Carboxylic Acids and Esters

• Carboxylic acid functional group = RCOOH

  has different properties than carboxyl and alcoholic compounds
• Carboxylic acids are weak acids, and solubility of carboxylic acids decreases as the chain length increases
• Why are carboxylic acids considered to be weak acids?

  The carboxyl group attracts electrons away from the hydroxyl group, wearing the OH bond and making it easier to break and release a proton
• When reacting, carboxylic acids form carboxylate salts
• Esters are formed by a reaction between carboxylic acids and alcohols, under reflux with an acid catalyst, usually H2SO4.

  Hydrolysis reverses this reaction, forming an alcohol and a carboxylic acid from an ester and water
• Ester hydrolysis can be acidic or alkaline:

  • acidic, makes a carboxylic acid and alcohol in ester hydrolysis
  • Alkaline, to completion, makes a carboxylic acid salt and alcohol in saponification
• Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol).
• When hydrolysed in acidic conditions, esters form alcohols and carboxylic acids.

  When hydrolysed in alkaline conditions, esters form carboxylic acid salts.
• Uses of Propane-1,2,3-triol:

  • pharmaceutical and cosmetic (attracts water and prevents drying out)
  • medical edible solvent (toothpaste, food colouring)
  • plasticiser, allows molecules to slip over eachother (cellophane)
• Vegetable oils and animal fats can be hydrolysed in alkaline conditions to give soap (salts of long-chain carboxylic acids) and glycerol.
• Biodiesel, a mixture of methyl esters of long-chain carboxylic acids, is produced by reacting vegetable oils with methanol in the presence of a catalyst.

  
• The structure of an acyl chloride: carbonyl group attached to a chlorine.

  
• The structure of an ester:

  
• carboxylic acid -(sulfur dichromate oxide)-> acyl chloride -(carb. acid salts)-> acid anhydride
• The structure of acid anhydride: 'W shaped' with two carbonyl groups

  
• The structure of an amide: RCONR'R''

  
• What reactions do acyl chlorides/acid anhydrides have with water, alcohols, ammonia and amines?

  Nucleophilic addition-elimination reactions where the C=O bond is broken and reformed
• Both acyl chlorides and acid anhydrides have good leaving groups so make the C=OR group positive.
• The carbon atoms of the carbonyl group is susceptible to nucleophilic attack as it's a densely negative region.
• When reacting with acyl chlorides and acid anhydrides:

  • ammonia -> amide + salt
  • amine -> n-sub amine + salt
• When acyl chlorides and acid anhydrides react with

  • water, carb acid + acid or 2 carb acids are produced
  • alcohol, ester + acid or ester + carb acid
• Why is ethanoic anhydride used more in industry than ethanoyl chloride?

  Ethanoic anhydride is cheaper than ethanoyl chloride. Ethanoic anhydride is safer to use, less corrosive and doesn't produce hydrogen chloride gas