Rates of solvolysis of p-dimethylamino benzyl halides
In the second compound, steric repulsions (denoted by the curly lines) between the N-methyl groups and the methyl groups on the ring occur:



This forces the $\ce{-NMe2}$ group to rotate such that the nitrogen lone pair is no longer parallel to the ring's $\pi$ system. As a result, the nitrogen lone pair can no longer contribute to the mesomeric stabilisation of the second carbocation.

The rate of solvolysis therefore increases in the order: $2 &lt; 1 &lt; 3$. The effect described above is called "steric inhibition of resonance".

A similar effect is observed in azo coupling:



In the second reaction, the nitrogen lone pair is not delocalised into the ring and the ring is therefore not nucleophilic enough to attack the diazonium ion.