Each step should involve no more than two molecules, ions, or radicals. Arrows flow one way and follow each other consecutively. Each species is a reactant or a product from an earlier step. Each step must be balanced in terms of atoms, electrons, and charges. A mechanism is a description of starting materials to products.
Arrow Drawing Skills
A double headed arrow shows movement of a pair of electrons. Arrows are drawn from an unshared or shared electron pair (tail of arrow) to new location of electron pair (head of arrow). Draw arrow from electron-rich species (nucleophile) to electron-poor species (electrophile). Nucleophiles have negative charges or have an unshared electron pair.
A single headed arrow shows movement of a single electron and is used for radicals. The arrow is drawn from a single radical to form a new bond.
Movement of Arrows with Electrons
An unshared electron pair becomes a shared electron pair. A shared electron pair becomes an unshared electron pair. A shared electron pair becomes another shared electron pair.
Intermediates can only come from starting reagents. Do not generate a species because you think it belongs there! Good intermediates include stable intermediates, conjugate acids or bases of reactants or products, neutral intermediate compounds, neutral organic molecules, carbocations, carbanions, free radicals, or carbenes. Characteristics of unlikely intermediates include two negative charges on the same molecule; two positive charges on the same molecule; opposite charges on the same molecule, except for recognized species; more than eight electrons on a first row element; or first row element with less than six electrons.