24 OUTLINES OF PHYSICAL CHEMISTRY
We therefore attribute the formula 2 H 3 O.OH to acetic acid.
But now the question arises, what is the constitution of the radical C 2 H 3 ?
The answer to this was given by KekuU, who observed that the dry distillation of a mixture of potassium acetate and caustic potash yields potassium carbonate and methane. Applying the above principle, we see that in acetic acid the three non-typical hydrogen atoms must be united to one and the same carbon atom :
CH3.COOK +KOH = K 2 C0 3 + CH 4 .
So that the constitution of the C 2 H 3 radical is CH 3 — CO— , and of acetic acid CH 3 — COOH.
Many reactions confirm this formula, notably the decom- position of trichloracetic acid with water :
CCl 3 COOH + HOH = CC1 3 H + C0 2 + H 2 0.
2. Another rule, which may become the principal one in cases where the application of the first is too difficult, is based on the following proposition: The chemical and physical properties of a substance depend on its molecular structu/re ; identical or analogous properties proceed from an identical or analogous structure of certain parts of the molecule.
Examples :
(a) The Alcohols. — The characteristic properties of alcohols may be fairly represented by those of methyl alcohol (only one formula possible), or by those of ethyl alcohol (formula known). The alcohols are characterised by the formation of alcoholates, simple ethers, saponifiable esters; by certain oxidation products (aldehydes and acids) ; by the action of phosphorus pentachloride, &c.
Whenever we have to do with an organic substance the empirical formula of which is compatible with the presence of the group — CH 2 OH, we consider it as a
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