Ortho vs para alkylation of chlorobenzene
In general para isomera are thermodynamically more stable than ortho isomers (except in cases when ther is hydrogen bonding in ortho isomer). There are several reasons:-
1. Steric hindrance makes para isomer more stable.
2.Benzene substituents rotate and change thier plane due to steric hindrance. It stops delecolaization of lone pair of electrons (if present in substituents) causing elevation in energy.
Both proper orientation (steric hindrance) and appropriate energy (temperature) is required for formation of product.
Para isomer satisfies these conditions readily and hence are major product.