Physical Properties of Amines

Classification of Amines

Have you ever wondered why toilets smell so bad? Well, ammonia and compounds of ammonia called amines are responsible for that foul smell. These compounds have a pungent smell, but they have an enormous significance in chemistry and our everyday lives. Amines are an essential class of organic compounds. Amines are an exciting subject to study, and it also has a major application in biology. Nitrogen is the only famous product, but many other products of ammonia get derived by changing the hydrogen atoms. In this article, you can learn about the classification of amines, its structure, and physical properties of amines.  


What are Amines?

Amines are the organic compounds, deriving from ammonia (NH3). Amines get derived by replacing its one or more hydrogen atoms with an alkyl or aryl group. Amines have a nitrogen atom with a lone pair of electrons. When you replace one or more hydrogen atoms by substituent groups like alkyl or aryl in ammonia, you get a new type of organic compound called amines. Amino acids, biogenic amine, trimethylamine, and aniline are some of the important amine structures.    


Classification of Amines

Amines get classified into four categories, such as primary, secondary, tertiary, and cyclic. A primary amine is the one where you replace one of the three hydrogen atoms with an alkyl or aryl group. When you replace two of the three hydrogen atoms, then you get a secondary amine. Likewise, by replacing all three hydrogen atoms, tertiary amines get formed. 


Keep in mind that only secondary or tertiary amines can be cyclic. Examples of primary amines are CH3NH2, methylamine and CH3NHCH3 or dimethylamine are examples of secondary amines. Example of the tertiary amine is trimethylamine and 3 - member ring aziridine is an example of cyclic amine.  


Physical Properties of Amines

The following are various amines physical properties that you should know.

General Properties:

  • Lower aliphatic amine often finds its gaseous state, and they have a fishy smell. 

  • Primary amines having three or four carbon atoms are in the liquid state at room temperatures, and higher ones get found in the solid-state.

  • Aniline and various arylamines are colourless. But, they do get coloured due to atmospheric oxidation when you keep them in the open.


Solubility Characteristics:

  • Lower aliphatic amines can form hydrogen bonds with water molecules. And thus, such amines are soluble in water. 

  • When there is an increase in the hydrophobic alkyl part of the amines, its molar mass goes up. As a result, such amines have a lower solubility in water. 

  • Amines are soluble in organic solvents like alcohol, ether, and benzene because alcohols have high polarity as compared to amines.


Boiling Points of Amines:

  • Primary and secondary amines get engaged in intermolecular association because of the hydrogen bonding between the nitrogen of one molecule with the hydrogen of another molecule.

  • Such intermolecular association is more in primary amines as compared to secondary amines because there are two hydrogen atoms present for bond formation. 

  • Tertiary amines don’t have an intermolecular association as it doesn’t have any hydrogen atom available for hydrogen bond formation.

  • That’s why the order of boiling points for isomeric amines goes like primary > secondary > tertiary.  

Those were some fundamental physical properties of amines.

FAQ (Frequently Asked Questions)

Question 1: What are the Types of Amines?

Answer: Depending on the number of carbon-containing groups attached to the amines, they can either be primary, secondary, or tertiary. The amine is primary if it has a single carbon-containing group like CH3NH2. If the amine has two carbon-containing groups, then it becomes secondary, and three groups make it tertiary. Incorporating the lone electron pair of nitrogen, sometimes it gets energetically favoured to use the nitrogen as a nucleophile and therefore, binds a fourth carbon-containing group to the amine. As a result, you get a quaternary ammonium ion. Also, an organic compound with multiple amino groups gets called as a diamine, triamine, tetramine, and so on.

Question 2: Arrange the below compounds in the decreasing order of their boiling points. 


a. C2H5OH   b. (CH3)2NH   c. C2H5NH2

  1. a > b > c

  2. b > a > c

  3. c > a > b

  4. c > b > a

Answer: Here, the correct order is option I. a > b > c. You must note that all the given compounds have intermolecular hydrogen bonding because of which they have high boiling points. Among all, C2H5OH has the strongest hydrogen bonding because O is more electronegative than N in other compounds. That’s why the compound with O has the highest boiling point. In (CH3)2NH and C2H5NH2, the latter one has a longer alkyl chain making the hydrogen bonding even weaker. Therefore, the correct order of decreasing boiling points is a > b > c.