A Quantitative Scale for the Extent of Conjugation of Substituted Olefines

The olefinic functional group is among the most important chemical building blocks, also playing an important role in organic and bioorganic reactions. The exact description and precise quantification of the degree of the olefinic conjugation in substituted alkenes is not trivial. The present work suggests a novel, yet simple, method toward quantifying the conjugation in a general olefin group (e.g., alkenes) on a linear scale, defined as the "olefinicity scale", achieved using the computed enthalpy of hydrogenation (DeltaH(H2)) of the compound examined. In the present conceptual work, the DeltaH(H2) value for allyl anion is used to define perfect conjugated character (olefinicity = +100%), while ethene represents complete absence of conjugation (olefinicity = 0%). The component DeltaH(H2) values were computed at different levels of theory, providing a near-"method-independent" measure of olefinicity. A total of 67 well-known olefinic compounds were examined to demonstrate the practicality of this protocol. For the compounds examined, a correlation has been made between the computed olefinicity percentage values and their associated proton affinities, as well as their reactivity values in a nucleophilic addition reaction; selected chemical reactions were also studied.