Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
Heteroatomic Functional Groups and Organic Nomenclature
3.12 Structure and Properties of Amines
Most of the organic compounds examined thus far all have primarily carbon, hydrogen, and oxygen atoms. In organic chemistry and biological chemistry, the nitrogen atom is another commonly encountered element. Amines are organic compounds that contain nitrogen; most have a distinct odor, and they are readily soluble in water due to their polarity. Some amines are toxic and cause cancer of the liver and other organs, but a large percentage of amines have pharmaceutical benefits. The amine functionality is found in some very useful everyday drugs, which are used routinely in medicine, some are shown below.
Amphetamine and metamphetamine are stimulants of the central nervous system.
Nitrogen is also found in proteins, nucleic acids, and other natural occurring molecules, which will be discussed in more detail in Chapter 20.
There are a large number of organic molecules that are amines and they are routinely classified based on the number of alkyl groups or hydrogens that are bonded to the nitrogen. Ammonia is a specific amine in that it has only hydrogens bonded to the nitrogen. Primary amines have two hydrogen atoms bonded to the nitrogen and one alkyl group. Secondary amines have one hydrogen and two alkyl groups bonded to the nitrogen atom. Tertiary amines do not have a hydrogen bonded to the nitrogen, but have three alkyl groups bonded to the nitrogen atom. Thus, the number of alkyl groups bonded to the nitrogen atom is used to classify amines as primary (1°), secondary (2°), or tertiary (3°). The general representations of these different classifications are shown below.
A fourth category exists if there are four alkyl groups bonded to the nitrogen; this category is classified as an ammonium salts, since it is ionic.
Shown below are examples of primary (1°) amines.
Shown below are examples of secondary (2°) amines.
Shown below are examples of tertiary (3°) amines.
Shown below are examples of ammonium ion salts.
Problem 3.20
Classify the following amines as primary (1°), secondary (2°), or tertiary (3°).
3.12.1 Nomenclature of Amines
In naming amines using the IUPAC system of nomenclature, the root name of the corresponding alkane that has the amine functionality is changed to alkanamine, i.e. the e of the alkane is changed to amine. In naming primary amines, identify the longest chain that contains the amine functional group and start numbering at the end closest to the amine group, as shown in the example below.
Next, the position of the amine functionality must be indicated with a number, as shown in the example below.
The locations of branches (substituents) are indicated by the numbers that are from the numbering system determined based on the presence of the amine functionality, as shown in the example below.
Problem 3.21
i. Give the IUPAC names for the compounds shown below.
ii. Give line-angle structures for the following molecules.
1. 3-Methyl-2-pentanamine
2. 2,4-Dimethyl-2-hexamine
In naming (2°) or tertiary (3°) amines, each alkyl group that is bonded to the amine nitrogen is named and their presence is signified by the prefix N. If the amine is secondary with only one alkyl group bonded to the nitrogen, then only one N is used to indicate its presence. On the other hand, if the amine is tertiary with two groups bonded, then both groups are identified and their presence is indicated by two Ns separated by a comma, as shown in the examples below.
Problem 3.22
i. Give the IUPAC names for the compounds shown below.
ii. Give line-angle structures for the following molecules.
1. N,N-Dimethyl-3-hexanamine
2. N-Ethyl-3-heptanamine
3. N,N-Dimethyl-2-propanamine
For amines that have cyclic structures, identify the cyclic system and if the nitrogen is not a part of the ring, name the amine as a derivative of the cyclic system; examples are shown below.
If the amine group is bonded to the phenyl ring (─C6H5), this compound has a special name, aniline. If groups are bonded to the phenyl ring, these compounds are named as substituted anilines, as shown in the examples below.
If the nitrogen is part of the ring system, then they are typically named as a derivative of a common name, such as shown below:
3.12.2 Nomenclature of Difunctional Amines
For most difunctional molecules that contain the amine functionality, the amino group is named as a substituent, based on the root name of another functional group that is present in the molecule. Examples are shown below.
Functional groups that are of low priority, such as the alkene or alkyne functionalities, are often incorporated in the nomenclature using the alkanamine system as shown in the example below.
Problem 3.23
i. Give the IUPAC names for the compounds shown below.
ii. Give line-angle structures for the following molecules.
1. N,N-Dimethylcyclopentanamine
2. 4-Amino-2-pentanone
3. 2-Fluoroaniline