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Properties of Simple Amines

Key Concepts

  • Amines contain the -NH2 functional group which is known as the amine group (amino group1).

  • Amines can be classified as :

    1. primary (1o) : general formula R-NH2

    2. secondary (2o) : general formula R-NH-R'

    3. tertiary (3o) : general formula R'-NR-R"
    where R, R' and R" represent an alkyl group (that is, a carbon chain)

  • Simple primary amines (alkanamines) are named by adding a locating infix and the suffix amine to the corresponding name of the parent hydrocarbon2:

    parent hydrocarbon -infix- suffix
    alkan -n- amine

    1. Identify the amine functional group.

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

    3. Name the longest carbon chain (parent hydrocarbon).

      Number of carbon atoms:12345678910
      Prefix:methethpropbutpenthexheptoctnondec

    4. Remove the "e" from the end of the name of the parent hydrocarbon

    5. Add the infix which indicates the location of the amine group

    6. Add the suffix -amine

  • Properties of Amines :
    1. Amines have a distinctive, often unpleasant, odour.

    2. Amines are polar molecules.
      Amines with low molar mass are soluble in polar solvents like water.
      The boiling point of amines lies between the comparable alkane and alkanol (alcohol).

    3. Amines are weak bases.
      In reactions with aqueous mineral acids or carboxylic acids, amines form salts.

    4. In reactions with hydrogen halides, amines form salts.

Naming Simple Primary Amines

  1. Identify the amine functional group.

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

  3. Name the longest carbon chain (parent hydrocarbon).

    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

  4. Remove the "e" from the end of the name of the parent hydrocarbon

  5. Add the infix which indicates the location of the amine group

  6. Add the suffix -amine

Example:

Name the compound shown below:

H
|
H
|
H
|
H-C-C-N-H
|
H
|
H
  1. Identify the amine 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 longest carbon chain (parent hydrocarbon).

    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

    2 carbon atoms in the longest alkane chain = ethane

  4. Remove the "e" from the end of the name of the parent hydrocarbon

    ethane becomes ethan

  5. Add the infix which indicates the location of the amine group

    amine (NH2) is covalently bonded to the first carbon atom so the infix is -1-

    ethan-1-

    Since there is only functional group and only 2 carbon atoms, the functional group will always be defined as being attached to the first carbon atom so it is not really necessary to include the infix.

    ethan

  6. Add the suffix amine

    ethanamine

The systematic IUPAC names of some amines are given below:

Preferred IUPAC Name3 (alternative name) formula general formula
methanamine (methylamine) CH3NH2 R-NH2
ethanamine (ethylamine) CH3CH2NH2 R-NH2
propan-1-amine (propylamine) CH3CH2CH2NH2 R-NH2
N-methylmethanamine (dimethylamine) (CH3)2NH R-NH-R'
N,N-dimethylmethanamine (trimethylamine) (CH3)3N R'-NR-R"

Classification of Amines

Amines can be thought of as derivatives of ammonia, NH3:

  H
|
 
H-N-H

formula NH3

Replacing one hydrogen atom (H) in the ammonia molecule with an alkyl group (R) results in a primary amine:

  H
|
 
H-N-R

general formula R-NH2

Replacing two hydrogen atoms (H) in the ammonia molecule with a alkyl groups (R and R') results in a secondary amine:

  H
|
 
R'-N-R

general formula R'-NH-R

Replacing all three hydrogen atoms (H) in the ammonia molecule with a alkyl groups (R and R' and R") results in a tertiary amine:

    R"
|
 
R'-N-R

general formula R'-NR"-R

Examples:

  1. Primary (1o) Amine: an alkyl group replaces 1 hydrogen atom in the ammonia molecule
    general formula R-NH2
      H
    |
    H
    |
    H
    |
    H-C-C-N-H
    |
    H
    |
    H

    A hydrogen atom (H) in the ammonia molecule has been replaced by an ethyl, CH3CH2, group

  2. Secondary (2o) Amine : alkyl groups replace 2 hydrogen atoms in the ammonia molecule
    The amine functional group (NH) occurs between 2 carbon atoms.
    general formula R-NH-R'
      H
    |
    H
    |
    H
    |
    H-C-N-C-H
    |
    H
    |
    H

    One hydrogen atom in the ammonia molecule has been replaced by a methyl, CH3, group and another hydrogen atom in the same ammonia molecule has been replaced by a different methyl, CH3, group.

  3. Tertiary (3o) Amine: alkyl groups replace all 3 hydrogen atoms in the ammonia molecule
    The nitrogen atom of the amine functional group (N) is bonded to 3 carbon atoms.
    general formula R-NR'-R"
      H
    |
    H
    |
    H-C-H
    |
    H
    |
    H-C-N-C-H
    |
    H
    |
    H

    One hydrogen atom in the ammonia molecule has been replaced by a methyl, CH3, group and the second hydrogen atom in the same ammonia molecule has been replaced by a different methyl, CH3, group and the third hydrogen atom in the same ammonia molecule has been replaced by a different methyl, CH3, group.

The classification of some amines is shown in the table below:

Preferred IUPAC Name (alternative name) formula general formula classification
methanamine (methylamine) CH3NH2 R-NH2 primary
ethanamine (ethylamine) CH3CH2NH2 R-NH2 primary
propan-1-amine (propylamine) CH3CH2CH2NH2 R-NH2 primary
N-methylmethanamine (dimethylamine) (CH3)2NH R-NH-R' secondary
N,N-dimethylmethanamine (trimethylamine) (CH3)3N R'-NR-R" tertiary

Physical 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
Preferred IUPAC Name
(alternative name)
formula solubility
in water
methanamine
(methylamine)
CH3NH2 soluble
N-methylmethanamine
(dimethylamine)
(CH3)2NH soluble
N,N-dimethylmethanamine
(trimethylamine)
(CH3)3N 91g/100g
ethanamine
(ethylamine)
CH3CH2NH2 soluble
N-ethylethanamine
(diethylamine)
(CH3CH2)2NH soluble
N,N-diethylethanamine
(triethylamine)
(CH3CH2)3N 14g/100g

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.
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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.
Preferred IUPAC Name
(alternative name)
formula boiling
point
(oC)
Trend
methanamine
(methylamine)
CH3NH2 -7.5 lower
B.P.
ethanamine
(ethylamine)
CH3CH2NH2 17
propan-1-amine
(propylamine)
CH3CH2CH2NH2 48
butan-1-amine
(butylamine)
CH3CH2CH2CH2NH2 78 higher
B.P.

Chemical Properties of Amines

Basicity of Amines

Amines are Brønsted-Lowry bases, amines can accept a proton from a Brønsed-Lowry acid such as water4:

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.

Preferred IUPAC Name
(alternative name)
formula Kb
methanamine
(methylamine)
CH3NH2 4.4 x 10-4
N-methylmethanamine
(dimethylamine)
(CH3)2NH 5.2 x 10-4
N,N-dimethylmethanamine
(trimethylamine)
(CH3)3N 5.0 x 10-5
ethanamine
(ethylamine)
CH3CH2NH2 4.7 x 10-4
propan-1-amine
(propylamine)
CH3CH2CH2NH2 5.1 x 10-4

Amines React with Mineral (Inorganic) Acids to Form Salts

General word equation: amine + mineral acid alkanaminium salt
(alkylammonium salt)
General chemical equation:R-NH2 + HX R-NH3+X-
Word equation example:methanamine
(methylamine)
+ hydrochloric acidmethanaminium chloride
(methylammonium chloride or methylamine hydrochloride)
Chemical equation example:CH3NH2 + HCl CH3NH3+Cl-

Amines React with Carboxylic (alkanoic) Acids to Form Salts

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


What would you like to do now?

1For more complex amines in which a functional group takes precedence over the amine group, the amine group is treated as a substituent and the amino prefix is added to the name of the parent hydrocarbon.

2This method results in the formation of a Preferred IUPAC Name.
An alternative IUPAC system for generating the names of amines is to add the name of the substituent alkyl group to the term "amine" used as a parent hydride. This alternative name will be given in (parentheses).
The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

3The italicized N is the locant. It indicates that alkyl groups are attached to a nitrogen atom.

4Amines are also Lewis bases because of the presence of the unshared pair of electrons.

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