Lewis Definition of bases.

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Today we would look at the idea of Lewis bases. This is the definition of bases as given by G.N Lewis in 1923. You would remember that we have previously dealt with the idea of Lewis acids and the fact that the Lewis acid must possess an empty orbital that can receive an electron pair.

We defined a Lewis acid in those lessons (see the page timeline) as any substance that is capable of accepting an electron pair. Where does the electron pair that the Lewis acid accept come from? This is the focus of today’s lesson.

A Lewis base is a substance that donates a pair of electrons to a Lewis acid. Thus, a Lewis base would have to be a neutral molecule that has a lone pair of electrons on one of its constituent atoms or an anion(a chemical specie that is negatively charged). Examples of species that can act as Lewis bases are; NH3, PH3, SH4, CO, NH2^-, S^2-, O^2-, SO3^2- etc.

To illustrate this idea of electron pair transfer, let us consider the reaction between NH3 and BF3. We would look at the electronic configuration of the central atom in each of the compounds which are N and B respectively (The idea of the central atom in a molecule was explained in the lesson on 29/06/2023 and can be found on the page timeline).

For N – 1s2 2s2 2px1 2py1 2pz1

For B – 1s2 2s2 2px1 2py0 2pz0

When ammonia is formed, three hydrogen atoms each having one electron pair with the electrons in the 2px, 2py and 2pz orbitals of nitrogen. These paired electrons in the molecules are called “bond pairs” or “shared pairs”. Recall that the 1s orbital is remote towards the nucleus and does not get involved in bonding when there are other higher energy orbitals present in the atom. The valence shell of nitrogen is n =2. The pair of electrons in the 2s orbitals are thus localized on the nitrogen atom and are not involved in bonding. This is the “lone pair”.

In boron, again the 1s orbital is remote towards the nucleus and we only consider the valence shell which is n =2. Let us think of how BF3 is formed for a minute. Imagine that one out of the pair of electrons in the 2s orbital of boron is promoted to the empty 2py orbital. We now have three singly filled orbitals of comparable energy; 2s1 2px1 and 2py1.

 The process of combining these orbitals which were originally at different energy sublevels or in some cases, even different principal energy levels so as to obtain orbitals that are of comparable energy and suitable for bonding is known as hybridization. Hence, in the formation of BF3, the boron central atom is said to be “sp2 hybridized”. The meaning of this is that one 2s orbital was mixed with two 2p orbitals when an electron formerly in a 2s orbital was promoted to 2py orbital to enable the sharing of electrons with three fluorine atoms each having only one unpaired electron.

We can see that after the sp2 hybridization of the central boron atom in BF3, we still have an empty 2pz orbital on the central boron atom. Since each orbital can hold a maximum of two electrons, the empty orbital can accept an electron pair from the nitrogen atom in NH3 to form a coordinate covalent bond. Thus, ammonia is now the lone pair donor (Lewis base) and BF3 is the lone pair acceptor (Lewis acid). I hope this clear? You can always get back to me at any time to ask for further clarification.

Finally, let me say this; acid – base reaction in the Lewis sense as described here is the basis of the vast sub-disciplines in chemistry known as; Coordination chemistry, Bioinorganic chemistry, Inorganic biochemistry and Organometallic Chemistry. In these disciplines, the Lewis acid is often a metal ion or a neutral metal atom. We shall look at a comprehensive introduction to coordination chemistry at a later time.

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