Carboxylic Acids

• Carboxylic acids are a homologous series containing the functional group COOH

  • general formula of CnH2n+1COOH
• The carboxylic acids behave like other acids
• They react with:

  • metals to form a salt and hydrogen
  • carbonates to form a salt, water and carbon dioxide gas
• Carboxylic acids also take part in neutralisation reactions to produce salt and water
• Ethanoic acid (also called acetic acid) - the acid used to make vinegar, which contains around 5% by volume of ethanoic acid
• The salts formed by the reaction of carboxylic acids all end –anoate

  • methanoic acid forms a salt called methanoate, ethanoic forms a salt called ethanoate etc.
• In the reaction with metals, a metal salt and hydrogen gas are produced
• The reaction of ethanoic acid with magnesium forms the salt magnesium ethanoate and hydrogen gas:2CH3COOH + Mg → (CH3COO)2Mg + H2
• In the reaction with hydroxides, salt and water are formed in a neutralisation reaction

  • e.g. the reaction between potassium hydroxide and propanoic acid forms the salt potassium propanoate and water: CH3CH2COOH + KOH → CH3CH2COOK + H2O
• • In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced
  • e.g. in the reaction between potassium carbonate and butanoic acid, the salt potassium butanoate is formed with water and carbon dioxide: 2CH3CH2CH2COOH + K2CO3 → 2CH3CH2CH2COOK + H2O + CO2
• 2 methods used to make carboxylic acids are:

  • Oxidation by fermentation
  • Using oxidising agents
• The fermentation of ethanol will produce a weak solution of vinegar (ethanoic acid)

  • occurs when a bottle of wine is opened as bacteria in the air will use atmospheric oxygen from air to oxidise the ethanol in the wine
  • C2H5OH (aq) + O2 (g) → CH3COOH (aq)+ H2O (l)
• The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid
• Oxidising agent potassium manganate(VII) can be used instead of fermentation

  • involves heating ethanol with acidified potassium manganate(VII) in the presence of an acid
• Heating ethanol is performed under reflux which involves heating the reaction mixture in a vessel with a condenser attached to the top

  • The condenser prevents the volatile alcohol from escaping the reaction vessel as alcohols have low boiling points
• Reflux diagram

  A) out

  B) in

  C) reactants

  D) water

  E) heat

  
• The equation for the reaction oxidising reaction is:

  • CH3CH2OH (aq) + [O]  →  CH3COOH (aq) + H2O (l)
  • The oxidising agent is represented by the symbol for oxygen in square brackets
• The solution will change from purple to colourless when using oxidising agents
• Alcohols and carboxylic acids react to make esters in esterification reactions
• Esters are compounds with the functional group R-COO-R

  • sweet-smelling oily liquids used in food flavourings and perfumes
• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

  • CH3COOH (aq) + C2H5OH (aq)  ⇌  CH3COOC2H5 (aq) + H2O (l)

  
• An ester is made from an alcohol and carboxylic acid

  • The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’
  • The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’
• Carboxylic acids are weak acids - only a small fraction of molecules break down to form H+ ions when added to water

  • only partially ionise in the water
• • Carboxylic acids added to water
  • 2 electrons are added to the carboxylic acid

  A) breaks

  B) H+

  C) electron

  D) electron

  
• Ethanoic acid is found in vinegar and is a weak acid

  • reacts with metals, bases and carbonations to form salts and with indicators
  • acid + based → salt and water
• Esters have fruity smells

  • occurs naturally as flavours in food flavourings and smells in flowers
• Esters are used in the food industry and in medicine
• The first part of the name of the ester comes from the carbons from the alcohol and the second part of the name comes from the H-C bond from the carboxylix acid

  
• Metal carbonate + acid = 


  metal salt + carbon dioxide + water
• Metal + acid =

  metal salt + hydrogen
• What are the properties of ethanoic acid?

  colourless, aqueous, has a pungent smell
• What are the products of a reaction between carboxylic acids and alcohol?

  Ester and water