The \(\text{NH}_2\) group, commonly known as the amino group, is a foundational chemical structure in organic and biological chemistry. It appears in molecules such as amino acids, the building blocks of proteins, and various neurotransmitters. Determining whether the \(\text{NH}_2\) group is polar or nonpolar depends on the symmetry and electron sharing within the structure. Polarity is defined by the uneven distribution of electrical charge across a molecule, which creates distinct positive and negative ends.
Understanding Molecular Polarity
Molecular polarity arises from the combined effects of bond polarity and molecular geometry. Bond polarity is a direct result of electronegativity, an atom’s inherent ability to attract a shared pair of electrons. When two atoms with different electronegativities bond, this unequal sharing establishes a bond dipole. The more electronegative atom gains a partial negative charge, while the less electronegative atom gains a partial positive charge.
The overall polarity of a larger group depends not just on the individual bond dipoles, but also on the three-dimensional arrangement of the atoms. If the molecule is perfectly symmetrical, the individual bond dipoles cancel each other out, resulting in a nonpolar structure.
Why the \(\text{NH}_2\) Group is Highly Polar
The \(\text{NH}_2\) group exhibits a distinct polarity, primarily due to the large difference in electronegativity between Nitrogen (N) and Hydrogen (H). Nitrogen possesses an electronegativity value of approximately 3.04 on the Pauling scale, while hydrogen is significantly lower at about 2.20. This difference of 0.84 is substantial enough to create highly polar N-H bonds, with the electron density being pulled strongly toward the nitrogen atom. This results in the nitrogen atom carrying a substantial partial negative charge, while the two hydrogen atoms carry partial positive charges.
Beyond the bond polarity, the three-dimensional structure of the \(\text{NH}_2\) group is the second determining factor. The nitrogen atom has five valence electrons, two of which form a non-bonding lone pair. This lone pair occupies space around the nitrogen atom and influences the group’s geometry. The combination of the lone pair and the two N-H bonds results in an asymmetrical, pyramidal-like arrangement.
Because the shape is asymmetrical, the dipoles created by the polar N-H bonds and the powerful dipole from the lone pair do not cancel each other out. Instead, they reinforce one another, pointing toward the nitrogen atom. This creates a significant net dipole moment for the entire \(\text{NH}_2\) functional group, confirming that the group is highly polar.
How Polarity Influences Chemical Behavior
The polar nature of the \(\text{NH}_2\) group dictates its chemical and physical interactions with other substances. A primary consequence is its ability to readily dissolve in polar solvents, such as water, following the rule “like dissolves like.” This high solubility is particularly important in biological systems, which are largely aqueous.
The partial charges allow the amino group to participate in hydrogen bonding. It acts as a hydrogen bond donor through its N-H bonds, and as a hydrogen bond acceptor via the lone pair on the nitrogen atom. Hydrogen bonding plays a large role in determining the structure of proteins, as the polar amino groups help larger molecules fold into their correct three-dimensional shapes.