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Carboxylic Acids and Derivatives

Learn and apply Carboxylic Acids and Derivatives in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Carboxylic Acids and Derivatives: Orientation

Carboxylic acids and their derivatives are tested by “reactivity + conditions”: predict products and write the right by-products (often HCl or H₂O). This lesson links esterification, acyl chloride reactions, and hydrolysis to the mark-scheme phrases examiners expect.

This topic gets cleaner if you cross-check Organic Mechanisms: Curly Arrows, Electrophiles, Nucleophiles while navigating from the Organic Chemistry hub.

Definitions (Must Know)

A. Carboxylic acid

A carboxylic acid contains the -COOH functional group (e.g. CH₃COOH).

B. Acid derivative (at this level)

Common derivatives you will meet:

  • Acyl chlorides: RCOCl
  • Esters: RCOOR'

C. Esterification

Esterification is the reaction of a carboxylic acid with an alcohol to form an ester and water (usually acid-catalysed and heated).

D. Hydrolysis

Hydrolysis is bond breaking by reaction with water (often with acid/base and heat), e.g. ester → carboxylic acid (or carboxylate) + alcohol.

Detailed Explanations

  • Oxidation of a primary alcohol (via aldehyde) to a carboxylic acid: CH₃CH₂OH → [O] CH₃CHO → [O] CH₃COOH
  • Hydrolysis of nitriles (often via an amide intermediate): R-CN + 2H₂O → [H + /heat] R-COOH + NH₄ +

B. Typical acid reactions (what to write)

  • With metals (salt + hydrogen): 2RCOOH + Mg → (RCOO)₂Mg + H₂
  • With bases (neutralisation): RCOOH + NaOH → RCOONa + H₂O
  • With carbonates (salt + water + carbon dioxide): 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂

C. Esterification (condensation)

General: RCOOH + R'OH ⇌ RCOOR' + H₂O

Conditions to quote:

  • concentrated H₂SO₄ catalyst
  • heat (often reflux)

D. Acyl chlorides (reactivity + key reactions)

1) Hydrolysis

RCOCl + H₂O → RCOOH + HCl

2) With alcohols (ester formation)

RCOCl + R'OH → RCOOR' + HCl

3) With ammonia / amines (amide formation)

RCOCl + 2NH₃ → RCONH₂ + NH₄Cl

Key explanation: Because Cl⁻ is a good leaving group and the carbonyl carbon is strongly electrophilic, therefore acyl chlorides react rapidly with nucleophiles (water, alcohols, ammonia).

E. Hydrolysis of esters

Acid hydrolysis (reversible): RCOOR' + H₂O ⇌ [H + /heat] RCOOH + R'OH

Alkaline hydrolysis (often treated as “complete”): RCOOR' + OH⁻ → RCOO⁻ + R'OH

F. Workflow: choosing the correct products (exam method)

  1. Identify the carbonyl derivative (acid / acyl chloride / ester).
  2. Identify the nucleophile: water, alcohol, ammonia/amine, or OH⁻.
  3. Write products by swapping the leaving group:
    • acyl chloride + water → carboxylic acid + HCl
    • acyl chloride + alcohol → ester + HCl
    • ester + OH⁻ → carboxylate + alcohol

Mini example: CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl (ethyl ethanoate forms).

G. Further conversions of acids and acyl chlorides

  • Convert a carboxylic acid to an acyl chloride using PCl₅: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl.
  • Reduce a carboxylic acid to a primary alcohol using LiAlH₄ in dry ether, followed by water.
  • An acyl chloride reacts with a primary amine to form an N-substituted amide; a second amine molecule removes the hydrogen chloride formed.
  • An acyl chloride reacts with phenol or phenoxide to form a phenyl ester. Benzoyl chloride with phenol gives phenyl benzoate.

H. Why different chlorides hydrolyse at different rates

Acyl chlorides hydrolyse rapidly because the carbonyl carbon is strongly δ + and chloride is a good leaving group. Halogenoalkanes hydrolyse more slowly by nucleophilic substitution, with rate depending on C–X bond strength and structure. Halogenoarenes such as chlorobenzene resist hydrolysis because their C–Cl bond has partial double-bond character and the usual SN1/SN2 routes are unfavourable.

In a comparison question, identify the type of chloride first, then explain electron distribution, bond strength and mechanism. “All contain chlorine” is not a chemical explanation.

Worked Examples

Modelled example 1

Prepare Ethyl Ethanoate

Core

Problem

State the reagents and conditions needed to make ethyl ethanoate from ethanol and ethanoic acid.
Study the worked solution

  1. Read the ester name

    Method

    Match ethyl to ethanol and ethanoate to ethanoic acid.

    Reason

    The alkyl part comes from the alcohol and the carboxylate part from the acid.

    Working

    ethanol + ethanoic acid.

  2. State catalyst and heating

    Method

    Heat under reflux with concentrated sulfuric acid.

    Reason

    The acid catalyses esterification while reflux permits sustained heating.

    Working

    C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O; concentrated H₂SO₄, reflux.

Guided practice 2

Hydrolysis of Ethanoyl Chloride

About 5 min

Problem

Write an equation for the reaction of ethanoyl chloride with water.

Choose both products

Organic product
Other product

Hints

Hint 1: replacement
Replace the acyl chloride’s Cl by OH.
Hint 2: balance
Use the remaining H from water with Cl to form HCl.
View solution step by step

  1. Form the carboxylic acid

    Method

    Replace Cl by OH at the acyl carbon.

    Reason

    Acyl chlorides hydrolyse readily with water.

    Working

    CH₃COCl → CH₃COOH.

  2. Complete the equation

    Method

    Add hydrogen chloride as the second product.

    Reason

    This conserves hydrogen and chlorine atoms.

    Working

    CH₃COCl + H₂O → CH₃COOH + HCl.

Common misconception 3

Alkaline Hydrolysis of an Ester

Find and correct the mistake

Learner claim

A learner says heating an ester with aqueous NaOH produces an alcohol and a carboxylic acid. Correct the products for alkaline conditions.

Account for the base

One product
Acyl-derived product

View solution step by step

  1. Split the ester

    Method

    Form an alcohol from the alkoxy part.

    Reason

    Hydrolysis cleaves the acyl–oxygen linkage.

    Working

    R'OH forms.

  2. Keep the acid deprotonated

    Method

    Form RCOO⁻, or RCOONa when the sodium salt is written.

    Reason

    A carboxylic acid is deprotonated in the alkaline reaction mixture.

    Working

    Products: R'OH + RCOONa.

Challenge 4

Ethanoyl Chloride with Ethanol

Minimal support

Nucleophile transfer

Transfer the ethanoyl-chloride reaction pattern from water to ethanol: write the equation and identify the organic product class.

Replace chloride using ethanol

Organic product
Product class

Hints

Hint 1: incoming group
Ethanol supplies -OC₂H₅ to replace Cl.
Hint 2: coproduct
The alcohol’s removed H combines with Cl as HCl.
View solution step by step

  1. Build the ester

    Method

    Replace the acyl chloride’s Cl by OC₂H₅.

    Reason

    The alcohol acts as the oxygen nucleophile in acyl substitution.

    Working

    Organic product: CH₃COOC₂H₅, ethyl ethanoate.

  2. Balance the equation

    Method

    Include hydrogen chloride.

    Reason

    The eliminated H and Cl account for the remaining atoms.

    Working

    CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl.

Mind Stretchers

Mind stretcher 1Extension

Explain why chloro-substitution on a carboxylic acid chain can increase acidity.

Show Hint

Track whether hydrolysis conditions leave the acid protonated or as a carboxylate salt.

Show Answer

Mark scheme:

  • Chlorine is electron-withdrawing (inductive effect).
  • It stabilises the carboxylate ion RCOO⁻ by dispersing negative charge.
  • Therefore dissociation is more favourable and the acid is stronger.

Mind stretcher 2: Comparing acidic and alkaline ester hydrolysisExtension

Question. Compare the organic products when ethyl ethanoate is heated with dilute acid and when it is heated with aqueous sodium hydroxide.

Show Hint

Both routes form ethanol; focus on the acid-derived product.

Show Answer

Acidic hydrolysis gives ethanol and ethanoic acid and is reversible. Alkaline hydrolysis gives ethanol and sodium ethanoate; carboxylate formation drives the reaction effectively to completion.