Lewis acids and bases

LEWIS ACIDS AND BASES

Key Notes

Lewis acids

Lewis acids are electron deficient molecules which are termed acidic because they will accept a pair of electrons (in the form of a bond) from an electron-rich species in order to fill up their valence shell. BF₃, AlCl₃, TiCl_4, and SnCl₄ are examples of Lewis acids.

Lewis bases

Lewis bases are ions or neutral molecules containing an atom with a lone pair of electrons. Lewis bases use a lone pair of electrons to form a bond to a Lewis acid.

Lewis acids

Lewis acids are ions or electron deficient molecules with an unfilled valence shell. They are classed as acids because they can accept a lone pair of electrons from another molecule to fill their valence shell. Lewis acids include all the Brønsted–Lowry acids we have already discussed, as well as ions (e.g. H⁺ , Mg²⁺ ), and neutral species such as BF₃ and AlCl₃.

Both Al and B are in Group 3A of the periodic table and have three valence elec-trons in their outer shell. This means that these elements can form three bonds. However, there is still room for a fourth bond. For example in BF₃, boron is sur-rounded by six electrons (three bonds containing two electrons each). However, boron’s valence shell can accommodate eight electrons and so a fourth bond is pos-sible if the fourth group can provide both electrons for the new bond. Since both boron and aluminum are in Group 3A of the periodic table, they are electropositive and will react with electron-rich molecules in order to obtain this fourth bond. Many transition metal compounds can also act as Lewis acids (e.g.TiCl₄ and SnCl₄).

Lewis bases

A Lewis base is a molecule which can provide a lone pair of electrons to fill the valence shell of a Lewis acid (Fig. 1). The base can be a negatively charged group such as a halide, or a neutral molecule such as water, an amine, or an ether, as long as there is an atom present with a lone pair of electrons (i.e. O, N, or a halogen).

All the Brønsted–Lowry bases discussed earlier can also be defined as Lewis bases. The crucial feature is the presence of a lone pair of electrons which is avail-able for bonding. Therefore, all negatively charged ions and all functional groups containing a nitrogen, oxygen, or halogen atom can act as Lewis bases.