Reactivity Trends in Allowed and Forbidden Reactions

Despite the above qualifications, reaction series where G dominates reactivity abound (2,4,5). Thus, in a series of Woodward—Hoffmann forbidden 2 + 2 

Of course, formally allowed and forbidden reactions, in the Woodward—Hoffmann sense, must be considered separately, as distinct reaction families, when correlating barriers to the G parameter. This happens because, as a rule, allowed reactions have comparatively larger resonance energies B relative to forbidden reactions, as will be shown below. 

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