Physical Properties of Nitrogenous Compounds

Figure 99.

You do not need to memorize the structures in figure 99 but you do need to have a knowledge of those in figure 98. Many of those seen in aromatic rings will have planar configuration with sp2 hybridization (pi-bonding) or will be in a shallower trigonal pyramidal shape. Those in singular bonded form with three hydrogen atoms or up to three alkyl groups will be in a trigonal pyramidal shape.

PHYSICAL PROPERTIES OF NITROGENOUS COMPOUNDS

The NH bond is polar by nature of the fact that there are electronegative differences between the two atoms. This leads to hydrogen bonding with itself and with other molecules, such as water. What this means is that there will be a higher melting point, high boiling points, and higher water solubility. There will be a higher partial positive charge on the hydrogen atom and a partial negative charge on the nitrogen atom in this bond.

The amine bond leads to an amine nitrogen atom that is a Lewis base. The alkyl ammonium has a pKa of about 10, while the aryl ammonium is more acidic, with a pKa of about 5, owing to the delocalization of electrons in the aromatic ring, making the nitrogen atom less likely to donate an electron pair. Because of the electronegativity of the nitrogen atoms in these types of bonds, they can act as bases or nucleophiles at the nitrogen atom. Removal of the proton, leads to the amide bond. The NH group itself is a

poor leaving group, which must change to a better leaving group before the nitrogen can be removed.

Ammonium ions of whatever sort leads to a nitrogen atom that is not nucleophilic. It has groups attached to it all over and it sits in the center of the molecule, leading to a low degree of nucleophilicity. It has no electrons to donate in this case. Because of the nature of NH3 as a possible bound group on an alkyl structure, it makes a good leaving group—as opposed to the NH2 molecule, which is infinitely more stable.

Amines are considered more basic than alcohols. The pKa of RH3+ is around 10, while the pKa of ROH2+ is about -3. Any factor, such as electronegativity and resonance, that affects the availability of the lone pair of electrons will affect the basicity of the nitrogen atom. Nitrogen is less electronegative than oxygen and will therefore be a better electron donor. As mentioned, aryl amines are stronger acids, less basic, than an alkyl or non-aromatic heterocyclic amine. The order in which this goes in terms of increasing basicity is ROH3+, aromatic amines, heterocyclic amines, and finally alkyl amines.

You should know that, when deprotonated, the amine group becomes the amide ion, which is NH2. These are important bases in organic chemistry, made by the reaction with sodium or potassium, which pulls off the hydrogen ion to give an amide group. You should know that the basicity of the amine group on the benzene ring will be increased by the presence of electron donating side chains on the aromatic compound, which will counteract the delocalization of the lone pair of electrons on the nitrogen molecule into the pi-system of the ring. It will be increased by electron withdrawing substances, especially ortho or para substituents as is seen in figure 100: