Chap 23A - Carboxylic acid

State general formula, functional group and lactic acid formula

• General formula of monocarboxylic acids: CnH2n+1COOH 

• Functional group: carboxyl group 

• Example: 2-hydroxypropanoic acid (lactic acid) 

Describe bp of carboxylic acid + why less volatile than alcohols

• Lower members are liquids with pungent odours 

• Less volatile (high bp) than alcohols of similar Mr 

  • Carbo has stronger HB than alcohols as -OH bond is more polarised due to the presence of electron–withdrawing inductive C=O group

  • Carbo also dimerise in the liquid state and in non–polar solvents, forming 2 HB between each pair of molecules 

  • In polar solvent: RCO2H ⇌ RCO2– + H+ 

Describe solubility of carboxylic acid

• Lower acids are completely miscible with water because  –CO2H can form HB with water

• As the length of the non-polar hydrocarbon chain increases -> solubility in water decreases but solubility in non-polar solvent increase

Draw resonance structures of RCOOH and RCO2-

Explain why carboxylate anion is stabalised

• Carbo is weak acid, dissociate partially: RCO2H ⇌ RCO2– + H+ 

• C–O bonds are equal in length -> negative charge is equally distributed between the 2 O atoms, leading to a stabilisation of the carboxylate anion

• An electron–withdrawing inductive group in RCO2– reduces the electron density of the negative charge on the carboxylate anion by charge dispersal -> stabilises the RCO2– and increases the strength of the acid

Why are carboxylic acids stronger acids than alcohols

• An electron–withdrawing inductive group in RCO2– reduces the electron density of the negative charge on the carboxylate anion by charge dispersal -> stabilises the RCO2– and increases the strength of the acid

• The RO- ion is destabilised by electron- donating inductive effect of the R group -> negative charge is intensified

• The first equilibrium position lies greater to the right compared to the second -> hence RCOOH is stronger acid than ROH 

State ways to prepare carboxylic acids

Oxidation of Primary Alcohols and Aldehydes

Hydrolysis of Nitriles

Side-chain oxidation of alkylbenzene to benzoic acid

Describe preparation of carboxylic acid (Oxidation of Primary Alcohols and Aldehydes) + equation

1. Oxidation of alcohol 

• Type of reaction: Oxidation

• Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), heat under reflux OR KMnO4(aq), H2SO4(aq), heat

• CH2OH -> COOH 
  

2. Oxidation of aldehydes 

• Type of reaction: Oxidation

• Reagents and conditions: K2Cr2O7(aq), H2SO4(aq), heat OR KMnO4(aq), H2SO4(aq), heat

Describe preparation of carboxylic acid (ACID hydrolysis nitrile) + equation

• Type of reaction: Hydrolysis

• Reagents and conditions: H2SO4(aq), heat

Describe preparation of carboxylic acid (ALKALINE hydrolysis nitrile) + equation

• Type of reaction: Hydrolysis

• Reagents and conditions: NaOH(aq), heat, followed by H2SO4(aq)

• Distinguishing test as NH3 produced turns damp red litmus paper blue 

Explain how alkaline hydrolysis can take place via amide (with equations)

 Reaction can take place via the amide

• RCN + H2O → RCONH2

• Same as above: 

  • RCONH2 + H2O → RCO2–NH4+

  • RCO2−NH4+ + H+ → RCO2H + NH4+

  • RCO2−NH4+ + OH– → RCO2– + NH3 + H2O

Describe side-chain oxidation of alkylbenzene to benzoic acid

• Type of reaction: Oxidation

• Reagents and conditions: KMnO4(aq), H2SO4(aq), heat OR KMnO4(aq), NaOH(aq), heat, followed by H2SO4(aq)

Explain why C=O does NOT undergo addition

• C=O does NOT undergo addition 

  • Reason: Lone pair of electrons in p orbital in O of -OH group interacts with the p orbital of the carbonyl C atom -> results in an increase in delocalisation of electrons and greater stability

  • This stability would be lost if a reagent is added to the carbonyl group -> carboxylic acids are more resistant to addition than aldehydes and ketones

Draw delocalisation of electrons in RCOOH