Which compound would give 3-methyl-6-oxoheptanal upon ozonolysis?

Solution

Chemistry ma’am · 3 steps

Join the two carbonyl carbons back into a C=C, then see where the second methyl sits on the ring.

1. What ozonolysis does. Ozone cuts the C=C and turns each alkene carbon into a C=O. In a ring, this opens the ring into one chain with a carbonyl at each end. An alkene carbon carrying CH₃ becomes a ketone (-CO-CH₃), and one carrying H becomes -CHO.
2. Rebuild the ring. 3-methyl-6-oxoheptanal is OHC-CH₂-CH(CH₃)-CH₂-CH₂-CO-CH₃. Join C1 (the CHO carbon) and C6 (the ketone carbon) with a double bond to get a six-membered ring. The ring methyl sits at C3, two carbons away from the CH= end, which puts it directly across from the methyl-bearing alkene carbon. That is 1,4-dimethylcyclohexene.

   Option	Alkene	Ozonolysis product
   (a)	1,4-dimethylcyclohexene	OHC-CH₂-CH(CH₃)-CH₂-CH₂-CO-CH₃, 3-methyl-6-oxoheptanal
   (b)	1,6-dimethylcyclohexene	OHC-CH₂-CH₂-CH₂-CH(CH₃)-CO-CH₃, 5-methyl-6-oxoheptanal
   (c)	1,3-dimethylcyclohexene	OHC-CH(CH₃)-CH₂-CH₂-CH₂-CO-CH₃, 2-methyl-6-oxoheptanal
   (d)	1,5-dimethylcyclohexene	OHC-CH₂-CH₂-CH(CH₃)-CH₂-CO-CH₃, 4-methyl-6-oxoheptanal

3. So. Only option (a), with the two methyls para to each other on the ring, opens up to give 3-methyl-6-oxoheptanal.

The idea to keep. To find the alkene from its ozonolysis product, erase the two C=O oxygens and join those carbons with a double bond. A keto-aldehyde from one molecule always means a ring alkene.

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