O- Substituted benzene derivative and aliphatic derivative do not obey the Hammett equation. Why?

When a reaction occurs at a position ortho to the substituent inside polar effect, steric effect also operates owing to the close position of the reaction centre and the substituent, the steric interaction takes place. m- and p- position being far away from reaction centre, the m- and p- substituent doesn’t have any steric effect on it. They have polar influence on it. Owing to the steric effect, o-substituted benzene derivative do not show the linear relationship i.e., Hammett equation.
The aliphatic derivative do not obey the Hammett equation because C- chains are very flexible and have zigzag structure. The substituents experience steric interaction to the reaction centre even it is far away from the substituent. Thus, like o- substituted benzene derivative, aliphatic derivative also do not obey the Hammett equation.

Note: 

The logarithm of the equilibrium constant of different reaction have a straight line, i.e., linear relationship. This implies that the substituent except their polar effect through changes of Potential Energy and no steric and entropies changes occur. Hence, Hammett equation is referred to have a Linear Free Energy Relationship (LFER).

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