Note 18 bis — Chapter IX · COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

If CO2 is the anhydride of a bibasic acid, and carboxyl corresponds with it, replacing the hydrogen of hydrocarbons, and giving them the character of comparatively feeble acids, then SO3 is the anhydride of an energetic bibasic acid, and sulphoxyl, SO2(OH), corresponds with it, being capable of replacing the hydrogen of hydrocarbons, and forming comparatively energetic sulphur oxyacids (sulphonic acids); for instance, C6H5(COOH), benzoic acid, and C6H5(SO2OH), benzenesulphonic acid, are derived from C6H6. As the exchange of H for methyl, CH3, is equivalent to the addition of CH2, the exchange of carboxyl, COOH, is equivalent to the addition of CO2; so the exchange of H for sulphoxyl is equivalent to the addition of SO3. The latter proceeds directly, for instance: C6H6 + SO3 = C6H5(SO2OH).

As, according to the determinations of Thomsen, the heat of combustion of the vapours of acids RCO₂ is known where R is a hydrocarbon, and the heat of combustion of the hydrocarbons R themselves, it may be seen that the formation of acids, RCO₂, from R + CO₂, is always accompanied by a small absorption or development of heat. We give the heats of combustion in thousands of calories, referred to the molecular weights of the substances:—

R = H₂ CH₄ C₂H₆ C₆H₆

68·4 212 370 777

RCO₂ = 69·4 225 387 766

Thus H₂, corresponds with formic acid, CH₂O₂; benzene, C₆H₆, with benzoic acid, C₇H₆O₂. The data for the latter are taken from Stohmann, and refer to the solid condition. For formic acid Stohmann gives the heat of combustion as 59,000 calories in a liquid state, but in a state of vapour, 64·6 thousand units, which is much less than according to Thomsen.