Page 14 - A TEXTBOOK OF ORGANIC CHEMISTRY

8 INTRODUCTION
and chloroform, CHCI,, and glycerine, CH,O,, from the triple
hydrochloric acid and water types:
] cy"], )o, (@gr)o,
C 0, ' H' H,
the assumption being made that the radicals (C,H,', (S0,)",
(CH)"", and (CH,)"" could replace a number of hydrogen atoms
corresponding with the number of accents (') marked upon
them, i.e. that they were monatomic, diatomic, &e. 'To the
above three types Kekul~ afterwards added a fourth, of especial
importance as regards the carbon compounds, viz.;
I) C, Manih-gae.

It was then found that many compounds could be referred
equally well to one or another of these types, methylamine,
for instance, either to CH, or to NH,, thus:
7-

The assumption, already mentioned, of the atomic groups
(radicals) which in these types replaced hydrogen, led further
to more exact investigations of the chemical value, i.e. the
replaceable value, of those groups as compared with that of
hydrogen. In this way chemists learnt to distinguish between
uni-, bi-, ter-, &o., valent groups, and, generally speaking, to
pay more attention to equivalent proportions,
As the outcome of his researches upon organo-metallic com-
pounds, Frankland formulated in 1852 (A., 85, 368) the law
that the elements nitrogen, phosphorus, arsenic, and antimony
tend to form compounds which contain three or five equiva-
lents of other elements.
Kekul~ then, in 1857-58 (A., 104, 129; 106, 129), proceeded
to show that a more profound idea (the " Type idea ") lay at
the root of the types themselves, viz. that there are uni-,
bi-, tri-, and quadrivalent, &e., elements, which possess a corre-
ponding replacing or combining value as regards hydrogen;
and that hydrogen is therefore univalent, oxygen bivalent,
nitrogen tervalent, carbon quadrivalent, and so on.
With the introduction of the CH, type by Kekl~, and the