Explain acidity in chlorine-substituted carboxylic acids
Use chlorine’s electron-withdrawing inductive effect to compare the acidity of substituted carboxylic acids.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 33. Focus on the named compounds, reagents and conditions in each outcome; apply the specified reaction type rather than extending it to unlisted cases. Written practice is self-checked
Before you begin
- Recall carboxylic acids, esters, phenols, alcohols, halogenoalkanes and electrophilic/nucleophilic reaction ideas from earlier topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does a chlorine substituent usually affect the acidity of a carboxylic acid?
What you will learn
- Compare acids with different numbers of chlorine substituents
- Relate chlorine position to its inductive effect
- Explain how electron withdrawal stabilises a carboxylate
Chlorine withdraws electron density
Chlorine is electronegative and withdraws electron density through σ bonds by the inductive effect. In a chlorine-substituted carboxylic acid, this helps stabilise the negative charge in the conjugate carboxylate ion.
A more stabilised conjugate base corresponds to a stronger acid. For otherwise comparable acids, adding chlorine substituents generally increases acidity.
Distance changes the effect
The inductive effect is strongest when chlorine is closer to the carboxyl group and weakens along the carbon chain. Thus 2-chloropropanoic acid is stronger than 3-chloropropanoic acid, which is stronger than propanoic acid.
When comparing isomers or substitution patterns, count both how many chlorine atoms are present and how far they are from –COOH. Do not explain this trend using resonance with the carboxyl group.
Pause and explain
Which is typically more acidic: 2-chloropropanoic acid or 3-chloropropanoic acid?
Worked example
Rank propanoic acid, 2-chloropropanoic acid and 3-chloropropanoic acid from strongest to weakest acid.
Show the worked solution
- Chlorine withdraws electron density inductively.
- The effect is stronger nearer the carboxyl group.
- Unsubstituted propanoic acid has no chlorine withdrawal.
Answer 2-chloropropanoic acid > 3-chloropropanoic acid > propanoic acid.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Rank ethanoic acid, 2-chloroethanoic acid and dichloroethanoic acid by increasing acidity.
Give me a hint
More nearby electron-withdrawing chlorine atoms stabilise the conjugate base more.
Compare my reasoning
- Ethanoic acid has no chlorine substituent.
- One chlorine increases the inductive effect.
- Two chlorines withdraw more strongly overall.
Ethanoic acid < 2-chloroethanoic acid < dichloroethanoic acid.
Look for these in your work
- Unsubstituted placed first
- One-Cl comparison
- Two-Cl comparison
Explain why 2-chloropropanoic acid is more acidic than 3-chloropropanoic acid.
Give me a hint
Compare the distance from chlorine to the carboxyl group.
Compare my reasoning
- Both have one chlorine substituent.
- Chlorine is closer to –COOH in the 2-isomer.
- Its inductive electron withdrawal is therefore stronger.
The chlorine in 2-chloropropanoic acid is nearer the carboxyl group, so its inductive effect stabilises the carboxylate more strongly and increases acidity.
Look for these in your work
- Substitution count compared
- Distance identified
- Conjugate-base effect stated
A student attributes the extra acidity of a chloroacid to chlorine donating a lone pair into the carboxylate. Correct the main explanation.
Give me a hint
Use the through-bond effect of electronegativity.
Compare my reasoning
- Chlorine is electronegative.
- It withdraws electron density through σ bonds by induction.
- This stabilises the carboxylate and makes the acid stronger.
The principal explanation is chlorine’s electron-withdrawing inductive effect, which stabilises the conjugate carboxylate; it is not donation into the carboxylate by resonance.
Look for these in your work
- Electron withdrawal corrected
- Inductive effect named
- Acidity connected
Common mistakes
- Saying chlorine donates electrons
- Ignoring the distance between Cl and –COOH
- Using resonance as the main explanation
- Assuming carbon-chain length alone explains the order
What can you explain now?
How does a chlorine substituent usually affect the acidity of a carboxylic acid?
Compare with the explanation
It increases acidity by withdrawing electron density
The inductive effect stabilises the carboxylate conjugate base.
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Make it stick
How do chlorine number and distance from –COOH affect carboxylic-acid strength?
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