Amides

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: amides

  • explain the properties within and between the homologous series of amides with reference to the intermolecular and intramolecular bonding present

  • investigate the differences between an organic acid and organic base

Amides: Structure and Properties

This video introduces the organic functional group amides including its structure and properties (boiling points and solubility in water).

 

Structure of Amides

An amide is a functional group where a nitrogen atom is connected to a carbonyl carbon (C=O). Like amines, an amide can be further classified as primary, secondary or tertiary amide depending on the total number of carbon atom(s) the nitrogen atom is bonded to.

 

 

Like in amines, the nitrogen atom in an amide occupies a trigonal pyramidal shape as its electron lone pair also repels adjacent covalent bonds away.

    Comparison of Amines and Amides

    Amines and amides both contain nitrogen atoms but should not be confused. Amides are characterised by the presence of carbonyl group adjacent to the nitrogen atom. 

    Nomenclature of Amides

    • Identify the longest carbon chain that contains the amine or amide functional group. This is used in conjunction with the functional group’s suffix to name the main part of the molecule.
    • Remaining alkyl groups attached to the amine or amide functional group are considered as substituents. Their positions are denoted by ‘N’.
    • Suffix -‘amide’
    • Prefix -‘amido’

     

     Nomenclature Priority

     

    • Amides have the third highest priority after carboxylic acids and esters, and before ketones and aldehydes.
    • In a molecule containing both ketone and amide functional groups, the ketone is denoted by its prefix, 'oxo', while the amide is denoted by its suffix, 'amide'.

    4-oxopentanamide

     

    • Amides have lower priority than carboxylic acid and ester. In a molecule where either of these functional groups is present, the amide is denoted by its prefix ‘amido’

     

     3-amidopropanoic acid 

    Properties of Amides

    • Unlike amines, amides are not bases.
    • In amides, the electron lone pair on the nitrogen atom is de-localised due to resonance stabilisation. This is caused by the electron-withdrawing effect of the oxygen atom in the carbonyl group (C=O).
    • Specifically, the resonance structure of an amide is formed when the electron lone pair of nitrogen moves to form a new bond with the carbonyl carbon atom, while a pair of electrons in the C=O bond moves to the oxygen atom to become one of its lone pairs.
    • The de-localisation of nitrogen’s electron lone pair interferes with its ability to accept a proton. Compared to amines, amides have a much lower tendency to accept protons.
    • Amides do not produce basic solutions when dissolved in water. 

     

     

    Figure: de-localisation of nitrogen’s electron lone pair causes amides to be very weak bases.

     

    Boiling and Melting Points of Amides

    • Amides have the highest boiling points compared to every other functional group with similar molecular weights.
    • Primary amides have higher boiling and melting points than secondary amides of the same molar mass. This is because primary amides can form more hydrogen bonds between molecules than secondary amides.
    • Like other functional groups, BP & MP of amide compounds increase with the number of carbon atom increases because the strength of dispersion forces increases with molecular mass.
    • Amides also have permanent dipoles due to the presence of nitrogen in their functional groups.

      

    • Amides vs carboxylic acid: amides generally have higher BP & MP than corresponding carboxylic acids of similar molecular mass.
    • The main reason for the difference is due to the ionic attraction between charged resonance structures of amides. Attractive forces due to ionic charges are stronger than hydrogen bonds.
    • Primary amides have substantially higher boiling point than corresponding carboxylic acids because there are two hydrogen atoms bound to the nitrogen. This enables more hydrogen bonds to be formed between amide molecules.

     

    Ionic attraction between amide molecules

    Figure: Resonance structures of amides allow formation for ionic bonds

     

    Compound

    Structure

    Molar mass

    (g mol–1)

    Boiling point

    (ºC)

    Methanamide (formamide)

    45

    210

    Methanoic acid

    (formic acid)

    46

    100.8
    • Secondary amides also have higher boiling point than corresponding carboxylic acids. While both compounds only contain one hydrogen atom that can partake in hydrogen bonding, the strength of attraction between secondary amide molecules are stronger due to the presence of ionic attraction.

     

    Compound

    Structure

    Molar mass

    (g mol–1)

    Boiling point

    (ºC)

    N-methylmethanamide

    59

    182.6

    Ethanoic acid

    60

    117.9

     

    • Tertiary amides vs similar molar mass carboxylic acids are difficult to predict. The general rule is:
      • Small amides have slightly stronger intermolecular forces than small carboxylic acids.
      • Large amides have lower intermolecular forces than large carboxylic acids. The difference becomes more apparent as the molar mass increases.

     

    Table: smaller tertiary amides have higher boiling point than corresponding carboxylic acids.

    Compound

    Structure

    Molar mass

    (g mol–1)

    Boiling point

    (ºC)

    N,N-dimethylmethanamide

    73

    153.0

    Propanoic acid

    74

    141.2

     

    Table: larger tertiary amides have lower boiling point than corresponding carboxylic acids.

    Compound

    Structure

    Molar mass

    (g mol–1)

    Boiling point

    (ºC)

    N,N-dimethylethanamide

     

    101

    176

    Pentanoic acid

    102

    186

     

    Solubility in Water

    • Small amides are polar molecules and therefore they are soluble in water.
    • All types of amides can form hydrogen bonds with water. Primary and secondary amides are more soluble due to their ability to donate and accept hydrogen bonds. Tertiary amides are less soluble as they can only accept hydrogen bonds from water molecules.
    • Like other functional groups, solubility amides decreases as the number of carbon atoms increases. This is because molecules become less polar with the addition of alkyl groups.

    Comparing with other functional groups

    • Amides are generally more soluble than corresponding amines (e.g. 1º amide vs 1º amine of similar molecular mass) because amides contain an additional oxygen atom whose two electron lone pairs allow the molecule to accept more hydrogen bonds from water.
    • Amides are more soluble than hydrocarbons as they are non-polar.
    • Solubility of amides vs other carbonyl groups
      • Solubility of primary and secondary amides are comparable with carboxylic acids as both functional groups have the ability to donate and accept hydrogen bonds to and from water.
      • Tertiary amides have similar solubility as esters, ketones and aldehydes as these molecules can only accept hydrogen bonds from water.

     

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