Chapter 2

Write the Lewis structures, including resonance structures where appropriate, for (a) the oxalate ion, \(\mathrm{C}_{2} \mathrm{O}_{4}{ }^{2-}\) (there is a \(\mathrm{C}\) - \(\mathrm{C}\) bond with two oxygen atoms attached to each carbon atom); (b) \(\mathrm{BrO}^{+}\); (c) the acctylide ion, \(\mathrm{C}_{2}^{2-}\). Assign formal charges to each atom.

Show how resonance can occur in the following organic ions: (a) acetate ion, \(\mathrm{CH}_{3} \mathrm{CO}_{2}{ }^{-}\); (b) enolate ion, \(\mathrm{CH}_{2} \mathrm{COCH}_{3}\) which has one resonance structure with \(\mathrm{a} \mathrm{C}=\mathrm{C}\) double bond and an \(-\mathrm{O}^{-}\)group on the central carbon atom; (c) allyl cation, \(\mathrm{CH}_{2} \mathrm{CHCH}_{2}^{*}\); (d) amidate ion, \(\mathrm{CH}_{3} \mathrm{CONH}^{-}\)(the \(\mathrm{O}\) and \(\mathrm{N}\) atoms are both bondod to the second \(C\) atom).

Which of the following members of cach pair is the stronger Lewis acid? Explain your reasoning. (a) \(\mathrm{BF}_{3}\) or \(\mathrm{NF}_{3}\); (b) \(\mathrm{Al}^{3 *}\) or \(\mathrm{K}^{+}\); (c) \(\mathrm{Mg}^{2+}\) in \(\mathrm{MgF}_{2}\) or \(\mathrm{Mg}^{2+}\) in \(\mathrm{MgCl}_{2}\).

Which of the following members of cach pair is the stronger Lewis base? Explain your reasoning. (a) \(\mathrm{CH}_{3}^{-}\)or \(\mathrm{CH}_{4}\); (b) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{~S}\); (c) \(\mathrm{NH}_{3}\) or \(\mathrm{NH}_{2}^{-}\).

An important principle in chemistry is the isolobal analogy. This very simple principle states that chemical fragments with similar valence orbital structures can replace each other in molecules. For example, \(\cdot \mathrm{C}-\mathrm{H}\) and \(\cdot \mathrm{S}-\mathrm{H}\) are isolobal fragments, cach having three electrons with which to form bonds besides the bond to \(\mathrm{H}\). An isolobal series of molecules would be HCCH, HCSiH, and HSiSiH. Similarly, a lone pair of clectrons can be used to replace a bond so that - \(\mathrm{N}\) : is isolobal with ' \(\mathrm{C}-\mathrm{H}\), with the lone pair taking the place of the \(\mathrm{C}-\mathrm{H}\) bond. The isolobal set here is HOCH, HCN, and NN. (a) Draw the Lewis structures for the molecules HCCH, HCSiH, HSiSiH, \(\mathrm{HCN}\), and NN. (b) Using the isolobal principle, draw Lewis structures for molecules based on the structure of benzenc, \(\mathrm{C}_{6} \mathrm{H}_{6}\), in which one or more \(\mathrm{CH}\) groups are replaced with \(\mathrm{N}\) atoms.

A common biologically active radical is the pentadicnyl radical, RCHCHCHCHCHR', in which the carbons form a long chain with \(R\) and \(R^{\prime}\) (which can be a number of different organic groups) at ench end. Draw three resonance structures for this compound that maintain carbon's valence of four.