Intermolecular forces influence the physical state and behavior of a substance. Methylamine (\(\text{CH}_3\text{NH}_2\)) is a simple organic molecule known as a primary amine. Understanding the forces between methylamine molecules is important for predicting its properties. The central question is whether this molecule participates in the strong intermolecular attraction known as hydrogen bonding.
Defining the Conditions for Hydrogen Bonding
Hydrogen bonding is a special, powerful type of dipole-dipole interaction that requires two specific structural features to occur. The first condition is the presence of a hydrogen atom covalently bonded to one of three highly electronegative elements: fluorine (F), oxygen (O), or nitrogen (N). When hydrogen is bonded to one of these atoms, the electron density is pulled strongly toward the electronegative atom, leaving the hydrogen atom with a significant partial positive charge (\(\delta+\)). This creates a highly polarized bond, which serves as the hydrogen bond donor.
The second condition is that a neighboring molecule must possess a lone pair of electrons on a small, highly electronegative atom, which acts as the hydrogen bond acceptor. The partially positive hydrogen atom from the donor molecule is then strongly attracted to this lone pair on the acceptor molecule. The small size of the hydrogen atom allows it to get very close to the acceptor atom, which contributes to the high strength of this particular force.
Analyzing Methylamine (\(\text{CH}_3\text{NH}_2\))
Methylamine is an ammonia derivative where one hydrogen atom has been replaced by a methyl (\(\text{CH}_3\)) group. Examination of its structure confirms that it possesses the necessary components to engage in hydrogen bonding. The molecule contains a nitrogen atom directly bonded to two hydrogen atoms, forming the amino (\(\text{NH}_2\)) functional group. These \(\text{N-H}\) bonds create the necessary partial positive charge on the hydrogen atoms, allowing them to act as hydrogen bond donors.
The nitrogen atom in methylamine also possesses one lone pair of electrons. This lone pair makes the nitrogen atom highly attractive to a partially positive hydrogen atom from a neighboring molecule, enabling methylamine to act as a hydrogen bond acceptor. Therefore, methylamine molecules can both donate and accept hydrogen bonds with other methylamine molecules.
Physical Consequences of Hydrogen Bonding
The ability of methylamine to form strong hydrogen bonds has direct and measurable consequences on its physical properties. These strong attractions require a substantial amount of energy to overcome, which is reflected in the compound’s boiling point. Methylamine has a boiling point of approximately \(-6.3^\circ \text{C}\), which is significantly higher than a non-hydrogen bonding molecule of a comparable size. For instance, ethane (\(\text{C}_2\text{H}_6\)), which has a similar molecular weight but only experiences London Dispersion Forces, boils at a much lower \(-88.6^\circ \text{C}\).
Hydrogen bonding also dramatically increases the solubility of methylamine in polar solvents, especially water. Water molecules are excellent hydrogen bond donors and acceptors, and they readily form strong hydrogen bonds with the \(\text{NH}_2\) group of methylamine. This allows the two substances to mix freely. The high solubility of methylamine is a characteristic shared by many compounds with \(\text{N-H}\) or \(\text{O-H}\) groups.