An ethylenic or double carbon union in the aliphatic hydrocarbons has, apparently, the same effect on the boiling-point as two hydrogen atoms, since the compounds C 0 H 2 „ +2 and CoH2n boil at about the same temperature.
It also acts as a chromogenic centre when double bonds or ethylenic linkages are present, as in fluorene ketone or fluorenone.
A more complex chromophoric group is the triple ethylenic grouping: C > C =, the introduction of which was rendered necessary by the discovery of certain coloured hydrocarbons.
From a study of the reduction of compounds containing two ethylenic bonds united by a single bond, termed a "conjugated system," E.
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