Aldehydes & Ketones

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

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.

 

    Functional group

    Suffix

    Prefix

    Generic structure

    Example

    Aldehyde

    -al

    Formyl-

    Ketone

    -one

    Oxo-

     

    Nomenclature priority

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

     

    Properties of Aldehydes and Ketones

    Boiling and Melting Points

    • Smaller aldehydes and ketones are polar molecules and there can form dipole-dipole forces on top of dispersion forces.
    • Aldehydes and ketones generally have stronger intermolecular forces than hydrocarbons of similar molecular mass. Thus, they have higher boiling and melting points.
    • Compared to alcohols, aldehydes and ketones generally have weaker intermolecular forces because they cannot form hydrogen bonds. Alcohol molecules contain hydroxyl (–OH) groups that can participate in hydrogen bonding as either a donor or acceptor.

     

     

    • In their own homologous series, boiling and melting points of aldehydes and ketones increase with molecular mass due to stronger dispersion forces.

     

    Solubility in water

    • Aldehydes and ketones are polar compounds, primarily due to the presence of an electronegative oxygen atom in their functional groups. Smaller aldehyde and keton molecules are soluble in water.
    • Aldehydes and ketones cannot form hydrogen bonds with themselves (among molecules of only aldehyde and ketone), they can however form hydrogen bonds with water molecules.
    • This enables small aldehyde and ketone molecules to dissolve in water (molecules with low number of carbon atoms).
    • As the length of non-polar carbon chain increases, the polarity of aldehydes and ketones decreases. This reduces the solubility of aldehydes and ketones in water.

     

    Table: melting and boiling points of aldehydes and ketones increase with molecular weight (size) while their solubilities decrease with molecular weight.

     

     

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

      

    Compound

    Functional group

    Molar mass

    (g mol–1)

    Type of intermolecular force

    Boiling point (ºC)


    Butane

    Alkane

    58

    Dispersion

    –1


    Butanal

    Aldehyde

    72

    Strong dipole

    49


    Butanone

    Ketone

    72

    Stronger dipole

    56


    Butanol

    Alcohol

    74

    Hydrogen bonding

    97

    Oxidation of Aldehydes and Ketones

    Oxidation of alcohols produces aldehydes and ketones according to the following table

       

      Reactant

      Reagent/catalyst/condition

      Product

      Primary alcohol

      Mild oxidising agent

      • pyridinium chlorochromate (PCC)

      Aldehyde

      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)

      Ketone

       

      • Oxidation of an aldehyde using a strong oxidising agent produces a carboxylic acid. For example propanal oxidises to form propanoic acid. 
      • Ketones cannot be oxidised

       

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