JEE Advanced 2020 · Paper 1 · Chemistry Q18

← JEE Advanced 2020 · Paper 1

ChemistryNumerical+4Question 36 of 54

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

What is the degree of unsaturation of Q?

P, methyl 2,4,6-triphenylbenzoate (a benzene ring carrying CO₂CH₃ with C₆H₅ groups at both ortho positions and the para position), is treated with conc. H₂SO₄ to give Q (a colored compound).

Your answer

Answer and worked solution

Official answer: 18

Ionic intermediate. The ester is flanked by two phenyl groups. In conc. $\ce{H2SO4}$ it loses methanol to give the acylium ion $\ce{ArC#O+}$ .

Cyclisation. The acylium ion attacks the neighbouring ortho phenyl ring (intramolecular Friedel-Crafts acylation). This closes a five-membered ring and gives 1,3-diphenylfluoren-9-one, which is yellow.

Degree of unsaturation of Q, $\ce{C25H16O}$ :
$\frac{2(25)+2-16}2=18.$
As a check: the fluorenone core has 10 (3 rings + 6 C=C + 1 C=O) and each phenyl has 4.

Answer: 18

Topics:Aromatic hydrocarbons and electrophilic substitution Reactive intermediates

← Previous question Next question →

JEE Advanced 2020 · Paper 1, Chemistry question 18