CHAPTER IX COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

they all, under one condition or another, yield carbonic anhydride, and can all be formed by means of it from substances destitute of acid properties. The following examples afford the best demonstration of this fact: if acetic acid, C 2 H 4 O 2 , the acid of vinegar, be passed in the form of vapour through a heated tube, it splits up into carbonic anhydride and marsh gas = CO 2 + CH 4 . But conversely it can also be obtained from those components into which it decomposes. If one equivalent of hydrogen in marsh gas be replaced (by indirect means) by sodium, and the compound CH 3 Na is obtained, this directly absorbs carbonic anhydride, forming a salt of acetic acid, CH 3 Na + CO 2 = C 2 H 3 NaO 2 ; from this acetic acid itself may be easily obtained. Thus acetic acid decomposes into marsh gas and carbonic anhydride, and conversely is obtainable from them. The hydrogen of marsh gas does not, like that in acids, show the property of being directly replaced by metals; i.e. CH 4 does not show any acid character whatever, but on combining with the elements of carbonic anhydride it acquires the properties of an acid. The investigation of all other organic acids shows similarly that their acid character depends on their containing the elements of carbonic anhydride. For this reason there is no organic acid containing less oxygen in its molecule than there is in carbonic anhydride; every organic acid contains in its molecule at least two atoms of oxygen. In order to express the relation between carbonic acid, H 2 CO 3 , and organic acids, and in order to understand the reason of the acidity of these latter, it is simplest to turn to that law of substitution which shows (Chapter VI.) the relation between the hydrogen and oxygen compounds of nitrogen, and permits us (Chapter VIII.) to regard all hydrocarbons as derived from methane. If we have a given organic compound, A, which has not the properties of an acid, but contains hydrogen connected to carbon, as in hydrocarbons, then ACO 2 will be a monobasic organic acid, A2CO 2 a bibasic, A3CO 2 a tribasic, and so on—that is, each molecule of CO 2 transforms one atom of hydrogen into that state in which it may be replaced by metals, as in acids. This furnishes a direct proof that in organic acids it is necessary to recognise the group HCO 2 , or carboxyl. If the addition of CO 2 raises the basicity, the removal of CO 2 lowers it. Thus from the bibasic oxalic acid, C 2 H 2 O 4 , or phthalic acid, C 8 H 6 O 4 , by eliminating CO 2 (easily effected experimentally) we obtain the monobasic formic acid, CH 2 O 2 , or benzoic acid, C 7 H 6 O 2 , respectively. The nature of carboxyl is directly explained by the law of substitution. Judging from what has been stated in Chapters VI. and VIII. concerning this law, it is evident that CO 2 is CH 4 with the exchange of H 4 for O 2 , and that the hydrate of carbonic anhydride, H 2 CO 3 , is CO(OH) 2 , that is, methane, in which two parts of hydrogen are replaced by two parts of the water radical (OH, hydroxyl) and the other two by oxygen. Therefore the group CO(OH), or carboxyl, HCO 2 , is a part of carbonic acid, and is equivalent to (OH), and therefore also to H. That is, it is a univalent residue of carbonic acid capable of replacing one atom of hydrogen. Carbonic acid itself is a bibasic acid, both hydrogen atoms in it being replaceable by metals, therefore carboxyl, which contains one of the hydrogen atoms of carbonic acid, represents a group in which the hydrogen is exchangeable for metals. And therefore if 1, 2 ... n atoms of non-metallic hydrogen are exchanged 1, 2 ... n times for carboxyl, we ought to obtain 1, 2 ... n -basic acids. Organic acids are the products of the carboxyl substitution in hydrocarbons. If in the saturated hydrocarbons, C n H 2n + 2 , one part of hydrogen is replaced by carboxyl, the monobasic saturated (or fatty) acids, C n H 2n + 1 (CO 2 H), will be obtained, as, for instance, formic acid, HCO 2 H, acetic acid, CH 2 CO 2 H, ... stearic acid, C 17 H 35 CO 2 H, &c. The double substitution will give bibasic acids, C n H 2n (CO 2 H)(CO 2 H); for instance, oxalic acid n = 0, malonic acid n = 1, succinic acid n = 2, &c. To benzene, C 6 H 6 correspond benzoic acid, C 6 H 5 (CO 2 H), phthalic acid (and its isomerides), C 6 H 4 (CO 2 H) 2 , up to mellitic acid, C 6 (CO 2 H) 6 , in all of which the basicity is equal to the number of carboxyl groups. As many isomerides exist in hydrocarbons, it is readily understood not only that such can exist also in organic acids, but that their number and structure may be foreseen. This complex and most interesting branch of chemistry is treated separately in organic chemistry.