Rules for Writing Reaction Mechanisms

Graphs depicting chemical structures are composed of lines (bonds) and letters (chemical symbols of elements from periodic table). Lone pair electrons are drawn either as two dots or as a bar over an atomic nucleus. Two dots together mean that electrons are in the same molecular orbital separated dots mean electrons occupy different orbitals. 

Electron flow is depicted by an arrow such that the arrowtail corresponds to the electron rich source and the arrowhead corresponds to the electron deficient sink. 

For two-electron movement, the arrowtail is drawn from (a) a lone pair of electrons (belonging to a nucleophilic center) occupying the same molecular orbital, or (b) from two electrons making up a bond. 

Apply grouping technique to locate substrate structures in the target product structure  

Correlate with correct combination of new bonds made and old bonds broken in reaction. 

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