Question 16.23: Identify the Lewis acid and Lewis base in each of the follow...
Identify the Lewis acid and Lewis base in each of the following reactions:
(a) \text{C}_2\text{H}_5\text{OC}_2\text{H}_5 + \text{AlCl}_3 \rightleftarrows (\text{C}_2\text{H}_5)_2\text{OAlCl}_3
(b) \text{Hg}^{2+}(aq) + 4\text{CN}^–(aq) \rightleftarrows \text{Hg(CN)}_4^{2–}(aq)
Strategy Determine which species in each reaction accepts a pair of electrons (Lewis acid) and which species donates a pair of electrons (Lewis base).
Setup
(a) It can be helpful to draw Lewis structures of the species involved:
(b) Metal ions act as Lewis acids, accepting electron pairs from anions or molecules with lone pairs.
(a) The \text{Al} is sp^2 -hybridized in \text{AlCl}_3 with an empty 2p_z orbital. It is electron deficient^*, sharing only six electrons. Therefore, the \text{Al} atom has the capacity to gain two electrons to complete its octet. This property makes \text{AlCl}_3 a Lewis acid. On the other hand, the lone pairs on the oxygen atom in \text{C}_2\text{H}_5\text{OC}_2\text{H}_5 make the compound a Lewis base:
(b) \text{ Hg}^{2+} accepts four pairs of electrons from the \text{ CN}^– ions. Therefore, \text{ Hg}^{2+} is the Lewis acid and \text{ CN}^– is the Lewis base.
^*Student Annotation: An electron-deficient molecule is one with less than a complete octet around the central atom.