Guidelines for drawing reaction mechanisms

Draw the structure of the reactants and include lone pairs on any heteroatoms. Show bond polarities by 8+ and 8—. 

Identify which reactant is the nucleophile and which is the electrophile, and identify the nucleophilic atom in the nucleophile and the electrophilic atom in the electrophile. 

Draw a double-headed curly arrow from the nucleophilic atom to the electrophilic atom to make a new bond. The arrow can start from a negative charge, a lone pair of electrons or a multiple bond. If a new bond is made to an uncharged H, C, N or O atom in the electrophile then one of the existing bonds should be broken and a second curly arrow should be drawn (pointing in the same direction as the first). 

The overall charge of the reactants should be the same as that of the products. Try to end up with a negative charge on an electronegative atom. 

Take care and check that a carbon atom does not have more than four bonds 

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