Which of the following compounds is the hemiacetal formed when $\mathrm{ethanol}$ reacts with $\mathrm{propanal}$?

• A. ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OCH}{2}_{\mathrm{}}\mathrm{CH}{3}_{\mathrm{}}\right)\mathrm{CHO}$
• B. ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OCH}{2}_{\mathrm{}}\mathrm{CH}{3}_{\mathrm{}}\right)\mathrm{CH}\left(\mathrm{OCH}{2}_{\mathrm{}}\mathrm{CH}{3}_{\mathrm{}}\right)$
• C. ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{CH}\left(\mathrm{OC}{H}_2\mathrm{CH}{3}_{\mathrm{}}\right)$
• D. ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{CH}\left(\mathrm{OH}\right)$

Answer: (C)

Verified step by step guidance

1. Identify the reactants: ethanol (CH3CH2OH) and propanal (CH3CH2CHO). Propanal is an aldehyde with the carbonyl group at the end of the chain.
2. Recall that a hemiacetal forms when an alcohol adds to the carbonyl carbon of an aldehyde or ketone. The nucleophilic oxygen of the alcohol attacks the electrophilic carbonyl carbon, leading to the formation of a tetrahedral intermediate.
3. In the hemiacetal, the original carbonyl carbon is bonded to both an -OH group and an -OR group (where R is the alkyl group from the alcohol). Specifically, the carbon will have one hydroxyl (-OH) and one alkoxy (-OCH2CH3) substituent.
4. Draw the structure of propanal with the carbonyl carbon now bonded to an -OH group and an -OCH2CH3 group, while the rest of the molecule remains unchanged (the methyl group and the methylene group attached to the carbonyl carbon).
5. Compare the drawn structure to the given options and select the one that shows the carbon bonded to both -OH and -OCH2CH3 groups, confirming it as the hemiacetal formed from ethanol and propanal.