Strength of Organic Bases

In the earlier articles on ionic equilibrium, I wrote about the definition of bases and how to make use of the base dissociation constants to determine the strength of the base. In Organic Chemistry, the Nitrogen Compounds are substances that act as bases. N atom when bonded contains a lone pair of electrons and this lone pair of electrons is available to accept protons.

Many would wonder if alcohol whose Oxygen atom also contains a lone pair of electrons, will be base. Generally, because N is less electronegative than O, this result in the N containing compounds to be more likely the base than O containing compounds. Hence, amines and ammonia are usually bases, while alcohols and water are generally not.

Like Organic acids, Organic bases are generally weak bases. They partially dissociates in water, form a cation and the OH^-. This cation is formed because the base donates its lone pair of electron to the proton and hence accepts a protons; resulting in an overall positive charge.

Hence, this article is about how we can use the properties of the substituents and predict the strength of the Organic base. When the substituent is electron donating, e.g. alkyl substituents, the substituents donate electron density via the induction and hence enriches the electron density of N, making it more willing to donate its electrons.

While, electron withdrawing substituents have two means of electron-pulling. If the N is attached to an electronegative atoms, e.g. F, the electron density from N can be pulled away via induction.

Alternatively, the electron withdrawing substituents can pull electron pairs from N via delocalisation due to resonance. This is apparent in phenylamine, where the p orbital of N and the p orbital of the C in benzene ring are overlapping. Hence, the lone pair on N is delocalised into the benzene ring via resonance.  Do note that this process will in-turn make the benzene ring more susceptible to electrophilic substitution. (You can make use of this picture from phenol to conceptualise the delocalisation. It is via the same process, making use of the same principle.)

Finally, the amides are N-compounds that are neutral. This is because their lone pair of electrons have been delocalised to the O of C=O and hence making it so unavailable that it does not have basic properties.

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^{Article written by Kwok YL 2010.}

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