A Bronsted-Lowry base has at least one pair of electrons in order to form a bond with a proton from a Bronsted-Lowry acid. The reactions that take place are called proton transfer reactions. Here we introduce Lewis bases and Lewis acids. A Lewis base is an electron donor while a Lewis acid is an electron acceptor. All Bronsted-Lowry bases are Lewis bases. In aqueous solutions, the Bronsted-Lowry definition is most useful. But there are other substances, that do not have protons, that are considered Lewis acids. These substances are electron deficient or they can accept an electron pair. For example, many metal cations have vacant orbitals that can accept an electron pair or pairs from a base. For example, the Fe3+ ion can react with cyanide ions, CN–, to form the ferricyanide ion.
The Lewis acid is the Fe3+ ion while the Lewis base is the CN–.
The substance BF3 also functions as a Lewis acid because it does not have an octet of electrons. The boron atom has three sp2 hybrid orbitals bonded to each of the F atoms. There is one empty 2p orbital that can accept an electron pair from a base.
Some substances that contain multiple bonds can also act as Lewis acids. Water and carbon dioxide can react to form carbonic acid (H2CO3). The carbon dioxide acts as a Lewis acid while water is the base.
The water attacks the carbon dioxide molecule resulting in the pi electrons of the double bond moving to an oxygen atom. This leaves an empty orbital on the carbon, and a bond is formed between the carbon and the oxygen of the water molecule. The product rearranges by transferring a proton from the oxygen in the water molecule to the oxygen in the carbon dioxide molecule. Carbonic acid is formed.
Exercises
Exercise 1. Identify the Lewis acid and the Lewis base in the following reaction.
Exercise 2. Identify the Lewis acids and bases in the following equations.
b) SO3 + OH– ⇄ HSO4–
c) (CH3)3N + BF3 ⇄ (CH3)3NBF3
Exercise 3. Which substance in each pair will produce the stronger aqueous acidic solution?
b) BF3 or BH3
c) Ca(NO3)2 or NiBr2
Back to Acids and Bases: Aqueous Equilibria
Back to Study Guides for General Chemistry 2