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

Key Concepts

• Amines contain the -NH₂ functional group which is known as the amine group (amino group⁽¹⁾).
• Amines can be classified as :

  ⚛ primary (1^o) : general formula R-NH₂

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

  ⚛ tertiary (3^o) : general formula R'-NR-R"

  where R, R' and R" represent an alkyl group (that is, a carbon chain)

• Simple primary amines are named by adding a locating infix and the suffix amine to the corresponding name of the parent hydrocarbon⁽²⁾:

  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:	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

• An alkanamine is a simple primary amine in which the parent hydrocarbon is an alkane.
• Properties of alkanamines :

  ⚛ Amines have a distinctive, often unpleasant, odour.

  ⚛ 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).

  ⚛ Amines are weak bases.

  In reactions with aqueous mineral acids or carboxylic acids, amines form salts.

  ⚛ In reactions with hydrogen halides, amines form salts.

  ⚛ In reactions with anhydrides or acyl halides, amines produce amides.

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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:

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:

Classification of Amines

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

formula NH₃

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

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:

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:

general formula R'-NR"-R

Examples of primary, secondary and tertiary amines are given below:

1. Primary (1^o) 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 (2^o) 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 (3^o) 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:

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?

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

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?

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:

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:

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⁾:

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

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

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.

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.

Primary and Secondary Amines React with Acyl Halides to Produce Amides

Primary and Secondary Amines React with Anhydrides to Produce Amides

The most common anhydride you are likely to meet in an introductory chemistry course is acetic anhydride, CH₃-CO-O-CO-CH₃, so we have used this in the equations below: