Carboxylic Acids

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

  • investigate the structural formulae, properties and functional group including:

– primary, secondary and tertiary alcohols
– aldehydes and ketones (ACSCH127)
– amines and amides
– carboxylic acids
  • explain the properties within and between the homologous series of carboxylic acids, amines and amides with reference to the intermolecular and intramolecular bonding present

  • investigate the differences between an organic acid and organic base

Carbonyl Compounds: Structure, Properties & Reactions

This video introduces a new group of organic compounds – carbonyl compounds, including the structure, properties and reactions of


Structure and Nomenclature

Aldehyde, ketone and carboxylic acids all contain a carbonyl carbon that is sp2 This means both functional groups contain a C=O bond, of which one is a reactive π-bond, the other is an unreactive s-bond. 

The carbonyl carbon in a carboxylic acid is also bonded to a –OH group. Note that this should not be referred to as an alcohol group. The –OH in a carboxylic acid functional group should not be considered separately, but as part of the entire group of atoms (–COOH). 


    Functional group



    Generic structure








    Carboxylic acid

    -oic acid



    Nomenclature priority

    - In order of decreasing priority: carboxylic acid, aldehyde, ketone, alcohol, alkene, alkyne and alkanes.

    - In the presence of carboxylic acid, aldehyde or ketone functional group, an alcohol will be referred to by its prefix ‘hydroxyl’ 


    Properties of Carboxylic Acid

    Acidity of Carboxylic Acids

    • Carboxylic acids are organic weak acids.
    • The deprotonation of hydrogen from a carboxylic acid forms a carboxylate ion (conjugate base).


    Why is carboxylic acid acidic?

    The proton or hydrogen atom attached to oxygen is acidic because:

    • O–H bond is polarised and weak due to oxygen’s high electronegativity
    • Resonance stabilisation of the conjugate base (carboxylate ion)


    What is resonance stabilisation? 

    Resonance stabilisation of carboxylate ion


      The negative charge of the carboxylate ion is delocalised. This means the extra electron lone pair of the negatively charged oxygen atom can move to the adjacent bond position to form a C=O bond with the adjacent carbon. When this occurs, the electrons in the old C=O moves to the other oxygen atom to become an electron lone pair. 

      The delocalisation of this electron lone pair is referred to as resonance stabilisation. The new chemical structure formed from resonance stabilisation is called the resonance structure of carboxylate ion.

      Resonance stabilisation of carboxylate ions explains why carboxylic acids are willing to give away their protons (and hence are acidic). It also explains why alcohols, despite having –OH group, do not behave as proton donors in water. The conjugate base of alcohols (alkoxide) does not have an additional oxygen atom to allow for resonance stabilisation. Therefore, alkoxides are more unstable than carboxylate ions.

      Strength of Carboxylic Acids

      When carboxylic acids are halogenated, the O–H bond becomes more polarised. This means pKa decreases and their acid strength increases.


        As the carbon chain of carboxylic acids increases in length, acidity decreases and pKa This is because alkyl groups have the opposite effect to that of halogens.




          Boiling and Melting Points of Carboxylic Acids

          • Carboxylic acids are polar compounds. They are generally considered more polar than aldehydes and ketones due to the presence of an additional electronegative oxygen atom.


          • Carboxylic acids can form hydrogen bonds – hydrogen atom attached to oxygen acts as a bond donor while an electron lone pair acts as a bond acceptor.
            • The electron lone pair can either be from –OH or C=O.


          • Carboxylic acids have much higher boiling and melting points than hydrocarbons, alcohols, aldehydes and ketones of similar molecular weight.


          • The formation of two hydrogen bonds between two molecules of carboxylic acid forms a dimer configuration which further increases the strength of dispersion forces between the two molecules.


          • As a result of this dimer configuration, carboxylic acids have higher boiling points than alcohols of similar molar mass, despite both functional groups being able to form hydrogen bonds.


          Hydrogen boding between molecules of carboxylic acids creates a dimer.


          Hydrogen bonding between molecules of ethanol does not produce dimers.




          Table: compounds that can form hydrogen bonds have, in general, stronger intermolecular force and higher boiling and melting points than those that do not.



          Functional group

          Molar mass

          (g mol–1)

          Type of intermolecular force

          Boiling point (ºC)









          Strong dipole





          Stronger dipole





          Hydrogen bonding



          Butanoic acid

          Carboxylic acid


          Hydrogen bonding



          Solubility in Water

          • Similar to alcohols, aldehydes and ketones, small carboxylic acids are soluble in water.

          - Carboxylic acids are more soluble in water than alcohol, aldehydes and ketones of similar molecular weight because they can form more hydrogen bonds.


          • Carboxylic acids’ solubilities in water decrease with molecular weight (number of carbons in its chain). The extension of the carbon chain decreases the overall polarity of the molecule.




          Oxidation of alcohols produces aldehyde, ketone and carboxylic according to the following table





            Primary alcohol

            Mild oxidising agent

            • pyridinium chlorochromate (PCC)


            Primary alcohol

            Strong oxidising agent

            • Acidified potassium permanganate (H+/KMnO4)
            • Acidified sodium dichromate (H+/NaCr2O7)
            • Jones Reagent (CrO3/H+)
            • Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)


            Carboxylic acid

            Secondary alcohol

            Any oxidising agent

            • Acidified potassium permanganate (H+/KMnO4)
            • Acidified sodium dichromate (H+/NaCr2O7)
            • Jones Reagent (CrO3/H+)
            • Tollens’ Reagent (Ag(NH3)2+) (silver mirror test)



            Oxidation of an aldehyde produces a carboxylic acid: 

            Carboxylic Acid and Base Reactions

            Carboxylic acids are weak acids that react with Arrhenius and Brønsted-Lowry bases. Each carboxylic acid functional group is monoprotic i.e. donates one proton. Some organic molecules may contain more than one carboxylic acid functional group in which case the substance may be polyprotic.

            Carboxylic acid + metal hydroxide →  salt + water

            Example: reaction between acetic acid (C2H4O2) and sodium hydroxide to produce sodium acetate and water


            C2H4O2(aq) + NaOH → NaC2H3O2(aq) + H2O(l)


            Carboxylic acid + metal carbonate → salt + carbon dioxide + water

            Example: reaction between acetic acid (C2H4O2) and sodium carbonate to produce sodium acetate, carbon dioxide and water


            2 C2H4O2(aq) + Na2CO3(aq) → 2 NaC2H3O2(aq) + CO2(g) + H2O(l)


            Carboxylic acid + metal hydrogen carbonate →  salt + carbon dioxide + water

            Example: reaction between acetic acid (C2H4O2) and sodium hydrogen carbonate to produce sodium acetate, carbon dioxide and water


            C2H4O2(aq) + NaHCO3(aq) → NaC2H3O2(aq) + CO2(g) + H2O(l)