# Comparison of basicity of o-phenanthroline and ammonia

I was trying to compare the basicity of o-phenanthroline and ammonia.

According to me, the factors that are seen in o-phenanthroline are:

1. o-Phenanthroline has two nitrogen atoms at the 1,10 positions.
2. It has a free lone pair on one nitrogen that is not delocalized due to resonance.
3. The nitrogens are $$\mathrm{sp^2}$$ hybridized which is more electronegative compared to the $$\mathrm{sp^3}$$ lone pair on ammonia.

However, would there be hydrogen bonding in its conjugate base? If so, then how do we compare its basicity with a base like ammonia?

A quote from here explains and give a good answer to your wondering:

Amines are the most basic of the common organic functional groups, but are still fairly weak bases. Protonation occurs on the non-bonded electron pair exclusively. The basicity of amines is directly dependent on the “electron density” at the nitrogen atom. Both inductive and resonance effects can alter the basicity of a nitrogen atom.

Hybridization on the $$\ce{N}$$ also affects basicity. An increase in $$\mathrm{s}$$ character on an atom increases the electronegativity of that atom which favors acidity and therefore disfavors basicity. Hence $$\mathrm{sp^3}$$-hybridized nitrogen is more basic than either $$\mathrm{sp^2}$$ or $$\mathrm{sp}$$ hybridized nitrogen.

The availability of this non-bonding lone pair is a factor of basisity. The 1,10-Phenanthroline is a pyridine derivative. Thus, both lone pairs of two $$\ce{N}$$ atoms in the ring system are contributed to the system's aromaticity. This make them not 100% available to incoming protons while the lone pair in ammonia is 100% available. Therefore, in aqueous solutions, the basicity of $$\ce{NH3} \ (\mathrm{p}K_\mathrm{a} = 9.3)\gt$$ basicity of 1,10-Phenanthroline $$(\mathrm{p}K_\mathrm{a} \approx 4.9)$$. This is similar to $$\mathrm{p}K_\mathrm{a}$$ of pyridine, which is $$5.2$$ in water (comparison with piperidine is depicted in following scheme):

Keep in mind that there are some other factors are effecting the basicity of amine as well. For example, the ring sizes of cyclic amines (Ref.1):

The order is 5-membered $$\ge$$ 4-membered $$\gt$$ 6-membered $$\gg$$ 3-membered.

References:

1. Scott Searles, Milton Tamres, Frank Block, Lloyd A. Quarterman, "Hydrogen Bonding and Basicity of Cyclic Imines," J. Am. Chem. Soc. 1956, 78(19), 4917–4920 (https://doi.org/10.1021/ja01600a029).