HSC Chemistry: Amines and 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: amines and amides

  • 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

Organic Bases: Structure, Boiling Point and Solubility of Amines

Structure of Amines 

Amines

  • An amine is a functional group where a nitrogen atom is connected to carbon and/or hydrogen atoms. Depending on the number of carbon atom(s) the nitrogen is connected to, an amine is further classified as primary (one carbon atom), secondary (two carbon atoms) and tertiary (three carbon atoms).

  • Nitrogen atom in an amine occupies a trigonal pyramidal shape as the negative electron lone pair repels the three adjacent covalent bonds, preventing the nitrogen from adopting a trigonal planar shape.

 

 

Nomenclature of Amines

 

  • 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’.

 

Amines

  • Suffix -‘amine’
  • Prefix -‘amino’

 

Nomenclature Priority

  • Amines have lower priority than most functional groups but hydrocarbons.

 

Properties of Amines

Amines are organic bases

  • Amines are organic Brønsted-Lowry bases. The nitrogen atom’s electron lone pair allows amines to accept protons (H+) from water or an acid to form its positively charged conjugate acid.
  • All organic bases are weak bases that partially ionise with water to form an equilibrium system (example shown below).

 

Boiling and Melting Points

Amines

  • Primary amines (1º) have higher boiling and melting points than secondary amines (2º) of the same molecular mass. This is because:
    • primary amines can form more hydrogen bonds between molecules than secondary amines
    • secondary amines have a smaller (weaker) molecular dipole due to the presence of nitrogen within the carbon chain as opposed to at the end in primary amine.

 

Hydrogen bonding between primary amine molecules

Hydrogen bonding between secondary amine molecules

Table: tertiary amines have much lower boiling (and melting) points than primary and secondary amines of the same molar mass. Secondary amines have lower boiling points than primary amines.

 

Compound

Molar mass(g mol–1)

Boiling point (ºC)

Ethanamine

(primary)

45

16.6

N-methylmethanamine

(secondary)

45

6.8

Propan-1-amine

(primary)

59

47.8

N-methylethanamine

(secondary)

59

33.5

N,N-dimethylmethanamine

(tertiary)

59

2.87

 

  • BP & MP of amine and amide compounds increase with the number of carbon atom increases because the strength of dispersion forces increases with molecular mass.
  • Amines and amides have permanent dipoles due to the presence of nitrogen in their functional groups.
  • Amines and amides have higher boiling and melting points than hydrocarbons of similar molecular weight.
  • Tertiary amines and amides cannot form hydrogen bonds. Thus, they have much lower boiling and melting points than primary and secondary counterparts.

 

Tertiary amines do not contain hydrogens that can partake in hydrogen bonding

 

  • Amines are usually compared with alcohols while amides are compared with carboxylic acids. This is because each pair of functional groups shares structural similarities.
  • Amines vs alcohol: amines generally have lower BP & MP than alcohols of similar molecular mass.
    • Nitrogen is less electronegative than oxygen. As a result, hydrogen bonds formed between amine molecules are weaker than those between alcohol molecules.
    • Tertiary amines have much lower boiling and melting points than alcohols of similar molecular mass because tertiary amines do not have a hydrogen atom bound to a nitrogen, oxygen or fluorine atom. Thus, they cannot form hydrogen bonds.

Solubility in Water

  • Small amines are polar molecules and therefore they are soluble in water.
  • All types of amines  can form hydrogen bonds with water.
    • Primary and secondary amines  are more soluble due to their ability to donate and accept hydrogen bonds.
    • Tertiary amines  are less soluble as they can only accept hydrogen bonds from water molecules.

 

Methanamine forms hydrogen bonds with H2O

Generic representation of hydrogen bonds between amide molecules and H2O.

 

  • Like other functional groups, solubility of amines  decreases as the number of carbon atoms increases. This is because molecules become less polar with the addition of alkyl groups.
  • Amines are more soluble than hydrocarbons as they are non-polar.

 

  • Solubility of amines vs alcohols
    • Primary amines are similar in solubility compared to corresponding alcohols. While primary amines are able to donate more hydrogen bonds than alcohols, alcohols can form stronger dipole-dipole forces and hydrogen bonds with water. Comparison of solubility between these two functional groups is therefore quite difficult.
    • Secondary and tertiary amines are less soluble than corresponding alcohols. Tertiary amines can only accept hydrogen bonds from water, so they much lower solubility than alcohols.


Amides: Structure, Boiling Point and Solubility in Water 

 

Structure of Amides 

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.

 

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’.

 

Amides

  • Suffix -‘amide’
  • Prefix -‘amido’

 

 Nomenclature Priority

  • Amides have the third highest priority, higher than ketones and aldehydes.

 

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

 

Amides are neutral

  • 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. Thus, amides are generally not considered as organic bases.
  • Compared to amines, amides have a much lower tendency to accept protons. Amides are considered very weak bases as they produce neutral 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

  • 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.
  • Formamide is a liquid at 25ºC whereas other ‘unsubstituted’ amides are solids at room temperature.

 

N-N-diethylethanamine

N,N-dimethylpropanamide

 

 

Table: tertiary amines have much lower boiling (and melting) points than primary and secondary amines of the same molar mass. Secondary amines have lower boiling points than primary amines.

 

Compound

Molar mass(g mol–1)

Boiling point (ºC)

Ethanamine

(primary)

45

16.6

N-methylmethanamine

(secondary)

45

6.8

Propan-1-amine

(primary)

59

47.8

N-methylethanamine

(secondary)

59

33.5

N,N-dimethylmethanamine

(tertiary)

59

2.87

 

  • Like amines, 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 have higher boiling and melting points than hydrocarbons of similar molecular weight.

 

  • 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.

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.
  • 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.