Question

Which alkene can be used for the synthesis of propanal exclusively using ozonolysis?

• A. Hex-3-ene
• B. Trans-hex-2-ene
• C. Cis-hex-2-ene
• D. Pent-2-ene

Hint:

For a single clean ozonolysis product, the double bond must sit at the center of a symmetric alkene.

Answer 1

The Correct Option is A

Step 1: Ozonolysis cleaves a carbon-carbon double bond and replaces it with two carbonyl groups, converting the alkene into two separate carbonyl compounds at the position of the original double bond. Step 2: For the product to be propanal, $CH_3CH_2CHO$, exclusively as a single product rather than a mixture, both fragments produced by cleavage must be identical, which means the double bond must lie exactly at the center of a symmetric alkene. Step 3: Consider hex-3-ene: $CH_3-CH_2-CH=CH-CH_2-CH_3$. The double bond is between C3 and C4, which is the exact center of the six-carbon chain. Step 4: Ozonolysis cleaves between C3 and C4, giving two identical three-carbon fragments, each of which is propanal. \[ CH_3CH_2CH=CHCH_2CH_3 \xrightarrow[Zn/H_2O]{1.\ O_3} 2\ CH_3CH_2CHO \] Step 5: In contrast, hex-2-ene (cis or trans) has its double bond between C2 and C3, off-center, so ozonolysis gives a mixture of acetaldehyde and butanal, not propanal exclusively. Similarly, pent-2-ene gives a mixture of acetaldehyde and propanal, not propanal alone. \[\boxed{\text{Hex-3-ene}}\]