Arrows, curly radical reaction mechanisms

By the end of the 1950s the main features of ionic and radical reactions were reasonably well understood, but pericyclic reactions were not even recognized as a separate class. Diels-Alder reactions, and a good many others, were known individually. Curly arrows were used to show where the bonds went to in these reactions, but the absence of a sense of direction to the arrows was unsettling. Doering provocatively called them no-mechanism reactions in the early 1960s. 

Robinson s curly arrow is used to show the movements of pairs of electrons in a reaction mechanism. The tail of a curly arrow starts at a mobile electron pair and its head points to the destination of the electron pair. Fishhook arrows indicate cleavage or movement of a single electron shown as a single-headed curved arrow. They are widely used in radical chemistry to represent the homolytic cleavage and reactions of radicals. ... 

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