Deep-Dive Guides · Reaction Mechanisms

Reaction Mechanisms — the complete deep dive

A mechanism is a bookkeeping of bonds and electrons through the reaction coordinate: which bonds break, which form, in what order, through which intermediates and transition states.

Free extract — the opening section of the guide, in full.

0. How chemists think about mechanism

A proposed mechanism is a hypothesis that must survive experimental tests. Two structures dominate the picture: the intermediate (a real species sitting in an energy well, however shallow — it can in principle be detected or trapped) and the transition state (a maximum on the reaction coordinate, not isolable, existing for one vibration). The rate-determining step (RDS) is the step with the highest transition state; only structural changes at or before the RDS affect the rate.

Fig 1. A two-step energy profile. The intermediate sits in a valley (detectable/trappable); the two transition states are maxima. Here TS1 is higher, so step 1 is rate-determining — only things affecting the reactant→TS1 stretch change the rate. (Hammond postulate: an early TS resembles reactants; a late TS resembles the intermediate/product.)

The three questions to ask of any mechanism claim: (1) What does the rate law say about the molecularity of the RDS (what is present in the TS)? (2) What is the stereochemical outcome (inversion, retention, racemisation, syn/anti)? (3) Do isotope, substituent (Hammett), and trapping experiments agree with the proposed intermediate? A mechanism is only "known" when these lines converge.

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The remaining 10 sections — Reactive intermediates, Substitution mechanisms, Elimination mechanisms, Substitution vs elimination — deciding and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.

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