Step-through 3D animations of organic reaction mechanisms: nucleophilic substitution (SN1, SN2), elimination (E1, E2), electrophilic addition, electrophilic aromatic substitution (EAS), free radical reactions, oxidation, and reduction.
Organic reaction mechanisms describe the step-by-step bond-breaking and bond-forming events that transform reactants into products. Understanding mechanisms — not merely memorizing reaction outcomes — is what separates excellent chemistry students from those who struggle under exam pressure. Mechanisms are the core of IChO organic chemistry and USNCO national part II.
The foundational tool is curved arrow notation: a full curved arrow shows the movement of two electrons (a bonding or lone pair); a half-headed arrow (fishhook) shows a single electron moving in radical reactions. Every mechanism step involves either breaking a bond (moving electrons away from it) or forming a bond (moving electrons toward an atom).
SN2 (substitution, nucleophilic, bimolecular) reactions occur in a single concerted step: the nucleophile attacks the electrophilic carbon from the back side of the leaving group, and the leaving group departs simultaneously. This gives inversion of configuration (Walden inversion) at the stereocenter. SN2 is favored for primary substrates with strong nucleophiles in polar aprotic solvents.
SN1 reactions proceed via a carbocation intermediate: the leaving group departs first (rate-determining step), forming a planar carbocation, which is then attacked by the nucleophile from either face — giving a racemic mixture at the former stereocenter. Tertiary and allylic/benzylic substrates stabilize carbocations via hyperconjugation and resonance, making them SN1-prone.
Elimination reactions (E1 and E2) compete with substitution. E2 is concerted: a base removes a proton anti-periplanar to the leaving group, forming the double bond in one step. E1 proceeds via the same carbocation intermediate as SN1, followed by proton removal. Zaitsev's rule predicts the more-substituted alkene as the major product; bulky bases favor the less-substituted Hofmann product.
Electrophilic aromatic substitution (EAS) — halogenation, nitration, Friedel-Crafts — follows a two-step mechanism: electrophile attack forms an arenium ion (Wheland intermediate), then deprotonation restores aromaticity. This 3D mechanism viewer animates every step with curved arrows, helping you internalize the mechanistic logic rather than merely memorize reaction equations.