OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 9

OCR A-Level Chemistry • Module 6: Organic Chemistry & Analysis

This multiple-choice question tests your understanding of organic synthetic routes to form esters, specifically focusing on:

Ester dissection: Identifying the acyl part (from the carboxylic acid derivative) and the alkoxy/aryloxy part (from the alcohol or phenol).
Reactivity of phenols vs aliphatic alcohols: Recognizing that phenols are not reactive enough to undergo direct esterification with carboxylic acids.
Acyl chlorides in ester synthesis: Knowing that acyl chlorides (or acid anhydrides) are required to react with phenols to synthesize phenyl esters.

Reagents: Propanoyl chloride ( CH₃CH₂COCl ) and Phenol ( C₆H₅OH )

Mark Scheme: 1 mark for selecting D.

Structure of the target: The target molecule is phenyl propanoate ( CH₃CH₂COOC₆H₅ ). The 3-carbon chain is part of the acyl group, and the benzene ring is attached via the ester oxygen (phenolic oxygen).
Phenol is less nucleophilic: The lone pair on the oxygen atom of phenol is partially delocalized into the aromatic π-system, reducing its nucleophilic strength compared to aliphatic alcohols.
Reaction barrier: Phenol will not react directly with carboxylic acids under standard reflux with an acid catalyst ( H₂SO₄ ).
The solution: A much more reactive acylating agent must be used—an acyl chloride (e.g., CH₃CH₂COCl ) or an acid anhydride.

Locate the ester link: The ester group is –C(=O)–O– .
Identify the carbonyl side (acyl donor): The carbonyl group –C(=O)– is bonded to CH₃CH₂– , giving a 3-carbon propanoyl backbone ( CH₃CH₂CO– ).
Identify the single-bonded oxygen side: The oxygen atom –O– is directly attached to a benzene ring ( –C₆H₅ ), indicating the aryl fragment originates from phenol.
Check synthetic feasibility:
• Option A pairs propanoic acid with phenol: No reaction occurs under normal esterification conditions.
• Options B and C produce the wrong constitutional isomers (ethyl benzoate and propyl benzoate, respectively), where the benzene ring contains the carbonyl carbon.
• Option D pairs propanoyl chloride with phenol: Forms phenyl propanoate + HCl vigorously at room temperature.

Choosing A (The "Classic" Trap): Students correctly identify the fragments as propanoic acid and phenol, but forget that phenols cannot be esterified by carboxylic acids alone.
Reversing the Ester Link (Choosing B or C): Confusing CH₃CH₂–COO–C₆H₅ (phenyl propanoate) with C₆H₅–COO–CH₂CH₃ (ethyl benzoate). Always read carefully from the carbonyl carbon!
Counting Carbons Carelessly: Option C provides a 3-carbon alcohol (propan-1-ol), which would yield propyl benzoate, having completely the wrong backbone connectivity.

"Look for the C=O": The side with C=O comes from the carboxylic acid/acyl derivative. The side with just –O– comes from the alcohol/phenol.
Recall the "Phenol Rule": Whenever you need to make an ester with a phenyl group on the single-bonded oxygen ( –O–C₆H₅ ), you must look for an acyl chloride ( –COCl ) or an acid anhydride.
Reaction equation:
CH₃CH₂COCl + C₆H₅OH → CH₃CH₂COOC₆H₅ + HCl

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