Amines are organic compounds derived from ammonia, where one or more hydrogen atoms are replaced by carbon-containing groups. Polarity in chemistry describes a molecule with an uneven distribution of electrical charge. Amines are generally considered polar molecules. This inherent polarity stems from the nitrogen atom’s unique electronic structure and significantly influences how amines behave chemically and physically.
Defining the Amine Functional Group
Amines are defined by the presence of a nitrogen atom bonded to one or more alkyl (carbon chain) or aryl (aromatic ring) groups, often represented as ‘R’ groups. This nitrogen atom always possesses a non-bonding electron pair, commonly referred to as a lone pair.
The classification of amines depends on how many R groups are directly attached to the nitrogen. A primary amine has the nitrogen bonded to one R group and two hydrogen atoms (R-NH\(_{2}\)). A secondary amine has the nitrogen bonded to two R groups and one hydrogen atom (R\(_{2}\)NH).
A tertiary amine has the nitrogen bonded to three R groups (R\(_{3}\)N). The nitrogen atom in all three categories retains its lone pair of electrons. This lone pair is a fundamental structural component that dictates the amine’s chemical properties.
The Basis of Polarity in Amines
The polarity of amines originates from the significant difference in electronegativity between the nitrogen atom and the atoms it is bonded to, such as carbon or hydrogen. Nitrogen is substantially more electronegative, meaning it has a stronger pull on the shared electrons. This difference creates polar covalent N-C and N-H bonds, with the electron density shifted toward the nitrogen atom.
The geometry of the amine molecule is also a determining factor for its overall polarity. The nitrogen atom in an amine is \(sp^{3}\) hybridized, adopting a tetrahedral electron geometry for its four electron domains. However, the molecular geometry, which only considers the position of the atoms, is trigonal pyramidal.
This asymmetrical, pyramid-like shape ensures that the individual bond dipoles created by the N-C and N-H bonds do not cancel each other out. The lone pair on the nitrogen atom further contributes a substantial dipole moment, pointing away from the base of the pyramid. This results in a net molecular dipole moment, confirming that amines are polar molecules.
The degree of polarity varies across the amine classifications. Primary and secondary amines, which possess the highly polar N-H bonds, typically exhibit greater polarity than tertiary amines. While all are polar due to the nitrogen atom and its lone pair, tertiary amines lack the N-H bond.
How Polarity Influences Amine Properties
The polar nature of amines profoundly influences their physical properties, particularly their ability to interact with other molecules. The presence of the polar N-H bond in primary and secondary amines enables a powerful intermolecular force known as hydrogen bonding. These amines can act as both hydrogen bond donors and acceptors.
This ability to form hydrogen bonds is a major reason why primary and secondary amines have significantly higher boiling points than nonpolar hydrocarbons of a similar molecular weight. More energy is required to overcome the stronger attractive forces of hydrogen bonding and dipole-dipole interactions. For example, a secondary amine like ethylmethylamine has a much higher boiling point than a tertiary amine of similar mass.
Tertiary amines lack the N-H bond, resulting in lower boiling points, similar to those of alkanes or ethers. However, all three classes of amines can form hydrogen bonds with water. The nitrogen’s lone pair allows them to readily interact with the highly polar water molecules.
This strong interaction with water explains why smaller amines are highly soluble. Generally, amines with six or fewer carbon atoms are miscible or very soluble. As the nonpolar hydrocarbon chain length increases, the nonpolar character of the molecule begins to dominate, which dramatically decreases the amine’s water solubility.