Which of the following molecules is an ester?

• A. ${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}{2}_{\mathrm{}}$
• B. ${\mathrm{CH}}_3-\mathrm{CH}=O$
• C. ${\mathrm{CH}}_3-\mathrm{COOH}$
• D. ${\mathrm{CH}}_3-\mathrm{COO}-{\mathrm{CH}}_3$

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the functional group of an ester. An ester has the general structure \(\mathrm{R{-}COO{-}R'}\), where a carbonyl group (\(\mathrm{C=O}\)) is directly bonded to an oxygen atom that is also bonded to another carbon chain.
2. Step 2: Analyze each given molecule to identify their functional groups. For example, check if the molecule contains a carbonyl group (\(\mathrm{C=O}\)) adjacent to an oxygen atom connected to another carbon.
3. Step 3: The first molecule, \(\mathrm{CH_3{-}CH=CH_2}\), is an alkene because it contains a carbon-carbon double bond but no carbonyl or ester group.
4. Step 4: The second molecule, \(\mathrm{CH_3{-}CH=O}\), is an aldehyde because it has a carbonyl group at the end of the chain but no oxygen linking to another carbon.
5. Step 5: The third molecule, \(\mathrm{CH_3{-}COOH}\), is a carboxylic acid, characterized by the \(\mathrm{-COOH}\) group, not an ester. The correct ester structure is \(\mathrm{CH_3{-}COO{-}CH_3}\), where the carbonyl carbon is bonded to an oxygen that is bonded to another carbon.