Spotting a nucleophile
Look for: lone pairs (Cl⁻, OH⁻, NH₃, H₂O), π bonds (alkenes, aromatic rings in EAS), or negative charges in general. Strong nucleophiles are charged and have a lone pair (CN⁻, OH⁻, RO⁻); medium ones are neutral with a lone pair (H₂O, NH₃, ROH); weak ones are π systems alone.
Spotting an electrophile
Look for: positive charges (carbocations, H⁺, metal cations), atoms with empty orbitals (BF₃, AlCl₃), or polar bonds putting a partial positive on carbon (the C of C=O, the C of C–X). The δ+ carbon of a carbonyl is the most common electrophile in organic mechanisms.
Strength trends
Down a column of the periodic table, nucleophilicity goes UP (the atom is more polarizable; the electrons are farther out). Across a row, nucleophilicity tracks basicity. In polar protic solvents, the trend reverses — small charged species get heavily solvated and slowed down, so I⁻ outpaces F⁻ as a nucleophile in water even though F⁻ is more basic.
Nucleophilicity vs basicity
They're related but not the same. Basicity measures bonding to H⁺ (a small, naked target). Nucleophilicity measures bonding to a carbon electrophile (bigger, hindered). Hindered bases like tert-butoxide are strong bases but poor nucleophiles — they're fine snatching a small H but too big to attack a substituted carbon.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.