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Notes/Chemistry/Paper 1/Carboxylic Acids and Derivatives
CAIEAS Level9701§18

Carboxylic Acids and Derivatives

Carboxylic acids at AS: how to make them, their reactions with metals, alkalis and carbonates, reduction with LiAlH₄, and how esters are made, named and hydrolysed.

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In the AS Carbonyl Compounds note you added HCN to aldehydes and ketones, and in the AS Hydroxy Compounds note you oxidised primary alcohols. Both roads lead here, to the carboxylic acids. They carry the carboxyl group, −COOH-\text{COOH}: a C=O and an –OH on the same carbon. Unlike alcohols, carboxylic acids release a few H+\text{H}^+ ions in water, so they behave as weak acids.

The note starts with structure and naming, then the ways to make acids, their reactions with metals, alkalis and carbonates, and their reduction by LiAlH4\text{LiAlH}_4. Next come esters: how an acid and an alcohol join to make one, how to name it, and how acid or alkali splits it again. The last section puts everything together in synthesis routes and distinguishing tests.

Before you start you should be able to

  • Oxidation of alcohols with acidified dichromate, and distillation versus reflux (AS Hydroxy Compounds)

  • The [O] and [H] shorthand in organic equations (AS Hydroxy Compounds and AS Carbonyl Compounds)

  • Making nitriles: KCN in ethanol with halogenoalkanes (AS Halogen Compounds) and HCN addition to carbonyl compounds (AS Carbonyl Compounds)

  • Reversible reactions and Le Chatelier's principle (AS Equilibria)

  • Naming organic compounds and reading skeletal formulae (AS Introduction to Organic Chemistry)

  • Moles and gas volumes (AS Atoms, Molecules and Stoichiometry)

By the end of this page you can

  • Recall how carboxylic acids are produced: oxidation of primary alcohols and aldehydes with acidified K₂Cr₂O₇ or KMnO₄ under reflux; hydrolysis of nitriles with dilute acid, or dilute alkali followed by acidification

  • Describe the redox reaction of carboxylic acids with reactive metals producing a salt and H₂(g), and predict how many moles of gas form per mole of acid

  • Describe the neutralisation reaction with alkalis producing a salt and water, and explain why NaOH deprotonates only the carboxyl O–H while sodium attacks every O–H

  • Describe the acid-base reaction with carbonates producing a salt, water and CO₂(g), and use it to distinguish acids from alcohols and esters

  • Describe reduction of carboxylic acids by LiAlH₄ to primary alcohols, writing balanced equations in [H] notation, and recall that NaBH₄ is not strong enough

  • Recall ester formation as the condensation reaction between an alcohol and a carboxylic acid, heated with concentrated H₂SO₄ as catalyst, name esters as alkyl alkanoates, and draw the ester link

  • Describe hydrolysis of esters by dilute acid (reversible) and by dilute alkali (complete, giving a carboxylate salt plus an alcohol), and deduce the products either way

  • Follow multi-step synthesis schemes that route through acids, nitriles and esters, reading them forwards and backwards, and choose tests that tell acids, alcohols, esters and carbonyl compounds apart

01

Meet the carboxylic acids

One carbon, two oxygens

A carboxylic acid carries the carboxyl group, −COOH-\text{COOH} (also written −CO2H-\text{CO}_2\text{H}). The name tells you the structure: a carbonyl group (C=O) and a hydroxyl group (–OH) on the same carbon. That carbon is bonded to three things:

  • an =O=\text{O} (the carbonyl oxygen),
  • an −OH-\text{OH} (the hydroxyl oxygen),
  • and either H\text{H} (methanoic acid, HCOOH\text{HCOOH}, the only one with no carbon neighbour) or the rest of the chain, R\text{R}.

The group can only sit at the end of a chain, so its carbon is always C1 and the group itself never needs a number. The name ends in -oic acid: ethanoic acid CH3COOH\text{CH}_3\text{COOH}, propanoic acid CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}, butanoic acid CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}. Branches are numbered from the COOH carbon: (CH3)2CHCOOH(\text{CH}_3)_2\text{CHCOOH} is 2-methylpropanoic acid, not 3-methylpropanoic acid.

Two other kinds appear often in exam schemes:

  • Dicarboxylic acids have a COOH at each end: ethanedioic acid HOOC–COOH\text{HOOC–COOH} (the simplest), propanedioic acid HOOC–CH2–COOH\text{HOOC–CH}_2\text{–COOH}, butanedioic acid HOOC–(CH2)2–COOH\text{HOOC–(CH}_2)_2\text{–COOH}. Each COOH reacts on its own, so one molecule reacts twice.
  • Substituted acids carry other groups on the chain: 2-hydroxypropanoic acid CH3CH(OH)COOH\text{CH}_3\text{CH(OH)COOH} (the acid in sour milk), 2-chloroethanoic acid. The COOH keeps its suffix; the other groups are numbered counting from it.
The carboxyl group −COOHRCOδ−carbonyl oxygenOHhydroxyl O–H:this H leaves as H⁺δ+end of chain ⇒ its carbon is always C1suffix −oic acid · branches numbered from hereOne group, four shapes you will meetbutanoic acidCH₃CH₂CH₂COOHstraight chain2-methylpropanoic acid(CH₃)₂CHCOOHbranchedbutanedioic acidHOOC(CH₂)₂COOHdicarboxylic2-hydroxypropanoic acidCH₃CH(OH)COOHsubstitutedtwo COOH groups ⇒ double the gas volumes in §03

Anatomy of the carboxyl group: one carbon carrying both =O and –OH, always C1 of the chain, and the four kinds of acid you will meet — straight-chain, branched, dicarboxylic and substituted.

The same-carbon rule

A carboxylic acid needs the =O and a real O–H on the same carbon. HOCH2CHO\text{HOCH}_2\text{CHO} has an –OH and a C=O on different carbons: it is a hydroxy-aldehyde, with the chemistry of the AS Carbonyl Compounds note, not this one. CH3COOCH3\text{CH}_3\text{COOCH}_3 has a C=O and an O on the same carbon, but that O is bonded to another carbon (a methyl), not to hydrogen: it is an ester (see “The ester family”), and it is not acidic. Check all three bonds on the carbon before you name.

Why carboxylic acids are acidic

In water, a small fraction of acid molecules give the H of their –OH to a water molecule:

CH3COOH  +  H2O⇌CH3COO−  +  H3O+\text{CH}_3\text{COOH} \;+\; \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COO}^- \;+\; \text{H}_3\text{O}^+

Carboxylic acids are weak acids: they are only partly dissociated. In 0.1 mol dm−30.1\ \text{mol dm}^{-3} ethanoic acid only about 1 molecule in 100 has dissociated, so the pH is about 3, not 1 as for 0.1 mol dm−30.1\ \text{mol dm}^{-3} hydrochloric acid.

Weak does not mean unreactive. There is enough H+\text{H}^+ to react with metals and carbonates. And when a base removes H+\text{H}^+, the equilibrium shifts to the right (Le Chatelier), so more acid dissociates until all of it has reacted. That is why a weak acid can still be neutralised completely.

The negative ion left behind, RCOO−\text{RCOO}^-, is called a carboxylate ion. An alcohol's O–H hardly dissociates at all: alcohols are neutral to indicators and do not react with carbonates. The C=O next to the –OH is what makes the carboxyl hydrogen come off more easily. (The detailed reason, and how acids compare with phenols, is in the A Level Carboxylic Acids note.)

Salts are named from their parent acid: ethanoic acid → ethanoate, propanoic → propanoate, ethanedioic → ethanedioate. So ethanoic acid with sodium hydroxide gives sodium ethanoate, CH3COO−Na+\text{CH}_3\text{COO}^-\text{Na}^+ (also written CH3COONa\text{CH}_3\text{COONa}).

Naming, classifying and the same-carbon rule

For each molecular formula and description, give the structure asked for and its systematic name:

(i) An unbranched monocarboxylic acid, C4H8O2\text{C}_4\text{H}_8\text{O}_2.

(ii) The only branched monocarboxylic acid, C4H8O2\text{C}_4\text{H}_8\text{O}_2.

(iii) A dicarboxylic acid, C4H6O4\text{C}_4\text{H}_6\text{O}_4, with a straight chain.

(iv) Another isomer of C4H8O2\text{C}_4\text{H}_8\text{O}_2 that has both oxygens on the same carbon but NO carboxyl group — name its family.

Show full working
  1. 1

    (i) Four carbons, COOH at C1: CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} — butanoic acid.

    The COOH carbon counts as chain carbon 1, so a straight-chain C4 acid has exactly three more carbons after it.

  2. 2

    (ii) A branched chain with the acid carbon still at C1: CH3CH(CH3)COOH\text{CH}_3\text{CH(CH}_3)\text{COOH} — 2-methylpropanoic acid. Numbering from the other end would give "3-methyl", which is wrong: the number counts FROM the carboxyl carbon.

    Numbering from the wrong end is the most common way to lose a naming mark for acids.

  3. 3

    (iii) Straight chain, one COOH at each end: HOOC–CH2–CH2–COOH\text{HOOC–CH}_2\text{–CH}_2\text{–COOH} — butanedioic acid.

    Two COOH groups use two of the four carbons, leaving a CH₂–CH₂ in the middle.

  4. 4

    Check the formula: 4 H on the two CH2\text{CH}_2 carbons + 2 H on the two –OH groups = 6 H, and 4 O → C4H6O4\text{C}_4\text{H}_6\text{O}_4 ✓.

    Students often forget the –OH hydrogens when counting a molecular formula.

  5. 5

    (iv) Two oxygens, no COOH, same formula as the acids: the ester family — e.g. methyl propanoate CH3CH2COOCH3\text{CH}_3\text{CH}_2\text{COOCH}_3 or ethyl ethanoate CH3COOCH2CH3\text{CH}_3\text{COOCH}_2\text{CH}_3.

    Esters have the same general formula as acids, CₙH₂ₙO₂, but no acidic O–H, so isomer questions often include both families.

Answer

(i) butanoic acid (ii) 2-methylpropanoic acid (iii) butanedioic acid HOOC(CH₂)₂COOH (iv) esters, e.g. methyl propanoate or ethyl ethanoate

Your turn

Naming and structure practice — every later section assumes you can do these quickly.

  1. 15 marks

    (a) Give the structural formulae of the THREE branched-chain monocarboxylic acids with molecular formula C5H10O2\text{C}_5\text{H}_{10}\text{O}_2, and name them.

    (b) A student draws CH3CH2COCH2OH\text{CH}_3\text{CH}_2\text{COCH}_2\text{OH} and names it "butanoic acid". Identify the functional groups actually present and give the correct name.

    (c) Write the equation for the partial dissociation of propanoic acid in water, and state what you would see if universal indicator were added to a 0.1 mol dm−30.1\ \text{mol dm}^{-3} solution.

    Stuck? Show hint

    (b) test the same-carbon rule on each carbon. (c) weak means PARTLY dissociated — what pH does that give?

    Show solution
    1. 1

      (a) The COOH carbon is C1. "Branched" means methyl branch(es) on the rest of the chain. One methyl on C2: 2-methylbutanoic acid, CH3CH2CH(CH3)COOH\text{CH}_3\text{CH}_2\text{CH(CH}_3)\text{COOH}.

      Fix the COOH at C1 first, then move one methyl along the chain.

    2. 2

      One methyl on C3: 3-methylbutanoic acid, (CH3)2CHCH2COOH(\text{CH}_3)_2\text{CHCH}_2\text{COOH}.

      A methyl on C4 would just lengthen the chain to pentanoic acid, which is unbranched.

    3. 3

      Two methyls on C2: 2,2-dimethylpropanoic acid, (CH3)3CCOOH(\text{CH}_3)_3\text{CCOOH}.

      After one methyl, try two. Stopping after one methyl is how students lose the third mark.

    4. 4

      (b) CH3CH2COCH2OH\text{CH}_3\text{CH}_2\text{COCH}_2\text{OH} has a C=O whose carbon is bonded to TWO carbons (a ketone), and a separate −CH2OH-\text{CH}_2\text{OH} (a primary alcohol). No carbon carries both =O and –OH, so it is not an acid. Correct name: 1-hydroxybutan-2-one.

      It has the same formula as butanoic acid (C₄H₈O₂), which is why the mistake is tempting — the same-carbon rule catches it.

    5. 5

      (c) CH3CH2COOH+H2O⇌CH3CH2COO−+H3O+\text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COO}^- + \text{H}_3\text{O}^+

      Use the reversible arrow: a weak acid is only partly dissociated.

    6. 6

      Universal indicator turns orange-red (pH about 3): acidic, but not the deep red of 0.1 mol dm−30.1\ \text{mol dm}^{-3} hydrochloric acid (pH 1).

      Comparing with a strong acid of the same concentration shows that 'weak' is about how much dissociates, not about concentration.

    Answer

    (a) 2-methylbutanoic, 3-methylbutanoic and 2,2-dimethylpropanoic acids (b) ketone + primary alcohol: 1-hydroxybutan-2-one (c) CH₃CH₂COOH + H₂O ⇌ CH₃CH₂COO⁻ + H₃O⁺; orange-red, pH about 3

  2. 23 marks

    (a) Draw the structural formula of 2-hydroxypropanoic acid and circle the hydrogen that is lost when it acts as an acid.

    (b) Name the sodium salt formed when propanedioic acid, HOOCCH2COOH\text{HOOCCH}_2\text{COOH}, reacts with an excess of sodium hydroxide, and give its formula.

    Stuck? Show hint

    (a) Only one of the two O–H groups is part of a carboxyl group. (b) Both COOH groups react with excess alkali.

    Show solution
    1. 1

      (a) CH3CH(OH)COOH\text{CH}_3\text{CH(OH)COOH}: C1 is the COOH carbon; C2 carries the –OH. The acidic hydrogen is the one on the –OH of the COOH group, not the one on C2's –OH.

      The C2 –OH is an alcohol group: like any alcohol, it barely dissociates in water.

    2. 2

      (b) Excess NaOH removes the H from both COOH groups. The salt is sodium propanedioate, NaOOCCH2COONa\text{NaOOCCH}_2\text{COONa} (Na2C3H2O4\text{Na}_2\text{C}_3\text{H}_2\text{O}_4).

      Name the salt from the parent acid: propanedioic acid → propanedioate. Two COOH groups → two Na⁺.

    Answer

    (a) CH₃CH(OH)COOH — the H on the COOH group's O (b) sodium propanedioate, NaOOCCH₂COONa

The rest of this note

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Can you do all of these?

  • Making an acid by oxidation: name the oxidising agent, say acidified, and say heat under reflux

  • Nitrile hydrolysis: dilute acid (or dilute alkali, then acidify), heat under reflux; the CN carbon becomes the COOH carbon

  • Count carbons across every arrow — only KCN or HCN steps add a carbon

  • NaOH removes only the carboxyl H; sodium metal reacts with every O–H

  • Gas amounts: ½ mol CO₂ per COOH with Na₂CO₃, 1 mol with NaHCO₃, ½ mol H₂ per O–H with Na

  • Carbonate effervescence identifies a carboxylic acid; alcohols and esters do not react

  • Reduction of an acid: LiAlH₄ in dry ether, 4[H] per COOH; NaBH₄ does not work

  • Esterification: acid + alcohol, conc. H₂SO₄, heat — a condensation; always show the water

  • Ester names: alkyl word from the alcohol, -oate word from the acid (includes the C=O carbon)

  • Hydrolysis: dilute acid → acid + alcohol, reversible; dilute alkali → carboxylate salt + alcohol, complete

Paper 1 · MCQ
Real past-paper questions, sorted by difficulty, with mark schemes
Paper 2 · Theory
Real past-paper questions, sorted by difficulty, with mark schemes