Main Difference – Primary vs Secondary vs Tertiary Amines
An amine is a derivative of ammonia. It is composed of one or more alkyl groups which replace the hydrogen atoms in ammonia (NH3) molecule. Therefore, the alkyl group is directly bonded to the nitrogen atom. According to the number of alkyl groups that have been attached to the nitrogen atom, amines are categorized into three broad groups as primary amines, secondary amines and tertiary amines. In primary amines, one alkyl or aryl group is attached to the nitrogen atom; in secondary amines, two alkyl or aryl groups are attached to the nitrogen atom whereas, in tertiary amines, three alkyl or aryl groups are attached to the nitrogen atom. This is the main difference between primary secondary and tertiary amines.
Key Areas Covered
1. What are Primary Amines
– Definition, Common Primary Amines, Basicity
2. What are Secondary Amines
– Definition, Basicity
3. What are Tertiary Amines
– Definition, Basicity
4. What is the Difference Between Primary Secondary and Tertiary Amines
– Comparison of Key Differences
Key Terms: Alkyl, Amine, Ammonia, Aryl, Bases, Basicity, Methenamine, Primary Amines, Secondary Amines, Tertiary Amines
What are Primary Amines
Primary amines are amines containing one alkyl or aryl group attached to the nitrogen atom. An ammonia molecule has three hydrogen atoms bonded to a central nitrogen atom. But one of these hydrogen atoms in a primary amine is replaced by an alkyl or aryl group. An alkyl group is a functional group that can be found in organic molecules. It is an alkane having a vacant point that is formed due to the loss of a hydrogen atom. Thus, this point can be attached to another atom; in this case, it is attached to a nitrogen atom. An aryl group always contains an aromatic ring. Aryl group is a simple aromatic compound where one hydrogen atom is removed from the ring, allowing it to get attached to another atom; here, it is attached to a nitrogen atom.
When naming a primary amine, the name of the alkyl or aryl group is also mentioned as a prefix. For example, if a methyl group is present in a primary amine, the compound is named as methylamine or methenamine (preferred IUPAC name). But if the amine has more functional groups, the substituent name used for primary amine group –NH2 is “amino”. Given below are some common primary amines.
Figure 1: Methylamine, Aniline and Alanine
Amines are bases due to the presence of a lone pair on the nitrogen atom, which can be donated to a proton. (When amines react with water, they can form OH– ions by donating the lone pair to a hydrogen atom of H2O molecule. Since bases are compounds that can release OH– ions, amines are also bases).
The basicity of amines is described by observing the stability of the compound formed after donating the lone pair. When the lone pair is donated, the nitrogen atoms gets a positive charge. Alkyl groups are electron withdrawing groups. In a primary amine, there is an alkyl group attached to the nitrogen atom. Therefore, the positive charge on the nitrogen atom is reduced due to the electron withdrawing property of the alkyl group (the electron density around the nitrogen atom is increased by alkyl group). However, since there is only one alkyl group present, primary amines are less basic than secondary amines.
What are Secondary Amines
Secondary amines are amines containing two alkyl or aryl group attached to the nitrogen atom. Secondary amines are formed by replacement of two hydrogen atoms of ammonia molecule by alkyl or aryl groups.
Secondary amines are more basic than primary amines and tertiary amines. The basicity of amines is described based on the stability of the compound formed after accepting a proton (donating the lone pair on nitrogen atom to a proton). Alkyl groups are electron donating groups. In a secondary amine consisting of two alkyl groups, the positive charge on the nitrogen atom is reduced due to this electron donating property of alkyl groups. Hence, the secondary amine is more basic than other amine forms.
Figure 2: General Formula for a Secondary Amine
When naming a secondary amine, the two alkyl or aryl groups attached to the nitrogen atom should be mentioned in the name. For example, N-ethyl-N-propylamine stands for a secondary amine consisting of a nitrogen atom bonded to a hydrogen atom, an ethyl group and a propyl group CH3CH2NHCH2CH2CH3.
What are Tertiary Amines
Tertiary amines are amines containing three alkyl or aryl group attached to the nitrogen atom. These amines are formed by replacement of all three hydrogen atoms of ammonia by alkyl or aryl groups. Therefore, there are no remaining hydrogen atoms bonded to the nitrogen atom.
Tertiary amines are more basic than primary amines and ammonia but are less basic than secondary amines. This is because there is a steric hindrance due to the presence of three alkyl groups around one nitrogen atom though these Alkyl groups reduce the positive charge on the nitrogen atom. This causes the reduction of basicity of the amine. The basicity of a tertiary amine is slightly higher or similar to the basicity of a primary amine.
Figure 3: General Structure of a Tertiary Amine
When naming a tertiary amine, the three alkyl or aryl groups attached to the nitrogen atom should be mentioned in the name. For example, tributylamine stands for a tertiary amine consist of a nitrogen atom bonded to three butyl groups; (CH3CH2CH2CH2)3N.
Difference Between Primary Secondary and Tertiary Amines
Definition
Primary Amines: Primary amines are amines containing one alkyl or aryl group attached to the nitrogen atom.
Secondary Amines: Secondary amines are amines containing two alkyl or aryl group attached to the nitrogen atom.
Tertiary Amines: Tertiary amines are amines containing three alkyl or aryl group attached to the nitrogen atom.
Alkyl (or Aryl) Groups
Primary Amines: Primary amines are composed of one alkyl group bonded to the nitrogen atom.
Secondary Amines: Secondary amines are composed of two alkyl groups bonded to the nitrogen atom.
Tertiary Amines: Tertiary amines are composed of three alkyl groups bonded to the nitrogen atom.
Basicity
Primary Amines: Primary amines are less basic than secondary and tertiary amines, but is more basic than ammonia.
Secondary Amines: Secondary amines are more basic than all amine forms including ammonia.
Tertiary Amines: Tertiary amines are more basic than ammonia and primary amines but are less basic than secondary amines.
Steric Hindrance
Primary Amines: There is no considerable steric hindrance in primary amines.
Secondary Amines: There is no considerable steric hindrance in secondary amines.
Tertiary Amines: Tertiary amines show steric hindrance due to the presence of three alky groups bonded to the same nitrogen atom.
Conclusion
Amines are nitrogen-containing organic compounds. There are three types of amines as primary amines, secondary amines and tertiary amines. The main difference between primary secondary and tertiary amines is that, in primary amines, one alkyl or aryl group is attached to the nitrogen atom and in secondary amines, two alkyl or aryl groups are attached to the nitrogen atom whereas, in tertiary amines, three alkyl or aryl groups are attached to the nitrogen atom.
Reference:
1. “Basicity of Amines.” TutorVista, Available here.
2. Reusch, William. Amine Reactivity. Available here.
3. Libretexts. “24.3 Basicity of Amines.” Chemistry LibreTexts, Libretexts, 25 Aug. 2017, Available here.
4. Block, Eric, and Peter A.S. Smith. “Amine.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 26 Dec. 2017, Available here.
Image Courtesy:
1. “Methylamine” By Rune.welsh at English Wikipedia (Public Domain) via Commons Wikimedia, “Aniline” By Calvero (Public Domain) via Commons Wikimedia and L-Alanin – L-Alanine” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia [Merged] 2. “Secondary-amine-2D-general” By Kes47 (?) – File:Secondary-amine-2D-general.png (Public Domain) via Commons Wikimedia
3. “Amine-(tertiary)” By Benjah-bmm27 assumed-Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
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