A specific oxidation method for estimating the valence states of carbon in coal is described. The method, which is also an important tool for more generally exploring the structure of coal, is based on the fact that sodium hypochlorite is a very selective oxidant capable of differentiating between Sp3-carbon and Spz-eromatic carbon. Proceeding from this recognition it is estimated that between 55 and 60% of the total carbon in coal exists in the Spbvalence state. This estimate is much higher than the accepted 'hydroaromaticity' of coals.
Attempts have been made to correlate the properties of coal with its structural features, and broad terminology (such as aromaticity, hydroaromaticity, etc.) is commonly used to define the basic structural parameters.
However, this terminology presupposes a structural model which is usually taken as a partly saturated polycondensed aromatic system containing alkyl, aryl and heteroaryl substituents, and may itself be misleading.
For example, if coal is in fact predominantly aromatic in character, then, since aromatic compounds can be regarded as Lewis bases and are soluble in acidic solvents, its solubility, instead of tending to increase with the basicity of the solvent, should tend to decrease.
Moreover, while aromatic and some mixed systems are generally very stable towards oxidation by air or oxygen, coal is easily oxidized by air. Accordingly, in order to avoid confusion, it is helpful to classify forms of carbon in coal by using a quantum-mechanical notation, especially when the chemical behaviour of a carbon atom is determined by its electron distributions or the states of hybridization in the molecule which render distinctive characters to C-C, C-H and C-O bonds.
This type of notation not only describes the valence state of carbon atoms, but also defies the molecular geometry.
(The abundance of Sp3-carbon in coal would, if the carbon is quaternary, indicate how far the molecular structure extends in 3-dimensional space.) The chemical reactivities towards different reagents can easily be described on the basis of this notation system. But this approach can only be followed if analytical tools for determining the valence state of carbon in coal are available.
One such tool is afforded by proton and carbon-13 n.m.r. spectroscopy and some data on coal and coal products have been reportedr*2.
Opinions about the reliability of quantitative data do, however, differ; the lack of solubility of coal and poor resolution of signals due to high noise level in carbon-13 n.m.r. spectra pose obvious problems.
Coal dehydrogenation studies3Y4 have also been utilized to estimate labile hydrogen in coal, but allow little * Contribution No.653 from the Research Council of Alberta,