Skip to content

Course home

Sign up

10.4 Alcohols and carboxylic acids

10.4.1 Alcohols and the -OH functional group

The first four alcohols and their formulae

Definition

Alcohol

A member of the homologous series whose functional group is -OH.

  1. Methanol is CH3OH\text{CH}_3\text{OH}CH3​OH.
  2. Ethanol is C2H5OH\text{C}_2\text{H}_5\text{OH}C2​H5​OH.
  3. Propan-1-ol is C3H7OH\text{C}_3\text{H}_7\text{OH}C3​H7​OH.
  4. Butan-1-ol is C4H9OH\text{C}_4\text{H}_9\text{OH}C4​H9​OH.
  5. Each has one more CH2\text{CH}_2CH2​ than the one before, so they form a homologous series.
Key Idea

Writing the formula as CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH}Cn​H2n+1​OH keeps the −OH-\text{OH}−OH group visible.

The functional group is -OH

Definition

Functional group

The group of atoms in a molecule that gives a homologous series its characteristic reactions.

  1. Every alcohol contains an oxygen joined to a hydrogen, written −OH-\text{OH}−OH.
  2. That group is attached to a carbon atom in the chain.
  3. It is what gives every alcohol its characteristic reactions.
  4. The 1 in propan-1-ol says the group sits on the first carbon of the chain.
  5. The rest of the molecule is an ordinary hydrocarbon chain.
Note

The −OH-\text{OH}−OH in an alcohol is covalently bonded, and is not the hydroxide ion of an alkali.

Drawing the structures

  1. Each carbon still shows four bonds and each hydrogen one.
  2. The oxygen shows two bonds, one to a carbon and one to a hydrogen.
  3. Methanol is one carbon carrying three hydrogens and one −OH-\text{OH}−OH.
  4. Ethanol is two carbons joined by a single bond, with the −OH-\text{OH}−OH on the end carbon.
  5. Counting the bonds on the oxygen as well as the carbons checks the structure.
Practical
  • Method: weigh a capped spirit burner, burn the alcohol under a metal can holding a measured mass of water, then cap it, let it cool and reweigh it.
  • The four alcohols used are ethanol, propanol, butanol and pentanol, with the mass of water, the wick length and the distance to the flame kept the same.
  • Compare them by the energy transferred per gram of fuel burned, found from the temperature rise and the mass of alcohol used up.
  • Heat loss to the surroundings is the largest source of error, so a draught shield and a lid help.
  • Soot on the can shows incomplete combustion, which lowers the energy released.

Alcohols can be dehydrated to alkenes

Definition

Alkene

An unsaturated hydrocarbon containing a carbon to carbon double bond, with the general formula CnH2n.

  1. Dehydration removes a molecule of water from the alcohol.
  2. The water comes from the −OH-\text{OH}−OH group and a hydrogen from the next carbon.
  3. The two carbons left behind form a double bond.
  4. Ethanol is dehydrated to ethene: C2H5OH→C2H4+H2O\text{C}_2\text{H}_5\text{OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O}C2​H5​OH→C2​H4​+H2​O
  5. The product is an alkene, so a saturated molecule has become unsaturated.
Self review
  • Give the formulae of methanol, ethanol, propan-1-ol and butan-1-ol.
  • What is the functional group in an alcohol?
  • Describe the structure of ethanol.
  • What is formed when ethanol is dehydrated?
  • Why are the alcohols a homologous series?

10.4.2 Carboxylic acids and the -COOH functional group

The first four carboxylic acids

Definition

Carboxylic acid

A member of the homologous series whose functional group is -COOH.

  1. Methanoic acid is HCOOH\text{HCOOH}HCOOH.
  2. Ethanoic acid is CH3COOH\text{CH}_3\text{COOH}CH3​COOH.
  3. Propanoic acid is C2H5COOH\text{C}_2\text{H}_5\text{COOH}C2​H5​COOH.
  4. Butanoic acid is C3H7COOH\text{C}_3\text{H}_7\text{COOH}C3​H7​COOH.
  5. Each differs from the next by CH2\text{CH}_2CH2​, so they are a homologous series.
Key Idea

Ethanoic acid is the acid in vinegar, which is the one met most often.

The functional group is -COOH

Definition

Functional group

The group of atoms in a molecule that gives a homologous series its characteristic reactions.

  1. Every carboxylic acid contains the group −COOH-\text{COOH}−COOH.
  2. That group is a carbon joined to an oxygen by a double bond and to an −OH-\text{OH}−OH group.
  3. It always sits at the end of the carbon chain.
  4. It is what gives the family its characteristic reactions.
  5. The −OH-\text{OH}−OH inside it is what releases the hydrogen ion in solution.
Note

Writing the formula as −COOH-\text{COOH}−COOH rather than −CO2H-\text{CO}_2\text{H}−CO2​H keeps the two oxygens distinct.

Drawing the structures

  1. The end carbon shows four bonds: two to one oxygen, one to the other oxygen, and one to the chain.
  2. The second oxygen carries a hydrogen, completing the −OH-\text{OH}−OH.
  3. Methanoic acid has a hydrogen in place of the carbon chain.
  4. Ethanoic acid has a CH3\text{CH}_3CH3​ group attached to the −COOH-\text{COOH}−COOH.
  5. Counting two oxygens in the group is the usual check on the structure.

Structural formula of ethanoic acid showing a methyl group (CH3) bonded to a carboxyl group (COOH).

Example
  • Methanoic acid: HCOOH\text{HCOOH}HCOOH, one carbon in all.
  • Ethanoic acid: CH3COOH\text{CH}_3\text{COOH}CH3​COOH, two carbons in all.

Their solutions show typical acidic properties

Definition

Acid

A substance that is a source of hydrogen ions when it dissolves in water.

  1. A carboxylic acid dissolves in water and releases hydrogen ions.
  2. Its solution therefore has a pH below 7 and turns litmus red.
  3. It reacts with a metal to give a salt and hydrogen: ethanoic acid+magnesium→magnesium ethanoate+hydrogen\text{ethanoic acid} + \text{magnesium} \rightarrow \text{magnesium ethanoate} + \text{hydrogen}ethanoic acid+magnesium→magnesium ethanoate+hydrogen
  4. It reacts with a carbonate to give a salt, water and carbon dioxide.
  5. It reacts with an alkali to give a salt and water, which is neutralisation.

A chemical equation and structural diagrams showing ethanoic acid dissociating in water to release a hydrogen ion, which forms a hydronium ion. The ethanoate ion is also produced.

Self review
  • Give the formulae of methanoic, ethanoic, propanoic and butanoic acids.
  • What is the functional group in a carboxylic acid?
  • Describe the structure of ethanoic acid.
  • Name the products when ethanoic acid reacts with magnesium.
  • What would you see if ethanoic acid were added to a carbonate?

10.4.3 Oxidation of alcohols and homologous series

Ethanol can be oxidised to ethanoic acid

Definition

Oxidation

The loss of electrons by a substance.

  1. Ethanol can be converted into ethanoic acid.
  2. The change is an oxidation: ethanol+oxygen→ethanoic acid+water\text{ethanol} + \text{oxygen} \rightarrow \text{ethanoic acid} + \text{water}ethanol+oxygen→ethanoic acid+water
  3. The −OH-\text{OH}−OH group of the alcohol becomes the −COOH-\text{COOH}−COOH group of the acid.
  4. The number of carbon atoms in the molecule does not change.
  5. The reagents used to bring this about are not required here.

A chemical equation showing the oxidation of ethanol to ethanoic acid. The structural formulas show the conversion of the hydroxyl group (-OH) in ethanol to a carboxyl group (-COOH) in ethanoic acid, using acidified potassium dichromate and heat under reflux.

Key Idea

The carbon chain is untouched: only the functional group changes.

The same change happens to other alcohols

  1. Methanol is oxidised to methanoic acid.
  2. Propan-1-ol is oxidised to propanoic acid.
  3. Butan-1-ol is oxidised to butanoic acid.
  4. In each case the acid has the same number of carbons as the alcohol.
  5. The name changes from -ol to -oic acid, keeping the same stem.
Example
  • Ethanol gives ethanoic acid, and propan-1-ol gives propanoic acid.
  • The stem stays: eth- stays eth-, prop- stays prop-.

A homologous series reacts the same way throughout

Definition

Homologous series

A family of compounds with the same general formula, differing by CH2 from one member to the next, with similar chemical properties and a gradual change in physical properties.

  1. Members of a series all contain the same functional group.
  2. Reactions happen at that group, so they are the same for every member.
  3. The carbon chain attached to it changes the physical properties, not the chemistry.
  4. A reaction shown for one member can therefore be extended to the others.
  5. This is what makes a homologous series worth learning as a family.
Note

Similar does not mean identical: a longer chain often reacts a little more slowly.

Predicting the product for another member

  1. Identify the functional group in the molecule given.
  2. Recall what that group does in the reaction being asked about.
  3. Apply the same change to the molecule in front of you.
  4. Keep the carbon chain the same unless the reaction is one that breaks it.
  5. Name the product using the stem of the starting material.
Exam technique
  • A prediction quotes the functional group as the reason, not the specific example learned.
  • Counting the carbons before and after catches a miscounted product name.
  • An answer that changes the chain length is almost always wrong for these reactions.
Self review
  • What is ethanol oxidised to?
  • Which functional group becomes which in that change?
  • What is propan-1-ol oxidised to?
  • Why do all the members of a homologous series react similarly?
  • Predict the product when pentan-1-ol is oxidised.

10.4.4 Fermentation and fractional distillation of ethanol

Fermentation converts carbohydrates into ethanol

Definition

Fermentation

The conversion of sugars into ethanol and carbon dioxide by the enzymes in yeast.

Definition

Enzyme

A biological catalyst that speeds up a reaction in a living organism.

  1. Yeast is added to a solution of carbohydrate in water.
  2. The yeast supplies the enzymes that bring the reaction about.
  3. The sugar is converted into ethanol and carbon dioxide: glucose→ethanol+carbon dioxide\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}glucose→ethanol+carbon dioxide
  4. The carbon dioxide escapes, so the mixture bubbles as the reaction runs.
  5. The carbohydrate must be in aqueous solution, since the enzymes work in water.
Key Idea

The enzymes in yeast are the catalyst, which is why the conditions suit a living organism.

The conditions fermentation needs

  1. A warm temperature is used, around body temperature.
  2. Too cold and the enzymes work too slowly to be useful.
  3. Too hot and the enzymes are damaged and stop working altogether.
  4. Air is kept out, because with oxygen the yeast respires aerobically and produces very little ethanol.
  5. Fermentation stops on its own once the ethanol concentration gets high enough to harm the yeast.
Common Mistake
  • Fermentation alone cannot give concentrated ethanol, because the yeast dies first.
  • Ethanol left standing in air later sours to ethanoic acid, but that is the work of other microorganisms.

Fractional distillation concentrates the ethanol

Definition

Fractional distillation

A method that separates miscible liquids with close boiling points, using a fractionating column in which vapours repeatedly condense and evaporate.

  1. The fermented mixture is mostly water, with ethanol dissolved in it.
  2. Ethanol boils at about 78 ∘C78\ ^{\circ}\text{C}78 ∘C and water at 100 ∘C100\ ^{\circ}\text{C}100 ∘C.
  3. Heating the mixture makes the ethanol evaporate first.
  4. The vapour passes up a fractionating column and through a condenser.
  5. The liquid collected is a far more concentrated solution of ethanol.
Note

The separation works because the two liquids have different boiling points.

Following the temperature during the distillation

  1. The thermometer bulb is placed level with the side arm, where the vapour leaves.
  2. The temperature steadies near 78 ∘C78\ ^{\circ}\text{C}78 ∘C while ethanol is distilling over.
  3. The liquid collected at that temperature is the ethanol-rich fraction.
  4. A rise towards 100 ∘C100\ ^{\circ}\text{C}100 ∘C shows that water is now coming over.
  5. Collection is stopped at that point, so that the product stays concentrated.
Self review
  • What does yeast supply in fermentation?
  • Write the word equation for the fermentation of glucose.
  • Why must air be kept out during fermentation?
  • Why does fermentation stop on its own?
  • Why does ethanol distil over before water?

How was this guide?

Teach Genie

Review 10.4 Alcohols and carboxylic acids by teaching Genie

Teach it back in your own words, spot gaps, and remember it better.

Start teaching
Genie and Baby Genie

Lesson

Recap your knowledge with an interactive lesson

9 minute activity

Start lesson

Alcohols contain the functional group −OH-\text{OH}−OH, which is covalently bonded to a carbon atom. Their general formula can be written as CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH}Cn​H2n+1​OH.

Carboxylic acids contain the functional group −COOH-\text{COOH}−COOH at the end of the carbon chain. In this group, one oxygen is double-bonded to carbon and the other forms an −OH-\text{OH}−OH group.

Members of either homologous series have the same functional group and similar chemical properties. Successive members differ by CH2\text{CH}_2CH2​ and show gradual changes in physical properties.

Questions

Put it into practice with exam-style questions

43 exam-style questions

Practice questions

Question 1

6 marks

Many hand sanitisers contain alcohols such as ethanol and propanol.

The table shows the names and formulae of the first four alcohols.

Name of alcoholFormula
methanolCH3OH\mathrm{CH_{3}OH}CH3​OH
ethanolX
propanolC3H7OH\mathrm{C_{3}H_{7}OH}C3​H7​OH
YC4H9OH\mathrm{C_{4}H_{9}OH}C4​H9​OH

Flashcards

Remember key concepts with flashcards

20 flashcards

Practice flashcards

What functional group gives alcohols their characteristic reactions?

10.4 Alcohols and carboxylic acids Revision Guide

  1. GCSE
  2. /Chemistry
  3. /10.4 Alcohols and carboxylic acids

Revision notes for Edexcel GCSE Chemistry 10.4 Alcohols and carboxylic acids: explanations and worked examples on 10.4.1 Alcohols and the -OH functional group, 10.4.2 Carboxylic acids and the -COOH functional group, 10.4.3 Oxidation of alcohols and homologous series, and 10.4.4 Fermentation and fractional distillation of ethanol.

Revision guides