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Amines

Key Concepts


  1. primary : general formula R-NH2
  2. secondary : general formula R-NH-R'
  3. tertiary : general formula R'-NR-R"

  • Amines are named by adding the suffix amine to the corresponding alkyl group of the molecule:

  1. Number the longest carbon chain giving the amine functional group the lowest number.

  2. Name the carbon chain.

  3. Add the suffix -amine

  • Properties of Amines :

  1. Amines are polar molecules and are soluble in water.
  2. Amines are weak bases.
  3. Amines have a distinctive, often unpleasant, odour.

  • Amines react with inorganic acids to form salts

Naming Amines

  1. Identify the amine's functional group.

  2. Number the longest carbon chain giving the amine functional group the lowest number.

  3. Name the carbon chain.

  4. Add the suffix -amine
Examples:
  1. Identify the amine's functional group:
    H
    |
    H
    |
    H
    |
    H-C-C-N-H
    |
    H
    |
    H

  2. Number the longest carbon chain giving the amine functional group the lowest number:
    H
    |
    H
    |
    H
    |
    H-C2-C1-N-H
    |
    H
    |
    H

  3. Name the carbon chain:
    2 carbon atoms in the chain = ethyl

  4. Add the suffix -amine:
    ethyl + amine = ethylamine
name formula general formula
methylamine CH3NH2 R-NH2
ethylamine CH3CH2NH2 R-NH2
propylamine CH3CH2CH2NH2 R-NH2
dimethylamine (CH3)2NH R-NH-R'
trimethylamine (CH3)3N R'-NR-R"

Classification of Amines

  1. Primary (1o) Amine: amine functional group occurs on a terminal carbon atom in the chain,
    general formula R-NH2
    example:H
    |
    H
    |
    H
    |
    H-C-C-N-H
    |
    H
    |
    H

  2. Secondary (2o) Amine : amine functional group occurs between 2 carbon atoms,
    general formula R-NH-R'
    example:H
    |
    H
    |
    H
    |
    H-C-N-C-H
    |
    H
    |
    H

  3. Tertiary (3o) Amine: the nitrogen atom of the amine group is bonded to 3 carbon atoms,
    general formula R'-NR-R"
    example:H
    |
    H
    |
    H-C-H
    |
    H
    |
    H-C-N-C-H
    |
    H
    |
    H
name formula general formula classification
methylamine CH3NH2 R-NH2 primary
ethylamine CH3CH2NH2 R-NH2 primary
propylamine CH3CH2CH2NH2 R-NH2 primary
dimethylamine (CH3)2NH R-NH-R' secondary
trimethylamine (CH3)3N R'-NR-R" tertiary

Properties of Amines

Solubility of Amines in Water

Amines with low molecular mass are water soluble because these amines can form hydrogen bonds with water molecules.
H
|
H
|
δ- repesents a partial negative charge
δ+ repesents a partial positive charge
....represents hydrogen bond
H-C-Nδ-.........δ+H-O-H
|
H
|
H
name formula solubility
in water
methylamine CH3NH2 soluble
dimethylamine (CH3)2NH soluble
trimethylamine (CH3)3N 91g/100g water
ethylamine CH3CH2NH2 soluble
diethylamine (CH3CH2)2NH soluble
triethylamine (CH3CH2)3N 14g/100g water

As the number of carbon atoms in the alkyl chain increases, the solubility of the amine in water decreases.
As the number of carbon atoms in the alkyl chains increases, (molecular mass of the amine increases), the weak intermolecular forces (London Forces or Dispersion Forces) become increasingly important as the long carbon chains which are non-polar are more attracted to each other than to the polar water molecules in the solvent.

Boiling Point of Amines

Amines can undergo hydrogen bonding though the N-H bond as described above.
Nitrogen is less electronegative than O, therefore the NH bond is less polar than an OH bond and the hydrogen bonding in amines is weaker than in compounds with OH groups such as alcohols and carboxylic acids.

Therefore the boiling points of amines falls between those for non-hydrogen bonded compounds like alkanes, and those for strongly hydrogen bonded compounds like alkanols (alcohols) or carboxylic (alkanoic) acids.

No. carbon atoms 1 2 3 4 Trend
Boiling Point
of Alkane (oC)
-162 -88.6 -42.1 -0.5 lower
B.P.
Boiling Point
of Amine (oC)
-7.5 17 48 78
Boiling Point
of Alkanol (oC)
65 78 97 117 higher
B.P.
Members should log-in to use the graph.
As the number of carbon atoms in the alkyl chain of the simple amines increase, (and the molecular mass of the amine increases), the weak intermolecular forces (London Forces or Dispersion Forces) become increasingly important as the long carbon chains which are non-polar are more attracted to each other than to the polar water molecules in the solvent.
name formula boiling
point
(oC)
Trend
methylamine CH3NH2 -7.5 lower
B.P.
ethylamine CH3CH2NH2 17
propylamine CH3CH2CH2NH2 48
butylamine CH3CH2CH2CH2NH2 78 higher
B.P.

Basicity of Amines

Amines are Brønsted-Lowry bases, amines can accept a proton from a Brønsed-Lowry acid such as water#:

amine
(base)
+ water
(acid)
conjugate acid
of the amine
+ conjugate base
of water
R-NH2 + H2O R-NH3+(aq) + OH-(aq)

Amines are weak bases, the base dissociation constant, Kb, is small.

name formula Kb
methylamine CH3NH2 4.4 x 10-4
dimethylamine (CH3)2NH 5.2 x 10-4
trimethylamine (CH3)3N 5.0 x 10-5
ethylamine CH3CH2NH2 4.7 x 10-4
propylamine CH3CH2CH2NH2 5.1 x 10-4

Amines React with Mineral (Inorganic) Acids to Form Salts

General word equation: amine + mineral acid alkylammonium salt
General chemical equation:R-NH2 + HX R-NH3+X-
Word equation example:methylamine + hydrochloric acidmethylammonium chloride
(methylamine hydroxhloride)
Chemical equation example:CH3NH2 + HCl CH3NH3+Cl-

Amines React with Carboxylic (alkanoic) Acids to Form Salts

General word equation: amine + alkanoic acid alkylammonium salt
General chemical equation:R-NH2 + HX R-NH3+X-
Word equation example:methylamine + ethanoic acid
(acetic acid)
methylammonium ethanoate
(methylammonium acetate)
(methylamine acetate)
Chemical equation example:CH3NH2 + CH3COOHCH3NH3+   -OOCCH3


What would you like to do now?

*For more complex amines, the amino group is treated as a substituent.

#Amines are also Lewis bases because of the presence of the unshared pair of electrons.

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