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Structure, Naming, Classification, and Properties of Amines Chemistry Tutorial

Key Concepts

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Naming Simple Primary Alkanamines

  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: 1 2 3 4 5 6 7 8 9 10
    Prefix: meth eth prop but pent hex hept oct non dec

  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 of naming an alkanamine:

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: 1 2 3 4 5 6 7 8 9 10
    Prefix: meth eth prop but pent hex hept oct non dec

    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 preferred IUPAC names and alternative names of some amines are given below:

Preferred IUPAC Name(3) (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"

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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 of primary, secondary and tertiary amines are given below:

  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

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Physical Properties of Amines

Amines are polar molecules. This is important when considering the physical properties of amines such as solubility in water, melting point and boiling point.

Solubility of Amines in Water

The table below gives the solubility of some amines in water.
Can you see a pattern, or trend, in the data?

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

Amines with low molecular mass are water soluble because these amines can form hydrogen bonds with water molecules as represented in the diagram below:

  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
   

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

As mentioned above, amines can undergo hydrogen bonding though the N-H bond.
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 alkanoic acids (carboxylic acids).

The boiling points of a number of alkanes, amines and alkanols are given in the table below. Can you see a pattern, or trend, in the data?

No. carbon atoms 1 2 3 4 Trend
Boiling Point of Alkane
(°C)
-162 -88.6 -42.1 -0.5 lower B.P.
Boiling Point of Amine
(°C)
-7.5 17 48 78
Boiling Point of Alkanol
(°C)
65 78 97 117 higher B.P.

The boiling point of an amine lies between the boiling points of the comparable alkane and alkanol.

The trend is even more obvious if you graph the boiling points of these alkanes, amines and alkanols, as shown below:

Boiling Point (°C) Boiling Point Comparison

No. carbon atoms in straight chain

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.

The trend of increasing boiling point with increasing length of alkyl chain on alkanamine is shown in the table below:

Preferred IUPAC Name
(alternative name)
formula boiling point
(°C)
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.

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Chemical Properties of Amines

Amines are weak bases.
The chemistry of amines can therefore be understood in terms of the ability of the amine group to accept a proton.

Basicity of Amines

Amines are Brønsted-Lowry bases, amines can accept a proton from a Brønsed-Lowry acid such as water(4):

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.

The value of the base dissociation constant, Kb, for some amines is given in the table below:

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

Amines React with Mineral Acids (Inorganic Acids) to Form Salts

Amines are bases, they can react with inorganic acids (mineral acids), such as hydrochloric acid, 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 acid methanaminium chloride
(methylammonium chloride
or
methylamine hydrochloride)
Chemical equation example: CH3NH2 + HCl CH3NH3+Cl-

Amines React with Carboxylic Acids (alkanoic acids) to Form Salts

Amines are bases, they can react with organic acid, such as acetic acid (ethanoic acid), 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 + CH3COOH CH3NH3+-OOCCH3

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Footnotes:

(1) For 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.

(2) This 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).

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

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