Reactions of alkenes and alcohols (chemistry only)

7.2.1 Structure and formulae of alkenes


  • Hydrocarbons with double C=C bond
  • General formula: CnH2n

Why are alkene molecules unsaturated?

  • They contain 2 fewer hydrogen atoms than alkane with same no of carbon atoms

What are alkenes used?

  • Produce polymers
  • As starting materials for production of other chemicals

What are the first 4 members of alkenes? (Money eat proper big peanut)

Ethene (C2H4)     Propene (C3H6)
Butene (C4H8) Pentene (C5H10)

7.2.2 Reactions of alkenes

Reactions of alkenes

  • Alkenes are more reactive than alkanes coz they contain a C=C double bond
  • Most of their reactions are addition reactions, where C=C bond is broken to form C-C bond

Why do alkenes burn in air with smoky flames when they react with oxygen in combustion reactions?

  • Incomplete combustion

Describe reactions & conditions for addition of hydrogen, water & halogens to alkenes.

  • C=C bond is broken to form C-C bond

Describe the colour change when alkenes react with bromine water. (1)

  • Orange to colourless

Why alkenes react with bromine water but alkanes don’t?

  • Alkenes’ double bond make them more reactive than alkanes

Burning alkanes

  • Burn with smoky flames when react with O2 in combustion reactions coz incomplete combustion

To make polymers and many other chemicals

7.2.3 Alcohols


  • Contain functional group -OH
  • General formula: CnH2n+1OH

What are the first 4 members of homologous series of alcohols?

Describe what happens when any first four alcohols react with sodium, burn in air, are added to water, react with an oxidising agent

Sodium Alcohols react with sodium producing bubbles of H2(g)
Burn in air Burn well with O2 and are good fuels
Added to water Alcohols mix with water & form a neutral solution with pH7
React with oxidising agent Oxidised to carboxylic acids if they are left standing in air or treated with a mild oxidising agent
Carboxylic acids Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters

What are the main uses of these alcohols?

  • Alcohols used as solvents & fuels
  • Ethanol in alcoholic drinks


What are produced when sugar solutions are fermented using yeast?

  • Aq of ethanol

What are the conditions used for fermentation of sugar using yeast?

  • Sugar dissolved in water
  • Yeast is added
  • Mixture kept at 30°C
  • Air kept out of mixture

7.2.4 Carboxylic acids

Carboxylic acids have the functional group –COOH

What are the first 4 members of a homologous series of carboxylic acids?

Describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, react with alcohols

  • React with carbonates to form salt eg sodium carbonate, water & carbon dioxide
  • Dissolves in water to form weak acidic solutions (pH4-6)
  • Alcohol + carboxylic acid → ester + water
Reaction What happens
  • Carboxylic acids dissolve in water to produce acidic solutions
  • Acids are substances that form H+ ions when added to water
  • Eg CH3COOH(aq) + H2O(l) ⇌ CH3COO (aq) + H3O+(aq)
  • They are weak acids so only a small fraction of molecules break into ions when added to water
Carbonates Alcohol + acid → ester + water

Ethanol + ethanoic acid → ethyl ethanoate + water

Alcohols Produce bubbles of CO2(g)

Explain why carboxylic acids are weak acids in terms of ionisation and pH?

  • Partially ionised in aq solution
  • pH4-6