Final answer:
The smallest ester molecule must contain at least three carbon atoms, forming methyl acetate (methyl ethanoate), since both the R and R' groups bonded to the oxygen must be carbon chains.
Step-by-step explanation:
The smallest molecule containing an ester functional group has to have at least three carbon atoms. This is because the ester is made of a carbonyl group (a carbon oxygen double bond) attached to another oxygen atom that is itself bonded to a carbon (R-CO-O-R'). For the smallest ester, we can have a methyl group (CH3) on both sides, resulting in methyl acetate, or methyl ethanoate in IUPAC nomenclature (CH3-COO-CH3).
Remember, if the 'R' group were a hydrogen, the compound would be a carboxylic acid instead of an ester. Therefore, the ester functional group requires a carbon chain on both sides of the oxygen that is bonded to the carbonyl carbon.