Which of the following molecules is classified as an ester?

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

Answer: (B)

Verified step by step guidance

1. Identify the functional groups present in each molecule by analyzing their structural formulas.
2. Recall that an ester functional group 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-containing group.
3. Examine the first molecule \(\mathrm{CH_3{-}CH=O}\), which contains an aldehyde group (a carbonyl bonded to a hydrogen and a carbon chain). This is not an ester.
4. Look at the second molecule \(\mathrm{CH_3{-}COO{-}CH_3}\), which has a carbonyl group adjacent to an oxygen atom bonded to another carbon group, matching the ester functional group pattern.
5. Check the remaining molecules: \(\mathrm{CH_3{-}COOH}\) is a carboxylic acid (carbonyl plus hydroxyl group), and \(\mathrm{CH_3{-}OH}\) is an alcohol (hydroxyl group attached to carbon). Neither is an ester.