Ozonolysis of an unknown organic compound gives formaldehyde (HCHO) as one of the carbonyl products. This observation confirms the presence of which structural unit in the original molecule?

Introduction / Context:
Ozonolysis is a powerful reaction in organic chemistry used to determine the position of carbon–carbon double bonds in alkenes. When an alkene is treated with ozone followed by a suitable workup, the double bond is cleaved, and carbonyl compounds such as aldehydes or ketones are formed. By analysing these products, chemists can deduce the original structure of the alkene. This question focuses on the special case where formaldehyde, HCHO, appears as one of the products, and asks what structural fragment must have been present in the original compound.

Given Data / Assumptions:

• The organic compound undergoes ozonolysis and produces formaldehyde as one of the products.
• Formaldehyde has the formula HCHO, with one carbon atom and one hydrogen directly attached to the carbonyl carbon.
• Ozonolysis of an alkene splits the carbon–carbon double bond and attaches an oxygen atom to each carbon, forming carbonyl compounds.
• We assume the original compound contains at least one carbon–carbon double bond.

Concept / Approach:
When an alkene with a terminal vinyl group –CH=CH2 is cleaved by ozonolysis, the terminal carbon that has two hydrogens becomes formaldehyde, HCHO. For example, CH2=CH–R yields formaldehyde from the CH2 end and an aldehyde or ketone from the other end. Internal double bonds, where neither double bond carbon has two hydrogens attached, cannot produce formaldehyde because they lead to larger aldehydes or ketones. Acetylenic triple bonds behave differently and are not cleaved to give formaldehyde in the same way. Hence, the presence of formaldehyde points strongly to a terminal vinyl group in the original structure.

Step-by-Step Solution:
Step 1: Recall that ozonolysis of an alkene converts the carbon–carbon double bond into two carbonyl groups, one on each carbon of the original double bond. Step 2: For formaldehyde, HCHO, to form, one of the original double bond carbons must become a one carbon aldehyde with two hydrogens attached to that carbon. Step 3: The only way this can happen is if, before ozonolysis, that carbon was part of a –CH2 group at a terminal position in the double bond, as in –CH=CH2 or CH2=CH– fragment. Step 4: Ozonolysis of a compound containing a vinyl group CH2=CH–R leads to HCHO from the terminal CH2 carbon and another carbonyl compound from the CH–R side. Step 5: Therefore, the appearance of formaldehyde as a product confirms that the original molecule contained a terminal vinyl group.

Verification / Alternative check:
Consider a specific example: propene, CH3–CH=CH2. Ozonolysis of propene yields formaldehyde (from the terminal CH2 carbon) and acetaldehyde (from the CH–CH3 carbon). Likewise, any longer chain alkene with a terminal CH2 group at the double bond end will produce formaldehyde upon ozonolysis. In contrast, internal alkenes such as but 2 ene, CH3–CH=CH–CH3, yield acetaldehyde only and no formaldehyde. Triple bonds, such as in acetylenes, undergo oxidative cleavage to give carboxylic acids or carbon dioxide rather than formaldehyde under typical conditions. These examples support the rule that formaldehyde as a product signals a terminal vinyl group in the starting alkene.

Why Other Options Are Wrong:
Option A, an acetylenic carbon–carbon triple bond, does not typically give formaldehyde as a major ozonolysis product under standard conditions, and triple bonds behave differently from double bonds. Option B, two ethylenic double bonds, could give various carbonyl fragments, but the key feature needed for formaldehyde is a terminal CH2 group on a double bond. Option D, an isopropyl group, is a saturated branched group and does not itself contain the required terminal double bond. Option E, an internal double bond, cannot produce formaldehyde because neither carbon of an internal double bond is CH2 with two hydrogens attached. Only option C correctly identifies the presence of a terminal vinyl group as the structural unit indicated by formaldehyde formation on ozonolysis.

Common Pitfalls:
Students sometimes memorise that ozonolysis gives aldehydes and ketones but do not link specific products to specific structural fragments. Another mistake is to assume that any double bond might produce formaldehyde, without considering the number of hydrogens on each carbon. To avoid these errors, always map each carbon in the double bond to its possible carbonyl product, keeping track of hydrogen attachments. Remember that a CH2 carbon in a double bond is the only one that can become the one carbon aldehyde formaldehyde, HCHO, upon ozonolysis.

Final Answer:
The presence of formaldehyde as an ozonolysis product confirms that the original molecule contained A terminal vinyl group (–CH=CH2) in the molecule.