Rates of solvolysis of p-dimethylamino benzyl halides

What will be the order of $\mathrm{S\_N1}$ solvolysis rates for these?


[![Compounds](https://i.stack.imgur.com/gQ3cm.png)](https://i.stack.imgur.com/gQ3cm.png)


I'm confused between the second and third options. $\ce{I-}$ is a better leaving group than $\ce{Br-}$, but the carbocation formed in **2** is more stable than **3**. So how are we supposed to decide between them?




In the second compound, steric repulsions (denoted by the curly lines) between the *N*-methyl groups and the methyl groups on the ring occur:


[![Steric clashes](https://i.stack.imgur.com/UVkJo.png)](https://i.stack.imgur.com/UVkJo.png)


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: $2 < 1 < 3$. The effect described above is called *"steric inhibition of resonance"*.


A similar effect is observed in azo coupling:


[![Azo coupling](https://i.stack.imgur.com/GhnrG.png)](https://i.stack.imgur.com/GhnrG.png)


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.


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