Carbonyls and Carboxylic Acids

Carbonyls and Carboxylic Acids

1. Aldehydes

   • CHO
   • Distilled from a primary alcohol
   • Reacts with acidified potassium dichromate (orange to green) producing a carboxylic acid.
   • Can be refluxed to produce a carboxylic acid.
   • Reacts with Tollens to produce a silver mirror
     • Ag+ is reduced and the aldehyde is oxidised.
   • With Fehlings colour changes from blue to brick red
     • Cu2+ is reduced and the aldehyde is oxidised.

2. Ketones

   • C=O
   • Full oxidation by distillation
   • Orange to green with acidified potassium dichromate.
   • There is no change with Tollens
   • There is no change with Fehlings.

3. Brady's reagent

   • Tests for the presence of a carbonyl group.
   • If an aldehyde or ketone is present, a yellow/orange precipitate is made.
   • 2-4 DNP is dissolved in methanol and sulphuric acid. This is Brady's reagent.
   • To distinguish between an aldehyde and ketone, test the melting points.
   • The yellow/orange precipitate can be filtered/recrystalised.

4. Carboxylic acids

   • They are weak
   • They are soluble in water. The shorter the chain, the more soluble it is due to polar bonds.
   • They react with carbonates to form a carboxylate salt, carbon dioxide and water.
   • They react with metal oxide to form a salt and water
   • React with a metal to produce a salt and hydrogen gas
   • They react with an alkali to produce a salt and water.
   • 3 main sites of reactivity: OH, C=O, C-O.
     • C-O undergoes substitution
     • C=O undergoes nucleophilic addition reactions
     • OH gives rise to acidity.

5. Esters

   • Alcohol + carboxylic acid
   • Requires concentrated H2SO4/HCl
   • Alcohol forms -yl
   • Carboxylic acid forms -ate
   • Water is produced as well
   • Hydrolysis of esters
     • By an acid or an alkali. In both cases heat under reflux with acid or alkali.
     • With acid, alcohol and carboxylic acid are produced.
       • Ester broken down by water, the acid is a catalyst.
     • With alkali, a salt and carboxylate salt are produced. It is an irreversible process.

6. Acyl chloride

   • Colourless fuming liquids
   • Strong smell - mixture of vinegar and the acrid smell of HCl gas.
   • Smell and the fumes are strong because ethanoyl chloride reacts with water vapour in the air.
   • COCl group
   • Can be used to produce esters. React acyl chloride with an alcohol. Produces hydrochloric acid too.
   • React an acyl chloride with ammonia/amine to produce an amide.
   • They react violently with cold concentrated ammonia solution.

7. Acid Anhydrides

   • Formed when water is removed from two carboxylic acids.
   • They react similarly to acyl chlorides. But they are less reactive so they may be safer to use in the lab.

8. NaBH4 - If you react this with a ketone or an aldehyde it will reduce them to their alcohols.

9. HCN - Increases the length of an aldehyde/ketones carbon chain.