1.8 Carboxylic acids

• Properties of carboxylic acids

  • Carboxylic acids have a sharp, irritating odour
  • Short chain carboxylic acids are liquids- the polar OH group causes hydrogen bonds between molecules, which require a lot of energy to break them
  • Short chain carboxylic acids are soluble in water- the polar OH group allows them to form hydrogen bonds with water molecules
  • Carboxylic acids are weak acids - in aqueous solution they are only partially ionised
• Formation of carboxylic acids

  1. Mild oxidation of primary alcohols and aldehydes

  2. Hydrolysis of esters

  3. Hydrolysis of nitriles
• Mild oxidation of primary alcohols and aldehydes

  Primary alcohols are oxidised first to aldehydes and then the aldehydes are oxidised to carboxylic acids
• Mild oxidation of primary alcohols and aldehydes

  • CH,CH,CH,OH+2/0) -> CH,CH,COOH + H₂O
• Hydrolysis of esters


  1. Acid-catalysed hydrolysis produces the carboxylic acid and the alcohol

  2. Base (alkali)-catalysed hydrolysis produces the salt of the carboxylic acid and the alcohol
• Hydrolysis of esters


  • CH,COOCH, +H,0-> CH,COOH + CH,OH

  CH,COOCH, + NaOH-> CH,COONa + CH,OH
• Hydrolysis of nitriles


  First stage: Formation of the amide

  Second stage: Formation of the acid with the removal of ammonia

  Acid-catalysed hydrolysis produces the carboxylic acid and the ammonium salt

  Base (alkali)-catalysed hydrolysis produces the salt of the carboxylic acid and ammonia
• Hydrolysis of nitriles


  • CH,CH,CN +2H,O-> CH,CH,COOH + NH,

  CH,CN + HCl + 2H₂O -> CH,COOH + NH,Cl

  CH,CN + NaOH -> CH,COONa + NH,
• Reactions of carboxylic acids

  • Reactions of the OH group

  Reactions as a weak acid

  Reduction reactions
• Reactions of the OH group

  Reaction with phosphorus pentachloride, (PCl)

  Esterification reactions
• Reaction with phosphorus pentachloride, PCl,

  • RCOOH + PCl, -> RCOCI + POCI, + HCl

  CH,COOH + PCl, -> CH,COCI + POCI, + HCl
• Esterification


  Esters are condensation products of the reaction between a carboxylic acid and an alcohol. They contain the COO group, often called an ester link.
• Esterification


  • CH,CH,COOH + CH,CH,OH -> CH,CH,COOCH,CH, + H₂O
• Reactions as a weak acid

  Reaction with sodium carbonate

  Reaction with sodium hydroxide

  Reaction with ammonia
• Reaction with sodium carbonate

  • 2CH COOH + Na₂CO, -> 2CH,COONa + CO₂ + H₂O
• Reaction with sodium hydroxide

  • CH,CH,COOOH + NaOH -> CH,CH,COONa + H₂O
• Reaction with ammonia


  • CH,COOH + NH₃ -> NH,COONH,
• More complex carboxylic acids

  • Dibasic and tribasic weak acids are used in questions with excess of a reagent such as sodium carbonate or even other carbonates. Common examples are ethanedioic acid (commonly called oxalic acid) and 2-hydroxypropane-1,2,3-tricarboxylic acid (commonly called citric acid).
• Reduction reactions


  Carboxylic acids are reduced to aldehydes and then further reduced to primary alcohols. The reduction is carried out by heating the carboxylic acid under reflux with lithal, LiAlH,, in dry ether.
• Reduction reactions


  • CH,COOH + 2[H) -> CH,CHO + H₂O

  CH,CH,COOH + 4[H] -> CH,CH,CH,OH + H₂O
• Preparation of a carboxylic acid from a primary alcohol

  Add water and concentrated sulfuric acid to a pear-shaped flask

  Swirl the solution and cool the flask

  Add potassium dichromate(vi) (or sodium dichromate(vi))

  Add anti-bump granules

  Add the alcohol (ethanol) slowly with a vertical condenser in place and cool the reaction vessel in a water bath

  Heat the mixture under reflux

  Distil off the acid
• Carboxylic acids


  1. COOH, contain the functional group -COOH
• Aldehydes and Ketones


  Contain the functional group C=O
• Carboxylic acids also contain C=O, and C-O-H but have their own distinctive and unique properties
• Carboxylic acid derivatives


  • Acyl chlorides R-COCI
  • Esters RCOOR'
  • Amides RCONHR
• Nomenclature (from AS)


  1. Count the total number of carbon atoms in the longest chain-including the -COOH carbon

  2. If you have side groups, always count from the -COOH carbon as being carbon1

  3. If an acid has 2 carboxylic acid groups, the name is based on the name of parent chain and the suffix dioic acid is used
• Boiling points


  Governed by the ability of carboxylic acids to form hydrogen bonds
• Boiling points of alcohols are higher than those of alkanes of similar size because the alcohols can form hydrogen bonds
• Boiling points of carboxylic acids of similar size are higher still than their similar sized alcohol counterparts due to intermolecular hydrogen bonding to form dimers
• Solubility


  In the presence of water, carboxylic acids form hydrogen bonds with water molecules instead of dimerising
• Carboxylic acids with up to four carbon atoms will mix with water in any proportion
• Solubility of larger carboxylic acids decreases rapidly due to the longer hydrocarbon "tails" breaking hydrogen bonds with water
• Acid dissociation


  Carboxylic acids are weak acids that partially dissociate in aqueous solution to form H+ ions
• Preparation of a Carboxylic Acid


  1. Oxidation of a primary alcohol

  2. Oxidation of an aldehyde

  3. Hydrolysis of esters or nitriles
• Acid and Base Catalysed Ester hydrolysis

  1. Acid catalyst: Reversible reaction, poor yield

  2. Base catalyst: Forms salt of weak acid, drives equilibrium to produce good yield
• Hydrolysis of a Nitrile


  1. Acid-catalysed: Forms primary amide then carboxylic acid

  2. Base-catalysed: Forms sodium carboxylate, then carboxylic acid on addition of acid
• Practical: Preparing a carboxylic acid from an alcohol
• Reactions of carboxylic acids


  • Acid-base reactions with bases like sodium carbonate, sodium hydroxide, ammonia
  • Reaction with reactive metals to form metal salt and hydrogen
• Esterification: Carboxylic acids react with alcohols in the presence of a strong acid catalyst to form esters
• Reduction (with LiAlH4): Carboxylic acids can be reduced to alcohols