Question

Considering the above reaction, the major product among the following is :

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Clemmensen reduction (Zn/HCl) converts the ketone to an alkane; \(\mathrm{Cr_2O_3}\), 773 K, 10-20 atm then aromatises a chain of six or more carbons (hydroforming).

Answer 1

The Correct Option is A

The ketone is 4-ethylhexan-3-one, \(\mathrm{(C_2H_5)_2CH-CO-CH_2CH_3}\), with 8 carbon atoms.
Step 1: Zn/HCl (Clemmensen reduction) converts \(\mathrm{C=O}\) to \(\mathrm{CH_2}\): \(\mathrm{(C_2H_5)_2CH-CH_2CH_2CH_3}\) = 3-ethylhexane.
Step 2: \(\mathrm{Cr_2O_3}\), 773 K, 10-20 atm causes aromatisation (dehydrocyclisation) of alkanes having six or more carbons in a chain.
The longest chain of 3-ethylhexane has 6 carbons (C1 to C6); C1 and C6 join to form the six-membered ring and 4 \(\mathrm{H_2}\) are lost, giving benzene with the ethyl group on C3.
Product: ethylbenzene.
Answer: (1) ethylbenzene