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11.7 Carboxylic acids

11.7.1 Reactions of ethanoic acid

Ethanoic acid as an acid

Definition

Salt

A compound formed when the hydrogen in an acid is replaced by a metal ion or an ammonium ion.

  1. Ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3​COOH, is a carboxylic acid, so its molecule contains the −COOH-\text{COOH}−COOH functional group.
  2. When ethanoic acid dissolves in water, some of its molecules release hydrogen ions, H+\text{H}^+H+, so the solution is acidic.
  3. Only the hydrogen atom in the −COOH-\text{COOH}−COOH group is released, while the three hydrogen atoms in the CH3\text{CH}_3CH3​ group stay attached to carbon.
  4. Aqueous ethanoic acid has a pH of about 3, turns blue litmus red and turns methyl orange red.
  5. Ethanoic acid is a weak acid, so its reactions are the same types as those of hydrochloric acid but happen more slowly at the same concentration.
  6. Ethanoic acid reacts with metals, bases and carbonates, and in every case it forms a salt.
  7. Vinegar is a dilute solution of ethanoic acid, which is why vinegar shows these reactions too.

Naming ethanoate salts

Definition

Ethanoate

A salt of ethanoic acid, formed when the hydrogen of the acid group is replaced by a metal ion.

  1. Salts made from ethanoic acid are called ethanoates.
  2. When the hydrogen ion leaves the −COOH-\text{COOH}−COOH group, the ethanoate ion, CH3COO−\text{CH}_3\text{COO}^-CH3​COO−, is left behind with a charge of 1−1-1−.
  3. The name of the salt is the name of the metal followed by ethanoate, such as sodium ethanoate or calcium ethanoate.
  4. The formula is worked out by balancing the charges so that the salt has no overall charge.
    1. A metal ion with a 1+1+1+ charge needs one ethanoate ion.
    2. A metal ion with a 2+2+2+ charge needs two ethanoate ions, so brackets are placed around the ethanoate group with a 2 outside.
  5. The formulae of common ethanoate salts are shown below.
    1. Sodium ethanoate, CH3COONa\text{CH}_3\text{COONa}CH3​COONa
    2. Potassium ethanoate, CH3COOK\text{CH}_3\text{COOK}CH3​COOK
    3. Magnesium ethanoate, (CH3COO)2Mg(\text{CH}_3\text{COO})_2\text{Mg}(CH3​COO)2​Mg
    4. Calcium ethanoate, (CH3COO)2Ca(\text{CH}_3\text{COO})_2\text{Ca}(CH3​COO)2​Ca
    5. Zinc ethanoate, (CH3COO)2Zn(\text{CH}_3\text{COO})_2\text{Zn}(CH3​COO)2​Zn
    6. Copper(II) ethanoate, (CH3COO)2Cu(\text{CH}_3\text{COO})_2\text{Cu}(CH3​COO)2​Cu
  6. The metal symbol may also be written first, as in Mg(CH3COO)2\text{Mg(CH}_3\text{COO)}_2Mg(CH3​COO)2​, and both forms show the same salt.
Example

Formula of calcium ethanoate

  • A calcium ion is Ca2+\text{Ca}^{2+}Ca2+ and an ethanoate ion is CH3COO−\text{CH}_3\text{COO}^-CH3​COO−.
  • Two ethanoate ions, each with a 1−1-1− charge, are needed to cancel the 2+2+2+ charge on one calcium ion.
  • The ethanoate group is placed in brackets with a 2 after it, so the whole group is doubled.
(CH3COO)2Ca (\text{CH}_3\text{COO})_2\text{Ca} (CH3​COO)2​Ca
  • In the same way, sodium ions have a 1+1+1+ charge, so sodium ethanoate needs only one ethanoate ion and no brackets.
CH3COONa \text{CH}_3\text{COONa} CH3​COONa

Reaction with metals

Definition

Effervescence

The rapid release of bubbles of gas from a liquid during a chemical reaction.

  1. Ethanoic acid reacts with reactive metals such as magnesium, zinc and calcium to form a salt and hydrogen.

    metal+ethanoic acid→metal ethanoate+hydrogen \text{metal} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{hydrogen} metal+ethanoic acid→metal ethanoate+hydrogen

  2. You see effervescence, because bubbles of hydrogen gas are given off.

  3. The metal gets smaller and finally disappears, leaving a colourless solution of the ethanoate salt.

  4. The fizzing is less vigorous than with hydrochloric acid of the same concentration, because ethanoic acid is a weak acid.

  5. The gas can be identified because it burns with a squeaky pop when a lighted splint is held at the mouth of the tube.

  6. Metals of low reactivity, such as copper, do not react with ethanoic acid.

Example

Magnesium and ethanoic acid

  • Magnesium forms Mg2+\text{Mg}^{2+}Mg2+ ions, so it reacts with two molecules of ethanoic acid.
magnesium+ethanoic acid→magnesium ethanoate+hydrogen \text{magnesium} + \text{ethanoic acid} \rightarrow \text{magnesium ethanoate} + \text{hydrogen} magnesium+ethanoic acid→magnesium ethanoate+hydrogen
  • The two acidic hydrogen atoms released from the two acid molecules join to form one H2\text{H}_2H2​ molecule.
2CH3COOH(aq)+Mg(s)→(CH3COO)2Mg(aq)+H2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg(aq)} + \text{H}_2\text{(g)} 2CH3​COOH(aq)+Mg(s)→(CH3​COO)2​Mg(aq)+H2​(g)
  • Zinc also forms 2+2+2+ ions, so it follows the same pattern to give zinc ethanoate.
2CH3COOH(aq)+Zn(s)→(CH3COO)2Zn(aq)+H2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Zn(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Zn(aq)} + \text{H}_2\text{(g)} 2CH3​COOH(aq)+Zn(s)→(CH3​COO)2​Zn(aq)+H2​(g)

Reaction with bases

  1. Ethanoic acid reacts with bases, which are metal oxides and metal hydroxides, to form a salt and water only.

    base+ethanoic acid→metal ethanoate+water \text{base} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{water} base+ethanoic acid→metal ethanoate+water

  2. This is a neutralisation reaction, in which the hydrogen ions from the acid react with the oxide or hydroxide ions of the base to form water.

  3. No gas is produced, so there is no fizzing.

  4. With sodium hydroxide solution, both reactants and products are colourless, so the only sign of reaction is a small rise in temperature.

  5. When black copper(II) oxide is warmed with ethanoic acid, the black solid disappears and the solution turns blue as copper(II) ethanoate forms.

Example

Sodium hydroxide and copper(II) oxide

  • Sodium ions have a 1+1+1+ charge, so one molecule of ethanoic acid reacts with one unit of sodium hydroxide.
sodium hydroxide+ethanoic acid→sodium ethanoate+water \text{sodium hydroxide} + \text{ethanoic acid} \rightarrow \text{sodium ethanoate} + \text{water} sodium hydroxide+ethanoic acid→sodium ethanoate+water CH3COOH(aq)+NaOH(aq)→CH3COONa(aq)+H2O(l) \text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} CH3​COOH(aq)+NaOH(aq)→CH3​COONa(aq)+H2​O(l)
  • Copper(II) ions have a 2+2+2+ charge, so two molecules of ethanoic acid react with one unit of copper(II) oxide.
2CH3COOH(aq)+CuO(s)→(CH3COO)2Cu(aq)+H2O(l) 2\text{CH}_3\text{COOH(aq)} + \text{CuO(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Cu(aq)} + \text{H}_2\text{O(l)} 2CH3​COOH(aq)+CuO(s)→(CH3​COO)2​Cu(aq)+H2​O(l)

Reaction with carbonates

  1. Ethanoic acid reacts with carbonates to form a salt, water and carbon dioxide.

    carbonate+ethanoic acid→metal ethanoate+water+carbon dioxide \text{carbonate} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{water} + \text{carbon dioxide} carbonate+ethanoic acid→metal ethanoate+water+carbon dioxide

  2. You see effervescence, because carbon dioxide gas is given off.

  3. A solid carbonate, such as calcium carbonate, gets smaller and disappears if enough acid is present.

  4. The gas turns limewater milky, which shows that it is carbon dioxide.

  5. The fizzing stops when either the carbonate or the ethanoic acid has been used up.

Example

Sodium carbonate and calcium carbonate

  • Sodium carbonate, Na2CO3\text{Na}_2\text{CO}_3Na2​CO3​, contains two sodium ions, so it gives two units of sodium ethanoate.
2CH3COOH(aq)+Na2CO3(s)→2CH3COONa(aq)+H2O(l)+CO2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} 2CH3​COOH(aq)+Na2​CO3​(s)→2CH3​COONa(aq)+H2​O(l)+CO2​(g)
  • Calcium carbonate, CaCO3\text{CaCO}_3CaCO3​, contains one Ca2+\text{Ca}^{2+}Ca2+ ion, so it gives one unit of calcium ethanoate.
calcium carbonate+ethanoic acid→calcium ethanoate+water+carbon dioxide \text{calcium carbonate} + \text{ethanoic acid} \rightarrow \text{calcium ethanoate} + \text{water} + \text{carbon dioxide} calcium carbonate+ethanoic acid→calcium ethanoate+water+carbon dioxide 2CH3COOH(aq)+CaCO3(s)→(CH3COO)2Ca(aq)+H2O(l)+CO2(g) 2\text{CH}_3\text{COOH(aq)} + \text{CaCO}_3\text{(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} 2CH3​COOH(aq)+CaCO3​(s)→(CH3​COO)2​Ca(aq)+H2​O(l)+CO2​(g)
Practical

Test-tube reactions of ethanoic acid

  • Aim: to observe how dilute ethanoic acid reacts with a metal, a base and a carbonate, and to identify any gas given off.
  • Apparatus: test tubes in a rack, test-tube holder, dilute ethanoic acid, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, 2 cm2\ \text{cm}2 cm strips of magnesium ribbon, copper(II) oxide powder, sodium carbonate powder, spatula, bung fitted with a delivery tube, limewater, wooden splints, Bunsen burner, heat-resistant mat and eye protection.
  • Method, acid and metal:
    • Pour 3 cm33\ \text{cm}^33 cm3 of dilute ethanoic acid into a test tube in the rack.
    • Add one strip of magnesium ribbon and watch the surface of the metal.
    • Hold a lighted splint at the mouth of the tube once bubbles have been forming for a short time.
  • Method, acid and base:
    • Add half a spatula of black copper(II) oxide to 3 cm33\ \text{cm}^33 cm3 of the acid in a clean tube.
    • Hold the tube in a test-tube holder and warm it gently in a Bunsen flame, shaking it and pointing the mouth away from everyone.
    • Stop heating before the liquid boils and note the colour of the solution.
  • Method, acid and carbonate:
    • Pour 3 cm33\ \text{cm}^33 cm3 of limewater into a second tube and stand it in the rack.
    • Put one spatula of sodium carbonate into a clean tube, add 5 cm35\ \text{cm}^35 cm3 of the acid and fit the bung straight away.
    • Let the gas bubble through the limewater.
  • Results:
    • Magnesium fizzes steadily, though less vigorously than in hydrochloric acid, and gradually disappears, and the gas pops with a lighted splint.
    • Copper(II) oxide disappears and the solution turns blue, with no bubbles at any stage.
    • Sodium carbonate fizzes and dissolves, and the limewater turns milky.
  • Conclusion: ethanoic acid behaves as a typical acid, forming an ethanoate salt with hydrogen from a metal, water from a base, and water and carbon dioxide from a carbonate.
  • Safety: wear eye protection, avoid getting the acid on skin or in eyes, keep splint flames away from reagent bottles, and never point a heated tube at anyone.
Common Mistake
  • Do not call the salt an "ethanoic salt" or "sodium ethanoic", because salts of ethanoic acid are named ethanoates.
  • Do not write the formula of magnesium ethanoate as CH3COOMg\text{CH}_3\text{COOMg}CH3​COOMg, because a 2+2+2+ ion needs two ethanoate ions.
  • Do not include hydrogen as a product of a reaction with a base or a carbonate, because hydrogen forms only when the acid reacts with a metal.
  • Do not leave water out of the products of the reactions with bases and carbonates.
Exam technique

Products, formulae and observations

  • Identify what the acid reacts with first, then list the products: metal gives salt + hydrogen, base gives salt + water, carbonate gives salt + water + carbon dioxide.
  • Work out the charge on the metal ion before writing the formula of the ethanoate, and use brackets when there are two ethanoate ions.
  • When asked what you would see, write bubbles or fizzing, the solid disappears, or a stated colour change, rather than naming the gas.
  • Give the test for the gas in full, such as "limewater turns milky" or "lighted splint gives a squeaky pop".
Self review
  • What are the products when ethanoic acid reacts with zinc?
  • Write the formula of sodium ethanoate and of calcium ethanoate.
  • What would you see when ethanoic acid is added to sodium carbonate?
  • Name the salt formed when ethanoic acid reacts with copper(II) oxide.
  • Why does magnesium fizz less vigorously in ethanoic acid than in hydrochloric acid of the same concentration?

11.7.2 Formation of ethanoic acid and esters

Oxidising ethanol

Definition

Oxidation (in terms of oxygen)

The gain of oxygen by a substance.

Definition

Oxidising agent

A substance that oxidises another substance and is itself reduced.

  1. Ethanoic acid can be made by oxidising ethanol, which means the ethanol gains oxygen.

  2. In the laboratory, ethanol is heated with acidified aqueous potassium manganate(VII).

    1. The potassium manganate(VII) solution is acidified by adding dilute sulfuric acid.
    2. Potassium manganate(VII) is the oxidising agent, because it supplies the oxygen that oxidises the ethanol.

  3. As the reaction happens, the solution changes colour from purple to colourless, because the manganate(VII) ions are reduced.

  4. The −CH2OH-\text{CH}_2\text{OH}−CH2​OH group of ethanol becomes the −COOH-\text{COOH}−COOH group of ethanoic acid, while the CH3\text{CH}_3CH3​ group is unchanged.

  5. The oxygen supplied by the oxidising agent is shown as [O][\text{O}][O] in the equation.

    CH3CH2OH+2[O]→CH3COOH+H2O \text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O} CH3​CH2​OH+2[O]→CH3​COOH+H2​O

  6. In this change ethanol gains one oxygen atom and loses two hydrogen atoms, which leave as part of a water molecule.

  7. The product smells of vinegar, which is a simple sign that ethanoic acid has formed.

Practical

Oxidising ethanol with potassium manganate(VII)

  • Aim: to oxidise ethanol to ethanoic acid using acidified potassium manganate(VII) and to observe the colour change.
  • Apparatus: boiling tube, test-tube rack, dropping pipettes, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, ethanol, dilute potassium manganate(VII) solution, dilute sulfuric acid, 250 cm3250\ \text{cm}^3250 cm3 beaker of hot water at about 60 ∘C60\ ^\circ\text{C}60 ∘C from a kettle, thermometer, and eye protection.
  • Method:
    • Pour 2 cm32\ \text{cm}^32 cm3 of dilute sulfuric acid into the boiling tube.
    • Add 5 drops of potassium manganate(VII) solution and swirl, so the mixture is clear purple.
    • Add 1 cm31\ \text{cm}^31 cm3 of ethanol and swirl again.
    • Stand the boiling tube in the beaker of hot water for about 5 minutes, watching the colour.
    • Remove the tube and carefully waft the vapour towards your nose to check the smell.
  • Results:
    • The purple colour fades and the solution becomes colourless.
    • The mixture smells sharp, like vinegar.
  • Conclusion: acidified potassium manganate(VII) oxidises ethanol to ethanoic acid and is itself reduced, which removes the purple colour.
  • Safety: wear eye protection, keep ethanol away from all naked flames because it is highly flammable, avoid skin contact with potassium manganate(VII) because it is an oxidiser that stains, and treat the sulfuric acid as an irritant.

Making vinegar

  1. Ethanoic acid also forms when ethanol is oxidised by bacteria, which is how vinegar is made.

  2. Wine, cider or another solution containing ethanol is left open to the air in warm conditions.

  3. Bacteria in the liquid use oxygen from the air to oxidise the ethanol to ethanoic acid.

    ethanol+oxygen→ethanoic acid+water \text{ethanol} + \text{oxygen} \rightarrow \text{ethanoic acid} + \text{water} ethanol+oxygen→ethanoic acid+water

  4. The balanced symbol equation is shown below.

    C2H5OH(aq)+O2(g)→CH3COOH(aq)+H2O(l) \text{C}_2\text{H}_5\text{OH(aq)} + \text{O}_2\text{(g)} \rightarrow \text{CH}_3\text{COOH(aq)} + \text{H}_2\text{O(l)} C2​H5​OH(aq)+O2​(g)→CH3​COOH(aq)+H2​O(l)

  5. This process is aerobic, because it needs oxygen, unlike fermentation, which must be kept away from oxygen.

  6. The product, vinegar, is a dilute aqueous solution of ethanoic acid, which is why vinegar tastes sour.

  7. The same reaction explains why an opened bottle of wine left exposed to air slowly turns sour.

Example

Balancing the bacterial oxidation

  • Ethanol, C2H5OH\text{C}_2\text{H}_5\text{OH}C2​H5​OH, reacts with oxygen to form ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3​COOH, and water.
C2H5OH+O2→CH3COOH+H2O \text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O} C2​H5​OH+O2​→CH3​COOH+H2​O
  • Carbon is balanced, with 2 atoms on each side.
  • Hydrogen is balanced, with 6 atoms on the left and 4+2=64 + 2 = 64+2=6 on the right.
  • Oxygen is balanced, with 1+2=31 + 2 = 31+2=3 atoms on the left and 2+1=32 + 1 = 32+1=3 on the right, so no extra numbers are needed.

Making esters

Definition

Esterification

The reaction of a carboxylic acid with an alcohol, in the presence of an acid catalyst, to form an ester and water.

  1. A carboxylic acid reacts with an alcohol to form an ester and water.

    carboxylic acid+alcohol⇌ester+water \text{carboxylic acid} + \text{alcohol} \rightleftharpoons \text{ester} + \text{water} carboxylic acid+alcohol⇌ester+water

  2. The conditions for the reaction are listed below.

    1. A few drops of concentrated sulfuric acid are added as an acid catalyst.
    2. The mixture is warmed, usually in a hot water bath.

  3. The reaction is reversible, so the mixture never turns completely into ester and water.

  4. The −OH-\text{OH}−OH from the acid and the H\text{H}H from the −OH-\text{OH}−OH group of the alcohol are removed to form the water molecule.

  5. The two remaining parts join through the ester group, −COO−-\text{COO}-−COO−.

  6. Ethanoic acid and ethanol react to form the ester ethyl ethanoate.

    ethanoic acid+ethanol⇌ethyl ethanoate+water \text{ethanoic acid} + \text{ethanol} \rightleftharpoons \text{ethyl ethanoate} + \text{water} ethanoic acid+ethanol⇌ethyl ethanoate+water

  7. The balanced symbol equation is shown below.

    CH3COOH+CH3CH2OH⇌CH3COOCH2CH3+H2O \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} CH3​COOH+CH3​CH2​OH⇌CH3​COOCH2​CH3​+H2​O

  8. The name of an ester has two parts, and the part from the alcohol comes first.

    1. The alcohol gives the first word, ending in -yl, so ethanol gives ethyl and methanol gives methyl.
    2. The acid gives the second word, ending in -oate, so ethanoic acid gives ethanoate and propanoic acid gives propanoate.

  9. Esters are volatile liquids with sweet, fruity smells, so they are used in flavourings and perfumes.

Displayed formula of ethyl ethanoate, the ester formed from ethanoic acid and ethanol, showing the ester group joining the two parts of the molecule

Example

Methyl ethanoate and ethyl propanoate

  • Methanol is the alcohol, so the first word is methyl, and ethanoic acid is the acid, so the second word is ethanoate.
CH3COOH+CH3OH⇌CH3COOCH3+H2O \text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} CH3​COOH+CH3​OH⇌CH3​COOCH3​+H2​O
  • The ester formed is methyl ethanoate, CH3COOCH3\text{CH}_3\text{COOCH}_3CH3​COOCH3​.
  • Ethanol is the alcohol, so the first word is ethyl, and propanoic acid is the acid, so the second word is propanoate.
CH3CH2COOH+CH3CH2OH⇌CH3CH2COOCH2CH3+H2O \text{CH}_3\text{CH}_2\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} CH3​CH2​COOH+CH3​CH2​OH⇌CH3​CH2​COOCH2​CH3​+H2​O
  • The ester formed is ethyl propanoate, CH3CH2COOCH2CH3\text{CH}_3\text{CH}_2\text{COOCH}_2\text{CH}_3CH3​CH2​COOCH2​CH3​.
Practical

Making ethyl ethanoate

  • Aim: to make a small sample of the ester ethyl ethanoate from ethanoic acid and ethanol.
  • Apparatus: boiling tube, dropping pipettes, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, ethanol, pure ethanoic acid, concentrated sulfuric acid, 250 cm3250\ \text{cm}^3250 cm3 beaker of water at about 70 ∘C70\ ^\circ\text{C}70 ∘C from a kettle, 100 cm3100\ \text{cm}^3100 cm3 beaker containing 50 cm350\ \text{cm}^350 cm3 of sodium hydrogencarbonate solution, eye protection and gloves.
  • Method:
    • Pour 1 cm31\ \text{cm}^31 cm3 of ethanol and 1 cm31\ \text{cm}^31 cm3 of pure ethanoic acid into the boiling tube.
    • Carefully add 3 drops of concentrated sulfuric acid as the catalyst.
    • Stand the boiling tube in the hot water bath for about 5 minutes.
    • Pour the warm mixture into the sodium hydrogencarbonate solution, which neutralises the leftover acids.
    • Waft the vapour from the beaker gently towards your nose.
  • Results:
    • The sodium hydrogencarbonate solution fizzes as it neutralises the unreacted acids.
    • A thin, oily, colourless layer of ester floats on the surface of the solution.
    • The mixture has a sweet, fruity smell, quite different from the sharp smell of ethanoic acid.
  • Conclusion: ethanoic acid and ethanol react, with concentrated sulfuric acid as the catalyst, to form the ester ethyl ethanoate.
  • Safety: wear eye protection and gloves, because concentrated sulfuric acid and pure ethanoic acid are corrosive, use a water bath instead of a flame because ethanol and the ester are highly flammable, and work in a well-ventilated room.
Common Mistake
  • Do not describe the colour change of potassium manganate(VII) as "purple to pink" or "clear", because the correct change is purple to colourless.
  • Do not call vinegar production fermentation, because bacteria oxidise ethanol using oxygen, while fermentation needs the absence of oxygen.
  • Do not put the acid part first when naming an ester, because the alcohol part comes first, as in ethyl ethanoate.
  • Do not leave water out of the esterification equation.
Exam technique

Writing about oxidation and esters

  • Name the oxidising agent in full as acidified (aqueous) potassium manganate(VII), including the Roman numeral.
  • Give the colour change with both colours, purple to colourless, when asked what you would see.
  • State the catalyst for esterification as concentrated sulfuric acid or an acid catalyst, and mention warming or heating.
  • To name an ester, take -yl from the alcohol and -oate from the acid, then check the total number of carbon atoms matches both parents.
Self review
  • Which reagent is used to oxidise ethanol to ethanoic acid in the laboratory?
  • What colour change is seen when ethanol is oxidised by acidified potassium manganate(VII)?
  • What is needed, besides ethanol, for bacteria to make vinegar?
  • Name the catalyst used to make an ester from a carboxylic acid and an alcohol.
  • Name the ester formed from ethanoic acid and methanol.
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Diagram showing the carboxyl group with carbonyl and hydroxyl parts labelled, plus methanoic, ethanoic, propanoic and butanoic acid structures

Carboxylic acids are organic compounds that contain the carboxyl group, −COOH-COOH−COOH. This functional group combines a carbonyl part, C=OC=OC=O, and a hydroxyl part, O−HO-HO−H, on the same carbon atom.

In a general formula, R−COOHR-COOHR−COOH, the RRR stands for the rest of the molecule. A compound is only a carboxylic acid if the whole −COOH-COOH−COOH group is present, so an −OH-OH−OH group by itself is not enough.

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