Aliphatic vs Aromatic Amines
The best and the most significant difference between aliphatic and aromatic amines is the structural difference between the two compounds. Aliphatic amines are the amine compounds in which Nitrogen is bonded to only alkyl groups, and aromatic amines are the amine compounds in which Nitrogen is bonded to at least one of the aryl groups. This structural difference leads to all other differences in their properties such as reactivity, acidity, and stability.
What are Aliphatic Amines?
In aliphatic amines, Nitrogen is directly bonded to only alkyl groups and Hydrogen atoms. The number of alkyl groups varies from one to three. Depending on the number of attached alkyl groups, they are called “primary amines” (only one alkyl group -1o), “secondary amines”(two alkyl groups – 2o), and “tertiary amines”(three alkyl groups – 3o).
All aliphatic amines are weak bases like ammonia, but they are slightly stronger bases than ammonia. They all have nearly the same base strength of Pkb = 3-4. Basicity increases as the hydrogen groups on Nitrogen atom are replaced by alkyl groups. Tertiary amines are more basic than primary and secondary amines.
When Nitrogen is one of the atoms in a ring, they are called heterocyclic amines. Piperidine and Pyrollidine are two examples for aliphatic heterocyclic amines.
What are Aromatic Amines?
In aromatic amines, Nitrogen is directly attached to at least one benzene ring. Depending on the number of groups attached to the nitrogen atom, they are categorized as “primary,” “secondary” and “tertiary” amines. “Aryl amines” is another name for aromatic amines. Similar to aliphatic amines, primary and secondary aromatic amines can form intermolecular hydrogen bonds. Therefore, boiling points of primary and secondary amines are relatively higher than the tertiary amines.
There are heterocyclic aromatic amines; pyrrole and pyrydine are two examples for them.
What is the difference between Aliphatic and Aromatic Amines?
• Alkyl amines do not contain benzene rings that are directly attached to Nitrogen atom.
• But, in aromatic amines, there is at least one benzene ring directly attached to the Nitrogen atom.
• Aliphatic amines can have aromatic rings as long as Nitrogen is directly attached to a Carbon atom.
• Aliphatic amines are stronger bases than aromatic amines. This is basically due to the stability of the cation that forms after the ionization. In other words, alkyl ammonium ions are more stable than aryl ammonium ions. Because, alkyl groups are electron-releasing groups and hence partially delocalize the positive charge on the Nitrogen atom.
• Aliphatic heterocyclic amines are also stronger bases than aromatic heterocyclic amines.
• Examples for aliphatic heterocyclic amines are Piperidine and Pyrollidine.
• Examples for heterocyclic aromatic amines are pyrrole and pyrydine.
Images Courtesy: Pyrollidine and Pyridine via Wikicommons (Public Domain)
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