Radicals electrophilic versus nucleophilic character

In summary, then, the diradical versus zwitterionic issue seems to have shifted in recent years, away from an either/or dichotomy and toward a more integrated view, one seeing tetramethylene intermediates as more or less dipolar diradicals. A small amount of zwitterionic character does not obviate the essentially diradical nature of these intermediates, any more than monoradicals forego radical character if they are more or less electrophilic or nucleophilic. 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns shown in Table 10.1. The relative reactivity of singlet dibromocarbene toward alkenes is more similar to that of electrophiles (bromination, epoxidation) than to that of radicals ( CC13). Carbene reactivity is strongly affected by substituents.61 Various singlet carbenes have been characterized as nucleophilic, ambi-philic, and electrophilic as shown in Table 10.2. This classification is based on relative reactivity toward a series of different alkenes containing both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituents to act as electron donors. For example, dimethoxycarbene is devoid of electrophilicity toward... 

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