Chapter 2, Problem 31P

Carbon monoxide is a toxic pollutant which arises from the partial combustion of carbon-based fuels. Complete combustion produces ${\text{CO}}_2$. The toxicity of CO is a result of its competition for the ${\text{O}}_2$-binding sites in blood, i.e. the iron present in haemoglobin (see Chapter 29). When CO binds to the iron, it prevents ${\text{O}}_2$ from being carried in the bloodstream. The following are resonance structures for CO:

$\text{:C = }\underset{··}{\overset{··}{\text{O}}}\leftrightarrow \text{:}\stackrel{-}{\text{C}}\equiv \underset{\oplus }{\overset{··}{\text{O}}}$

(a) Comment on these structures in terms of octet rule. (b) How is the right-hand resonance structure related to a Lewis structure for ${\text{N}}_2$? (c) A primary interaction between CO and iron in haemoglobin involves a lone pair of electrons on the carbon atom. Using MO theory, explain how this lone pair arises. (d) Without treatment, severe CO poisoning is fatal. Explain why a hyperbaric chamber containing pure ${\text{O}}_2$ at a pressure of 1.4 bar is used to treat a patient with severe CO poisoning. Normal air contains 21% ${\text{O}}_2$.