Aromatic Amine and Bases

Question

How to determine which aromatic amine is a strong base using topics like amine basicity, polarity/EN, and resonance?

Answer 1

Hi! When looking at acidity and basicity, (specifically Brønsted-Lowry), it's always critical to understand relative pKa values of the conjugate acid base pairs in order to make comparisons. If you know relative pKas, you can determine strong vs weak bases pretty easily. But here I'll provide a brief review of acidity and basicity, and then at the end I'll add some specific commentary about aromatic amine bases.

Review of Acidity and Basicity:

Recall the following statements:

1. The pKa of a strong acid will be low and the pka of a weak acid will be high.
2. A weak acid has a strong conjugate base
3. A strong acid has a weak conjugate base
4. A weak base has a strong conjugate acid
5. A strong base has a weak conjugate acid

So when trying to determine the basicity of a strong base, what you're looking for is a weak conjugate acid.

We determine acidity based on the ability of the conjugate base to accommodate the electrons leftover from deprotonation. This translates to: how "happily" does the conjugate base accept the electrons, and relates to the stability of the conjugate base

Consider the following factors that relate to acidity, and you can use the same logic to translate it to basicity.

Charge: A proton is more acidic if attached to an atom that is already positively charged, because removing it gives a neutral (more stable) compound.

The Atom: Acidity increases across a row, which relates to the increasing electronegativity of the conjugate base. The more electronegative, the more comfortable an atom will be accepting electrons. Acidity also increases down the column of the periodic table, which relates to the increasing the size of conjugate base. When an atom is bigger, it results in weaker orbital overlap between the larger outer shell orbitals and H 1s orbital. When you have larger outer-shell orbitals the electrons are able to occupy more space, which ultimately reduces electron repulsion.

Resonance: If you can draw several resonance forms of the conjugate base of an acid using the electrons left over from deprotonation, this means that the electrons are delocalized and ultimately happier (read: lower in energy) so you have a weaker base (remember, weak conj base = strong acid, strong conj base = weak acid).

Induction: Electronegative atoms like to hoard electrons, so by nature of having an electronegative atom, electron density is pulled away from the adjacent protons, increasing their acidity and stabilizing the left over negative charge.

Orbitals: When the orbital accepting the lone pair from deprotonation has more s character, the electrons get to be closer to the positively charged nucleus. This inherently increases the stability of the leftover charge. This means that in order of acidity: sp>sp2>sp3. The opposite is true of basicity, sp3>sp2>sp.

---

Aromatic Amine Bases

With all of that being said, aromatic amine bases seem a little strange on first glance because you have to consider which orbital is accepting the lone pair and whether it has any possible resonance forms.

Take pyridine for example: Pyridine is C5H5N, and it is aromatic because it is

1. cyclic
2. planar
3. fully conjugated
4. has 4n+2 electrons

If you consider the hybridization of the N in pyridine, it is sp2 hybridized. There are 2 sp2 orbitals that bond to a carbon each, and one P orbital that is part of the aromatic system. The N also has a lone pair, and we can come to the conclusion that the lone pair is in an sp2 orbital. Because the lone pair is in an sp2 orbital, it is not part of the network of p orbitals that give it its aromaticity. This means that the lone pair available to act as a base does not have resonance forms with the aromatic system! Recall: more resonance forms = less basic, so this makes pyridine more basic than you would expect on first glance if you were to erroneously assume that it had multiple resonance forms.

So for other aromatic amine bases, make sure to consider the aforementioned factors that relate to acidity and basicity, and then pay extra attention to what orbital the lone pair is in. If it can resonate with the aromatic system, it will and it is considered to be in a P orbital. If it cannot, like with pyridine, then determine what orbital it is in and see how that relates to acidity and basicity to draw your conclusions.

Hope this helps!