carboxylic acids- reactions

why are carboxylic acids called weak acids?

• in aqueous solution they are only slightly ionised, to give low concentrations of hydronium ions and alkanoate ions (often called carboxylate ions)

• This partial ionisation in solution means that carboxylic acids are weak acids

• This means that the position of the equilibrium lies to the left and that the concentration of H⁺ is much smaller than the concentration of the carboxylic acid

• However, the concentration of hydrogen ions is sufficient to react with an aqueous solution of sodium carbonate or sodium hydrogen carbonate to produce carbon dioxide

how aqueous solutions of sodium carbonate or sodium hydrogen carbonate can be used as a test for presence of a carboxylic acid

• Sodium carbonate: 2RCOOH + Na₂CO₃ → 2RCOO^-Na⁺ + CO₂ + H₂O

• Ionic equation with carbonates: 2RCOOH + CO₃^2- → 2RCOO^- + CO₂ + H₂O

• Sodium hydrogen carbonate: RCOOH + NaHCO₃ → RCOO^-Na⁺ + CO₂ + H₂O

• Ionic equation with hydrogen carbonates: RCOOH + HCO₃^- → RCOO^- + CO₂ + H₂O

reactions of carboxylic acid with LiAlH4 in dry ether

• Carboxylic acids can undergo reduction when they react with a reducing agent such as lithium tetrahydridoaluminate, LiAlH₄, suspended in dry ether at room temperature 

• A carboxylic acid will be reduced to a primary alcohol, for example

CH₃CH₂COOH (l) + 4[H] → CH₃CH₂CH₂OH (l) + H₂O (l)

• Addition of water at the end will destroy any excess lithium tetrahydridoaluminate

• reduction reaction takes place

reaction of carboxylic aid with bases

• a neutralisation reaction occurs

• Carboxylate/carboxylic acid salt formed

• Carboxylic acids can form salts with metals, alkalis and carbonates.

• In the reaction with metal oxides a metal salt and water are produced

• 2CH₃COOH (aq) + MgO (s) → (CH₃COO)₂Mg (aq) + H₂O (l) 

  • In the reaction with alkalis a salt and water are formed in a neutralisation reaction

    • For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:

  CH₃COOH (aq) + KOH (aq) → CH₃COOK (aq) + H₂O (l)

  • In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced

    • For example in reaction with potassium carbonate the salt potassium ethanoate is formed:

  2CH₃COOH (aq) + K₂CO₃ (s) → 2CH₃COOK (aq) + H₂O (l) + CO₂ (g)

reaction of carboxylic acid with PCl5

• Carboxylic acids react with solid phosphorus(V) chloride to form an acyl chloride

• For example, propanoic acid will react with phosphorus(V) chloride to form propanoyl chloride, phosphorus trichloride oxide and hydrogen chloride 

CH₃CH₂COOH (l) + PCl₅ (s) → CH₃CH₂COCl (l) + POCl₃ (l) + HCl (g)

• In this reaction, steamy fumes of HCl are produced 

• The Acyl chlorideand POCl3 liquid products can be separated by fractional distillation

reaction of carboxylic acid with alcohols

• When carboxylic acids react with alcohols an ester is formed

• Conditions: Warm/acid catalyst, H+ eg concentrated H₂SO₄ as catalyst

• Esters are compounds with an -COOR functional group and are characterised by their sweet and fruity smells

• They are prepared from the condensation reaction called esterification between a carboxylic acid and alcohol

• The first part of the ester’s name comes from the alcohol and the second part of the name comes from the carboxylic acid

  • E.g. Propanol and ethanoic acid will give the ester propyl ethanoate

what are acyl chlorides

• acyl chlorides are derivatives of carboxylic acids by substitution of the -OH group by a chlorine atom

• They are named by identifying the parent hydrocarbon chain and adding the suffix -oyl chloride. They can also be named by removing the -oic acid from the carboxylic acid and adding -oyl chloride

what are acyl anhydrides?

• Acid anhydrides are also derivatives of carboxylic acids formed by substitution of the -OH group by an alkanoate

  • Acid anhydrides are named by identifying the parent hydrocarbon chain and adding the suffix -oic anhydride

  • They can also be named by removing the -oic acid from the carboxylic acid and adding -oic anyhydride

reactivity of acyl chlorides

• the electronegativity of the O and Cl make acyl chlorides very reactive

• the S+ carbon is open to nucleophilic attack

• In nucleophilic addition-elimination reactions, the nucleophilic addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule

• all reactions take place at room temperature. The reactions are vigorous and do not need a catalyst