isopropyl chlorobenzenes was 63/5/32.
To cut the long story short, under the conditions describe above, the ortho isomer prevailed.