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A-Level Chemistry May/June 2025 Q10(c): When propanoic acid is treated with chlorine gas in the presence of ultraviolet light,…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 10(c) · [2 marks]
When propanoic acid is treated with chlorine gas in the presence of ultraviolet light, a mixture of products is formed. One of these products is 2,2-dichloropropanoic acid. Explain why 2,2-dichloropropanoic acid is stronger than propanoic acid. Refer to the structure of each compound in your answer.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The two chlorine atoms in 2,2-dichloropropanoic acid are highly electronegative. They are electron-withdrawing groups, pulling electron density away from the rest of the molecule via the inductive effect.
This withdrawal of electron density polarises and weakens the O–H bond in the carboxylic acid group, making the proton (H⁺) easier to lose compared to in propanoic acid. Therefore, 2,2-dichloropropanoic acid is a stronger acid.
How the marks are awarded
- M1 — The answer correctly identifies that chlorine atoms are electronegative and therefore electron-withdrawing.
- M2 — The answer links the electron-withdrawing effect to the weakening of the O–H bond, which explains the increased acidity. An alternative correct answer would be linking the effect to the stabilisation of the conjugate base.
Common mistakes
- Simply stating that chlorine is present without explaining its electronic property (i.e., its high electronegativity and electron-withdrawing effect).
- Correctly identifying chlorine as electron-withdrawing (M1) but failing to explain the consequence, i.e., how this leads to a weaker O–H bond or a more stable conjugate base (M2).
- Describing the alkyl group (CH₃CH₂–) in propanoic acid as electron-withdrawing instead of correctly identifying it as having a positive inductive (electron-donating) effect.
- Confusing the effect of chlorine atoms in the structure with the reactivity of chlorine gas (Cl₂).
Examiner tip: For questions on relative acidity, always connect the electronegativity of substituent atoms to the resulting inductive effect and its impact on either O–H bond strength or conjugate base stability.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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