Oxides and oxoacids of carbon

Unlike the later elements in group 14, carbon forms stable, volatile monomeric oxides: CO and CO₂. A comment on the difference between CO₂ and SiO₂ can be made in the light of the thermochemical data in Table 1.1: the C=O bond enthalpy term is more than twice that for the C^_O bond, while the Si=O bond enthalpy term is less than twice that of the Si^_O bond.

In rationalizing these differences, there is some justification for saying that the C=O bond is strengthened relative to Si=O by (p–p)π contributions, and, in the past, it has been argued that the Si^_O bond is strengthened relative to the C^_O bond by ( p–d)π-bonding.

    Irrespective of the interpretation of the enthalpy terms however, the data indicate that (ignoring enthalpy and entropy changes associated with vaporization) SiO₂ is stable with respect to conversion into molecular O=Si=O, while CO₂ is stable with respect to the formation of a macromolecular species containing 4-coordinate C and C^_O single bonds. However, an extended solid phase of CO₂ has recently been prepared by laser-heating a molecular phase at 1800K and under 40 GPa pressure; the vibrational spectrum of the new phase indicates that it is structurally similar to quartz .  Carbon monoxide is a colourless gas, formed when C burns in a restricted supply of O₂. Small-scale preparations involve the dehydration of methanoic acid (as in below equation). CO is manufactured by reduction of CO₂ using coke heated above 1070K or by the water–gas shift reaction.

Carbon monoxide is almost insoluble in water under normal conditions and does not react with aqueous NaOH, but at high pressures and temperatures, HCO₂H and Na[HCO₂] are formed respectively. Carbon monoxide combines with F₂, Cl₂ and Br₂ sulfur and selenium. The high toxicity of CO arises from the formation of a stable complex with haemoglobin (see Section 28.3) with the consequent inhibition of O2 transport in the body. The oxidation of CO to CO₂ is the basis of quantitative analysis for CO with the I₂ formed being removed and titrated against thiosulfate. CO is similarly oxidized by a mixture of MnO₂, CuO and Ag₂O at ambient temperatures and this reaction is used in respirators.

The thermodynamics of the oxidation of carbon is of immense importance in metallurgy. Selected physical properties of CO and CO₂ are given in B Table 1.2; bonding models . The bond in CO is the strongest known in a stable molecule and confirms the efficiency of ( p–p)π-bonding between C and O. However, considerations of the bonding provide no simple explanation as to why the dipole moment of CO is so low. In an excess of O₂, C burns to give CO₂. Solid CO₂ is called dry ice and readily sublimes (Table 1.2) but may be kept in insulated containers for laboratory use in, e.g. lowtemperature baths (Table 1.3).

    Supercritical CO₂ has become a much studied and versatile solvent. Small-scale laboratory syntheses of gaseous CO₂ usually involve reactions.

Carbon dioxide is the world’s major environmental source of acid and its low solubility in water is of immense biochemical and geochemical significance. In an aqueous solution of carbon dioxide, most of the solute is present as molecular CO2 rather than as H₂CO₃, as can be seen from the value of K ≈ 1.7 × 10^-3 for the equilibrium:

Aqueous solutions of CO₂ are only weakly acidic, but it does not follow that H₂CO₃ (carbonic acid) is a very weak acid. The value of pKa(1) for H₂CO₃ is usually quoted.

This evaluation, however, assumes that all the acid is present in solution as H₂CO₃ or [HCO₃]^- when, in fact, a large proportion is present as dissolved CO₂. By taking this into account, one arrives at a ‘true’ pKa(1) for H₂CO₃ of ≈3.6. Moreover, something that is of great biological and industrial importance is the fact that combination of CO₂ with water is a relatively slow process. This can be shown by titrating a saturated solution of CO₂ against aqueous NaOH using phenolphthalein as indicator. Neutralization of CO₂ occurs by two routes. For pH<8, the main pathway is by direct hydration which shows pseudo-first order kinetics. At pH>10, the main pathway is by attack of hydroxide ion.

    Until 1993, there was no evidence that free carbonic acid had been isolated, although an unstable ether adduct is formed when dry HCl reacts with NaHCO₃ suspended in Me₂O at 243 K, and there is mass spectrometric evidence for H₂CO₃ being a product of the thermal decomposition of [NH₄][HCO₃]. However, IR spectroscopic data now indicate that H₂CO₃ can be isolated using a cryogenic method in which glassy MeOH solution layers of KHCO₃ (or Cs₂CO₃) and HCl are quenched on top of each other at 78K and the reaction mixture warmed to 300 K. In the absence of water, H₂CO₃ can be sublimed unchanged. It remains a fact that, under ambient conditions, H₂CO₃ is not a readily studied species.

  The carbonate ion is planar and possesses D₃h symmetry with all C^_O bonds of length 129 pm. A delocalized bonding picture involving ( p–p)π-interactions is appropriate, and VB theory describes the ion in terms of three resonance. The C^_O bond distance in [CO₃]^2- is longer than in CO₂ (Table 1.1) and is consistent with a formal bond order of 1.13. Most metal carbonates, other than those of the group 1 metals are sparingly soluble in water. A general method of preparing peroxo salts can be used to convert K₂CO₃ to K₂C₂O₆; the electrolysis of aqueous K₂CO₃ at 253K using a high current density produces a salt believed to contain the peroxocarbonate ion.

 An alternative route involves the reaction of CO₂ with KOH in 86% aqueous H₂O₂ at 263 K. The colour of the product is variable and probably depends upon the presence of impurities such as KO₃. The electrolytic method gives a blue material whereas the product from the second route is orange. Peroxocarbonates are also believed to be intermediates in the reactions of CO₂ with superoxides.

    A third oxide of carbon is the suboxide C₃O₂ which is made by dehydrating malonic acid, CH₂(CO₂H)₂, using P₂O₅ at 430 K. At room temperature, C₃O₂ is a gas (bp 279 K), but it polymerizes above 288K to form a red-brown paramagnetic material. The structure of C₃O₂ is usually described as ‘quasi-linear’ because IR spectroscopic and electron diffraction data for the gaseous molecule show that the energy barrier to bending at the central C atom is only 0.37 kJ mol^_1, i.e. very close to the vibrational ground state. The melting point of C₃O₂ is 160 K. An X-ray diffraction study of crystals grown just below this temperature confirms that the molecules are essentially linear in the solid state. However, the data are best interpreted in terms of disordered  bent molecules with a C^_C^_C bond angle close to 1708, consistent  with a ‘quasi-linear’ description.

The species