Abstract
AbstractWhen R2 ≠ R3, cis/trans isomerism exists in trisubstituted ethylenes R1HC = CR2R3. A previously1 described and tested concept of rehybridization makes possible the prediction of certain infrared frequency shifts as a function of sterical hindrance. In this paper it is shown that rehybridization accounts also reasonably well for certain frequency shifts of those tri‐alkylated ethylenes where cis/trans isomerism is absent. A number of rules are then developed which make possible the assignment of the cis or trans configuration when such isomerism is present.It is proposed that the cis/trans labelling of the National Bureau of Standards' standard samples of the 3‐methyl‐2‐pentenes and 3,4‐dimethyl‐2‐pentenes be reversed. The geranyl group is given the cis configuration and neryl the corresponding trans configuration, contrary to the hitherto accepted structures. The Δ4 (trans, Δ6 (7) (cis) structure has been assigned to the high‐boiling alloocimene and the all‐trans configuration to the corresponding low‐boiling isomer.
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