Into two groups: (r) those exhibiting properties closely analogous to the aliphatic series - the polymethylenes, and (2) a series exhibiting properties differing in many respects from the aliphatic and polymethylene compounds, and characterized by a peculiar stability which is to be associated with the disposition of certain carbon valencies not saturated by hydrogen - the " aromatic series."
And >- CH, the first characterizing the polymethylene and reduced heterocyclic compounds, the second true aromatic compounds.
A great impetus was given to the study of polymethylene derivatives by the important and unexpected observation made by W.
Von Baeyer has sought to explain the variations in stability manifest in the various polymethylene rings by a purely mechanical hypothesis, the " strain " or Spannungs theory (Ber., 1885, p. 2277).
Other hydrocarbon nuclei generally classed as aromatic in character result from the union of two or more benzene nuclei joined by one or two valencies with polymethylene or oxidized polymethylene rings; instances of such nuclei are indene, hydrindene, fluorene, and fluoranthene.
His chief researches deal with the polymethylene compounds, the alkaloids, in particular hydrastine and berberine, and the camphors and terpenes.