Carbonyls and carboxylic acids

• Carboxylic acids


  Have the functional group –COOH
• Carboxylic acids


  • Partially dissociate in water, making them weak acids
  • Can react with carbonates to liberate CO2
  • Can react with alkalis and metal oxides as acids do
• Reaction of carboxylic acids with carbonates

  2CH3COOH (aq) + Na2CO3 (s) → 2CH3COO–Na+ (aq) + CO2 (g) + H2O (l)
• Carboxylic acids with short carbon chains


  • Are soluble because they can form hydrogen bonds with water molecules
  • The C=O and O-H bond in carboxylic acids are polar bonds, allowing for the hydrogen bonds to form
  • As the chain length increases, the solubility decreases, because the size of the hydrophobic non-polar part of the molecule increases
• Esters


  Have the general formula RCOOR'
• Ester formation


  Esters can be made when alcohols and carboxylic acids are reacted in the presence of a sulphuric acid catalyst
• Ester naming


  The alcohol gives the first part of the name and the acid gives the second part of the name
• Acid catalysed hydrolysis of esters


  Forms a carboxylic acid and an alcohol
• Base catalysed hydrolysis of esters


  Forms a carboxylate salt and an alcohol
• Brady's reagent


  A mixture of methanol, sulfuric acid and a solution of 2,4-dinitrophenylhydrazine (2,4-DNP), used to test for the presence of carbonyls
• When Brady's reagent is added to an aldehyde or a ketone


  An orange transparent solution forms a yellow/orange precipitate of a 2,4-DNP derivative
• The observation is not seen with carboxylic acids and esters despite these compounds also having C=O bonds
• Tollen's reagent


  Ammoniacal silver nitrate, a colourless weak oxidising agent which reacts with aldehydes, but not ketones
• When Tollen's reagent is added to an aldehyde

  A silver mirror is observed
• When Tollen's reagent is added to an aldehyde


  The silver ions are reduced to silver metal and the aldehyde is oxidised to a carboxylic acid
• No observation is made when Tollen's reagent is added to a ketone
• Acyl chlorides


  A derivative of carboxylic acids, where the –OH group is replaced by a –Cl group
• Preparation of acyl chlorides

  By reacting a carboxylic acid with SOCl2
• Reaction of acyl chlorides with alcohols


  Forms esters in a nucleophilic addition–elimination reaction
• Acyl chlorides will react with a variety of nucleophiles
• Aldehydes and ketones


  Carbonyl compounds containing the C=O functional group
• Aldehyde


  Carbonyl group at the end of the carbon chain, suffix -al, functional group is -CHO
• Ketone


  Carbonyl group is not at the end of the carbon chain, suffix -one, functional group is -CO
• Oxidation of aldehydes


  Aldehydes can be oxidised to form carboxylic acids by heating under reflux with potassium dichromate (oxidising agent) and concentrated sulphuric acid
• Carbonyls


  • Have a permanent dipole making them susceptible to nucleophilic addition reactions
• Reduction of aldehydes and ketones

  Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols using NaBH4 as the reducing agent
• Nucleophilic addition of cyanide ions


  Can be used to extend the length of a carbon chain
• Reaction of carbonyls with KCN and dilute acid

  Produces a hydroxynitrile
• Aldehydes and unsymmetrical ketones form two optical isomers, as there is an equal chance of either isomer being formed. This is a racemic mixture
• Cyanide-containing compounds can be toxic or irritants