Modern inorganic chemistry

178 GROUP IV

(p. 398) reacts with carbon monoxide in preference to oxygen so

preventing the haemoglobin from acting in its normal capacity as

an oxygen carrier.

Carbon monoxide is formed by the incomplete combustion of

carbon. It is prepared in the laboratory by dropping methanoic

(formic) acid into warm concentrated sulphuric acid; the latter

dehydrates the methanoic acid:

HCOOHFil^^4 COt + H 2 O

The gas is passed through caustic soda solution to remove any

sulphur dioxide or carbon dioxide produced in side reactions.

Carbon monoxide is also obtained when an ethanedioate (oxalate)

is heated with concentrated sulphuric acid:

c 2 oj- + H 2 so 4 -> cot + co 2 | + H 2 o + sor

The carbon dioxide is removed by passage of the gas through a

mixture of sodium and calcium hydroxides. Very pure carbon

monoxide is produced by heating nickel tetracarbonyl (see p. 179):

Ni(CO) 4 -> Ni + 4COt

The commercial production of carbon monoxide in the form of

water gas is now largely obsolete. The production by the reaction

between steam and hydrocarbons is considered later (p. 180).

The structure of carbon monoxide can be represented as a reson-

ance hybrid between two structures

C=O and C=O

i.e. JC*O: and xC-O*;

X?

(a) (b)

In structure (a) each atom has a complete octet; in the actual
molecule, the carbon-oxygen bond length is greater than would be
expected for a triple bond, and the molecule has a much smaller
dipole moment than would be expected if the oxygen was donating
electrons to the carbon as in (a); hence structure (b) must contribute
to the actual structure. A simplified orbital picture of structure is
shown at the top of the next page, where nl is formed by sharing
electrons from both carbon and oxygen and n 2 is formed by electrons
donated from oxygen only.
This structure indicates that carbon monoxide should have donor
properties, the carbon atom having a lone pair of electrons. Carbon