second step is rate-determining for this reaction. However, some other reactions are believed to involve rapid pre-equilibria prior to the rate-determining step, as shown below. Nucleophilic substitution Another example is the unimolecular nucleophilic substitution (SN1) reaction in organic chemistry, where it is the first, rate-determining step that is unimolecular. A specific case is the basic hydrolysis of tert-butyl bromide () by aqueous sodium hydroxide. The mechanism has two steps (where R denotes the tert-butyl radical ): Formation of a carbocation R−Br → + . Nucleophilic attack by hydroxide ion + → ROH. This reaction is found to be first-order with r = k[R−Br], which indicates that the first step is slow and determines the rate. The second step with OH− is much faster, so the overall rate is independent of the concentration of OH−. In contrast, the alkaline hydrolysis of methyl bromide () is a bimolecular nucleophilic substitution (SN2) reaction in a single bimolecular step. Its rate law is second-order: r = k[R−Br][]. Composition of the transition state A useful rule in the determination of mechanism is that the concentration factors in the rate law indicate the composition and charge of the activated complex or transition state. For the –CO reaction above, the rate depends on []2, so that the activated complex has composition , with 2 entering the reaction before the transition state, and CO reacting after the transition state. A multistep example is the reaction between oxalic acid and chlorine in aqueous solution: + → 2 + 2 + 2 . The observed rate law is which implies an activated complex in which the reactants lose 2 + before the rate-determining step. The formula of the activated complex is + − 2 − + , or (an unknown number of water molecules are added because the possible dependence of the reaction rate on was not studied, since the data were obtained in water solvent at a large and essentially unvarying concentration). One possible mechanism in which the preliminary steps are assumed to be rapid pre-equilibria occurring prior to the transition state is + HOCl + + + HOCl + → + + 2 Reaction coordinate diagram In a multistep reaction, the rate-determining step does not necessarily correspond to the highest Gibbs energy on the reaction coordinate diagram. If there is a reaction intermediate whose energy is lower than the initial reactants, then the activation energy needed to pass