 Solution

ID: past-exam-of-the-chemistry-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2023/paper-1/1/b/ii/solution

C gives spectrum 3. For allyl acetate,

C H_{3} C (= O) OC H_{2} CH = C H_{2}

, assign

δ_{C} (C = O) = 170.6, δ_{C} (COC H_{3}) = 20.9, δ_{C} (OC H_{2}) = 65.1, δ_{C} (CH =) = 132.3, δ_{C} (= C H_{2}) = 118.1 ppm .

The 65.1-ppm resonance is the oxygen-bound methylene. Its attached proton test phase matches the terminal alkene methylene and the nonprotonated carbonyl, while the alkene CH and acetyl methyl have the opposite phase. The carbonyl is not conjugated with the alkene because oxygen and a saturated methylene interrupt the carbon–carbon conjugated path. This explains its higher carbonyl shift relative to B without requiring the two esters to have identical electronic environments.

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