JEE Main 2026 — Biomolecules Question with Solution

Step 1: The starting material is an aldohexose (like D-glucose). On prolonged heating with $HI$, it undergoes reduction to form n-hexane. This indicates that all six carbon atoms are linked in a straight chain.
$\text{CHO}-(\text{CHOH}{)}_4-{\text{CH}}_2\text{OH}\stackrel{\text{HI, }\Delta }{\to }{\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$ (n-hexane)

Step 2: n-Hexane, when passed over heated $V_2{O}_5$ at $773\text{ K}$ and $10-20\text{ atm}$ pressure, undergoes aromatization (dehydrogenation and cyclization) to form benzene.
$\text{n-hexane}\stackrel{V_2{O}_5,\text{ 773 K, 10-20 atm}}{\to }{\text{C}}_6{\text{H}}_6$ (Benzene)

Step 3: Benzene reacts with benzoyl chloride (${\text{C}}_6{\text{H}}_5\text{COCl}$) in the presence of anhydrous ${\text{AlCl}}_3$ to form benzophenone. This is a Friedel-Crafts acylation reaction.
${\text{C}}_6{\text{H}}_6+{\text{C}}_6{\text{H}}_5\text{COCl}\stackrel{{\text{Anhyd. AlCl}}_3}{\to }{\text{C}}_6{\text{H}}_5-\text{CO}-{\text{C}}_6{\text{H}}_5$ (Benzophenone)

Step 4: Analyze the product (X), which is benzophenone.
The structure of benzophenone consists of two phenyl rings attached to a carbonyl group.
- Each phenyl ring contains $3$ $\pi$ bonds, so two rings have $6$ $\pi$ bonds.
- The carbonyl group ($\text{C=O}$) contains $1$ $\pi$ bond.
- Total number of $\pi$ bonds = $6+1=7$.
- Total number of $\pi$ electrons = $7\times 2=14$.
- The oxygen atom in the carbonyl group has $2$ lone pairs of electrons.
- Total number of lone pair electrons = $2\times 2=4$.

Total number of electrons in the $\pi$ bonds and lone pairs = $14+4=18$.

Answer: $18$