Skip to content

Course home

7.2.3 Alcohols

7.2.3 Alcohols

Alcohols: the -OH\text{-OH}-OH functional group defines the homologous series

Definition

Alcohol

An organic compound containing the hydroxyl functional group, -OH\text{-OH}-OH.

  1. Alcohols contain the functional group -OH\text{-OH}-OH, which gives them their characteristic reactions.
  2. Methanol, ethanol, propanol and butanol are the first four members of one homologous series.
  3. Each member has one more CH2\text{CH}_2CH2​ unit than the one before it.
  4. Alcohol names end in -ol, which helps you spot them.
  5. A condensed structural formula groups atoms together but still shows the -OH\text{-OH}-OH group.
    1. Methanol is CH3OH\text{CH}_3\text{OH}CH3​OH.
    2. Ethanol is CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}CH3​CH2​OH.
    3. Propanol is CH3CH2CH2OH\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}CH3​CH2​CH2​OH.
    4. Butanol is CH3CH2CH2CH2OH\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}CH3​CH2​CH2​CH2​OH.
Key Idea
  • The prefix gives the carbon count: meth- one, eth- two, prop- three, but- four.
  • To recognise an alcohol, look for an -OH\text{-OH}-OH group on a carbon chain, or a name ending in -ol.

The first four alcohols dissolve in water and work as fuels and solvents

Definition

Aqueous solution

A mixture formed when a substance is dissolved in water.

  1. The first four alcohols are soluble in water, though less dissolves as the carbon chain gets longer.
  2. Their aqueous solutions are neutral.
  3. They can be used as fuels, because they release energy when they burn.
  4. They can be used as solvents in products such as perfumes and cleaning fluids.
  5. Ethanol is the alcohol found in alcoholic drinks.
Example
  • Ethanol is blended with petrol and burned as a fuel.
  • Ethanol dissolves fragrance chemicals when it is used as a solvent in perfume.
  • Methanol is poisonous, so not every alcohol is safe to drink.

Alcohols fizz with sodium and burn to release energy

  1. Any of the first four alcohols reacts with sodium, fizzing as it releases hydrogen gas.
  2. Any of them burns in air and transfers energy to the surroundings, so combustion is exothermic.
  3. Complete combustion gives carbon dioxide and water.
    1. 2CH3OH(l)+3O2(g)→2CO2(g)+4H2O(l)2\text{CH}_3\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 4\text{H}_2\text{O}(l)2CH3​OH(l)+3O2​(g)→2CO2​(g)+4H2​O(l)
    2. CH3CH2OH(l)+3O2(g)→2CO2(g)+3H2O(l)\text{CH}_3\text{CH}_2\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(l)CH3​CH2​OH(l)+3O2​(g)→2CO2​(g)+3H2​O(l)
    3. 2CH3CH2CH2OH(l)+9O2(g)→6CO2(g)+8H2O(l)2\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}(l) + 9\text{O}_2(g) \rightarrow 6\text{CO}_2(g) + 8\text{H}_2\text{O}(l)2CH3​CH2​CH2​OH(l)+9O2​(g)→6CO2​(g)+8H2​O(l)
    4. CH3CH2CH2CH2OH(l)+6O2(g)→4CO2(g)+5H2O(l)\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}(l) + 6\text{O}_2(g) \rightarrow 4\text{CO}_2(g) + 5\text{H}_2\text{O}(l)CH3​CH2​CH2​CH2​OH(l)+6O2​(g)→4CO2​(g)+5H2​O(l)
Exam technique
  • You need balanced equations for combustion only, not for the reactions with sodium or oxidising agents.
  • Balance by changing the coefficients in front of formulae, never the alcohol's formula, and write oxygen as O2\text{O}_2O2​.

Oxidising agents turn alcohols into carboxylic acids

Definition

Oxidising agent

A substance that causes another substance to gain oxygen.

  1. An oxidising agent adds oxygen to an alcohol, turning it into a carboxylic acid.
  2. The alcohol is said to be oxidised.
  3. Methanol becomes methanoic acid, ethanol becomes ethanoic acid, propanol becomes propanoic acid and butanol becomes butanoic acid.
  4. Acidified potassium dichromate(VI) is a common oxidising agent that turns from orange to green as the alcohol is oxidised.
Example
  • If acidified potassium dichromate(VI) turns from orange to green when warmed with ethanol, the ethanol has been oxidised to ethanoic acid.
  • In general, oxidising an alcohol gives a carboxylic acid.

Fermenting sugar with yeast makes aqueous ethanol

Definition

Yeast

A microorganism whose enzymes can convert sugar into ethanol and carbon dioxide during fermentation.

  1. Fermentation uses yeast to turn sugar into ethanol and carbon dioxide.
  2. Sugar solution: glucose is dissolved in water.
  3. Yeast: enzymes in the yeast carry out the reaction.
  4. Temperature: kept warm at about 30∘C30^\circ\text{C}30∘C; too cold is slow and too hot kills the yeast.
  5. No oxygen: air is kept out so fermentation happens without oxygen.
  6. The word equation is glucose →\rightarrow→ ethanol + carbon dioxide.
  7. The product is an aqueous solution of ethanol, because the ethanol stays mixed with water.
Self review
  • What functional group is present in every alcohol?
  • Give the names and condensed formulae of the first four alcohols.
  • Which gas forms when an alcohol reacts with sodium?
  • What are the products of the complete combustion of an alcohol?
  • What conditions are needed to ferment sugar solution with yeast?
PreviousNext

How was this guide?

Teach Genie

Review 7.2.3 Alcohols 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

8 minute activity

Start lesson

Alcohols are organic compounds containing the hydroxyl functional group, -OH\text{-OH}-OH. Their names end in -ol, and the prefix shows the number of carbon atoms: meth- means one, eth- means two, prop- means three, and but- means four.

The first four alcohols are methanol, ethanol, propanol and butanol. Each successive member has one additional CH2\text{CH}_2CH2​ unit.

The condensed structural formulae are methanol, CH3OH\text{CH}_3\text{OH}CH3​OH; ethanol, CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}CH3​CH2​OH; propanol, CH3CH2CH2OH\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}CH3​CH2​CH2​OH; and butanol, CH3CH2CH2CH2OH\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}CH3​CH2​CH2​CH2​OH. These formulae show the increasing carbon-chain length across the homologous series.

Alcohols react with sodium to form a sodium alkoxide and hydrogen. They undergo complete combustion in oxygen to form carbon dioxide and water. Under suitable conditions, primary alcohols can be oxidised to carboxylic acids using acidified potassium dichromate(VI). Secondary alcohols are oxidised to ketones, while tertiary alcohols do not readily undergo the same oxidation.

Flashcards

Remember key concepts with flashcards

26 flashcards

Practice flashcards

Which functional group is present in every alcohol?

7.2.3 Alcohols Revision Guide

  1. GCSE
  2. /Chemistry
  3. /7.2.3 Alcohols

Revision notes for AQA GCSE Chemistry 7.2.3 Alcohols. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.