Reaction of methyl triphenylbenzoate with concentrated sulphuric acid

In the following reaction, compound Q is obtained from compound P via an ionic intermediate.


[![enter image description here](https://i.stack.imgur.com/CX2GO.png)](https://i.stack.imgur.com/CX2GO.png)


What is the degree of unsaturation of Q?


This question was asked in [JEE ADV 2020](https://www.jeeadv.ac.in/past_qps/2020_1_English.pdf)




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I thought that the ester would undergo hydrolysis to form a carboxylic acid but the product Q is
[![enter image description here](https://i.stack.imgur.com/Cw2du.png)](https://i.stack.imgur.com/Cw2du.png)


which has degree of unsaturation = 18


Does this happen because of the phenyl groups?


**What is the mechanism for this reaction and does this have a name?**




The conditions are concentrated sulfuric acid so there is no water present to hydrolyse the ester. If it was dilute sulfuric then you would get the hydrolysis. This reaction proceeds by protonation of the ester group, loss of MeOH creating R-C=O+ cation which then does an electrophilic attack on an adjacent Ph group. This is generally called acid-catalysed aromatic acylation.


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