If the compound shown below is the product of a Fischer esterification, which of the following compounds is the starting reagent with oxygen-18 isotopic substitution for this reaction?

Question

Quizplus · Accepted Answer

11ecba2d_1283_1d41_a3bf_058ffc356368_MD0008_00 An atom in a compound can be substituted with an isotope, which can be "followed" in a reaction. This procedure is called labeling. The final position of the isotopic label in the reaction product gives information about how the reaction occurs. Esters are carboxylic acid derivatives that can be formed under acidic conditions through a Fischer esterification using a carboxylic acid and an alcohol. The reaction begins with protonation of the carbonyl oxygen, followed by nucleophilic attack of the carbonyl carbon by the alcohol. A proton is then transferred from the alcohol oxygen to the hydroxyl group of the carboxylic acid, resulting in water (a good leaving group). Lastly, the carbonyl oxygen is deprotonated to yield an ester. The ester given in the question is formed from butanoic acid and methanol. The isotopically labeled oxygen (¹⁸O) in the ester is bonded to the carbonyl carbon and a methyl (−CH₃) group. Because the ester oxygen atom bonded to the carbonyl carbon and the alkyl group (methyl, in this instance) always originates from the alcohol reagent, the labeled starting material must be the alcohol. Therefore, the ¹⁸O originates from ¹⁸O-labeled methanol (CH₃−¹⁸OH). (Choice B) Although it is correct that the ¹⁸O in the ester comes from an alcohol, this structure has too many carbon atoms. The four-carbon chain in the ester comes from a carboxylic acid (butanoic acid) rather than an alcohol. (Choice C) It is true that the four-carbon chain originates from a carboxylic acid, but the ¹⁸O in the ester comes from the alcohol rather than the carboxylic acid. (Choice D) A carboxylic acid is involved in a Fischer esterification, but this structure has the incorrect number of carbon atoms, and the ¹⁸O in the ester comes from the alcohol starting reagent. Educational objective: Esters are carboxylic acid derivatives that can be formed from a carboxylic acid and an alcohol through a Fischer esterification. The ester oxygen bonded to both the carbonyl carbon and the alkyl group originates from the alcohol, a fact that can be confirmed by isotopically labeling the hydroxyl oxygen of the alcohol starting reagent.

If the Compound Shown Below Is the Product of a Fischer