History of Chemistry, Volume 2 (of 2) / From 1850 to 1910

by T. E. Thorpe

Sulphuryl
nitryl chloride.

Sulphuric acid.

These formulæ serve to show how the several substances are mutually related, and that they may be derived from one another by the substitution of atoms of chlorine for hydroxyl (OH), or nitryl (NO₂), or vice versa.

It was pointed out in 1851 by Williamson, and subsequently by Gerhardt, that these groups are characterised by differences in their power of combining with or replacing atoms of hydrogen, or of groups or elements which, like chlorine, are chemically equivalent to hydrogen. Such a radical or residue as ethyl (C₂H₅) is chemically equivalent to one atom of hydrogen, as is shown when we compare the formula for ether, as established by Williamson, with that of ordinary alcohol:

Alcohol.

Ether.

Sulphuryl, SO₂, is chemically equivalento two atoms of hydrogen; phosphoryl, PO, as suggested by Odling, to three atoms of hydrogen. Gerhardt therefore proposed to designate these and similar groups as monatomic, diatomic, triatomic, according to their respective hydrogen-replacing power.

This conception of the definite atom-fixing or replacing power of groups or compound radicals was extended by Frankland, in 1852, so as to include the simple radical—that is, the elements. In the memoir in which he announced the existence of the organometallic compounds he pointed out that the elements may be classified according to their combining power, or, as he expressed it, according as “their affinities are best satisfied.” This idea was independently developed by Couper and Kekulé in 1858; it is from that period that the definite introduction of the conception of atomicity, atomic-value, or valency, into chemical doctrine may be said to date.

The memoir in Liebig’s Annalen der Chemie und Pharmacie, in which Kekulé announced his views, deals particularly with the tetravalency of carbon and the doctrine of linking of atoms in terms of their valency. As formulated by Kekulé and as subsequently developed in his famous text-book, this doctrine exercised an immediate effect on the progress of the chemistry of carbon compounds. Like every fruitful hypothesis, it stimulated inquiry, and brought out analogies; and the more it was applied the more apparent became its suggestiveness and utility. The scope of chemical formulæ was greatly extended. Rational formulæ grew into dissected or constitutional formulæ; and on the system of constitutional formulæ have been grafted successive attempts to elucidate the manner in which the constituents of a molecule are grouped and held together. It is interesting to note that the proximate effect of the theory of chemical structure which grew out of Kekulé’s doctrine was to assimilate what was sound in the seemingly antagonistic theories of types and radicals. As a mode of exposition, Kekulé used models to illustrate the manner in which the affinity-values of compounds are satisfied; these were not intended to represent the actual spatial distribution of the atoms in a molecule, but they nevertheless familiarised the mind with the idea first clearly recognised by Wollaston and Berzelius that this is the ultimate aim of chemistry. It was probably their use, either actually or by visualisation, that led Kekulé in 1865 to his theory of the constitution of benzene, as developed in his paper on the constitution of the aromatic compounds—a theory no less fruitful in its consequences than that of the tetravalency of carbon and of the linkage of atoms. Such models, too, in the hands of Van ’t Hoff, subsequently served to elucidate the connection between optical characters and crystalline form, and to explain the isomerism of certain organic substances.