Which of the following is an example of conjugated diketone?

Solution

Chemistry ma’am · 3 steps

Walk from one C=O to the other: if every carbon on the way is sp², the two carbonyls are conjugated.

1. What conjugated means. Two C=O groups are conjugated with each other when double and single bonds alternate all the way between them. A single sp³ carbon (a CH₂ or CH) in the path breaks the overlap of p orbitals.
2. Check each path. Trace the shortest chain of carbons joining the two carbonyl carbons in each option. Then look for any sp³ carbon on it.

   Option	Path between the two C=O	sp³ carbons in the path	Conjugated diketone?
   (a) heptane-2,6-dione	C(=O)-CH₂-CH₂-CH₂-C(=O)	3	No
   (b) 4-(2-oxopropyl)cyclohexanone	ring C(=O)-CH₂-CH₂-CH-CH₂-C(=O)	4	No
   (c) p-benzoquinone	C(=O)-CH=CH-C(=O)	0, all sp²	Yes
   (d) 1-phenylpentane-1,4-dione	C(=O)-CH₂-CH₂-C(=O)	2	No

3. So. Only in p-benzoquinone does the system O=C-C=C-C=O alternate without a break, so (c) is the conjugated diketone.

The idea to keep. Conjugation needs an unbroken run of alternating double and single bonds. One sp³ carbon in between is enough to cut it, however close the two groups look.

Answer from the NTA final answer key. Solution by PrepWiser.