Transition state unimolecular nucleophilic substitution

Section 4.9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process. The diagram for the reaction of a secondary or tertiary alcohol with a hydrogen halide is characterized by two intermediates and three transition states. The reaction is classified as a unimolecular- nucleophilic substitution, abbreviated as SnI. 

The fact that the rate law depends only on the concentration of tert-butyl chloride means that only tert-butyl chloride is present in the transition state that determines the rate of the reaction. There must be more than one step in the mechanism because the acetate ion must not be involved until after the step with this transition state. Because only one molecule pert-butyl chloride) is present in the step involving the transition state that determines the rate of the reaction, this step is said to be unimolecular. The reaction is therefore described as a unimolecular nucleophilic substitution reaction, or an SN1 reaction. 

Nucleophilic catalysis in the unimolecular mechanism is straightforward, since there is but one reactant that can bring the nucleophile into the transition state. If, however, a bimolecular substitution is found to be subject to nucleophilic catalysis and if it is concluded that, say, one molecule of the nucleophile, B, is involved in the transition state, then two main possibilities exist. The nucleophile may be brought into the transition state either by the organometallic substrate as in process (22) or by the electrophile as in process (23), viz. 

This type of substitution is called an SN1 reaction, for Substitution, Nucleophilic, unimolecular. The term unimolecular means there is only one molecule involved in the transition state of the rate-limiting step. The mechanism of the SnI reaction of tert-butyl bromide with methanol is shown here. Ionization of the alkyl halide (first step) is the rate-limiting step. 

A process following the rate law shown in Eq. (20.65) is said to be an SN1 (substitution, nucleophilic, unimolecular) process. The term unimolecular refers to the fact that a single species is required to form the transition state. Because the rate of such a reaction depends on the rate of dissociation of the M-X bond, the mechanism is also known as a dissociative pathway. In aqueous solutions, the solvent is also a potential nucleophile, and it solvates the transition state. In fact, the activated complex in such cases would be indistinguishable from the aqua complex [ML H20] in which a molecule of H20 actually completes the coordination sphere of the metal ion after X leaves. This situation is represented by the dotted curve in Figure 20.1 where the aqua complex is an intermediate that has lower energy than [ML,]. The species [ML H20] is called an intermediate because it has a lower energy than that of the activated complex, [MLJ. 

The first step, which is rate-determining, is an ionization to a carbocation intermediate that reacts with the nucleophile in the second step. Because the transition state for the rds includes R-X but not Y, the reaction is unimolecular and is labeled S l (substitution nucleophilic unimolecular). First-order kinetics are observed, with the rate being independent of the nucleophilic identity and concentration. 

In a unimolecular substitution reaction, a group departs initially with a pair of electrons leaving an electron deficient carbonium ion intermediate, which is subsequently attacked by an incoming nucleophile. Note that this is a two step reaction, in which the intermediate can, on occasions, be isolated. In contrast, in a bimolecular substitution reaction, the leaving group departs simultaneously as the nucleophile approaches. In this case, the reaction occurs in one step, with only a transition state and no proper intermediate. 

Nucleophilic substitution at silicon, as described in previous sections, usually proceeds through five- or possibly six-coordinated silicon intermediates or transition states. For silicon bearing the common alkyl or aryl ligands, the barrier to formation of such extracoordinated intermediates is quite low. In these examples bimolecular substitution is so facile that unimolecular substitution, even in the most favourable cases, is so slow by comparison as to be unobservable. 

In reactions of tertiary halides, unimolecular processes dominate in protic solvents (especially water and aqueous solvent mixtures) where substitution is usually faster than elimination. Substitution involves nucleophilic attack directly at a cationic center, whereas elimination involves removal of an acidic hydrogen two bonds removed from the cationic atom, explaining the difference. Clearly, a nucleophilic species is strongly attracted to the most positive center and that should lead to the major product. In aprotic solvents, bimolecular substitution is not observed for tertiary halide due to the high energy required to form the pentacoordinate transition state (sec. 2.7.A.i). Under conditions that favor bimolecular reactions and in the presence of a suitable base, elimination is the dominant process. 

Furthermore, these results indicate that the transition state governing the rate of reaction involves only molecules of tert-huty chloride, and not water or hydroxide ions. The reaction is said to be unimolecular (first-order) in the rate-determining step, and we call it an S , reaction (substitution, nucleophilic, unimolecular). In Section 6.15 we shall see that elimination reactions can compete with S l reactions, leading to the formation of alkenes, but in the case of the conditions used above for the experiments with tert-huty chloride (moderate temperature and dilute base), S l is the dominant process. 

The effects of solvents on reaction rates have been studied most extensively on unimolecular solvolysis reactions (S l) and on bimolecular nucleophilic substitution reactions (Sj 2). Absolute rate theory specifies that the activated complex in the transition state is at equilibrium with the reactants and is formed on provision of the activation (Gibbs) energy, AG. In other words, the energy barrier that the reaction must pass to proceed has to be overcome. The specific rate constant is given by ... 

We have seen that the reaction between bromomethane and hydroxide ion is an 8 2 reaction. The reaction between 2-bromo-2-methylpropane and water is an S l reaction, where S stands for substitution, N stands for nucleophilic, and 1 stands for unimolecular. Unimolecular means that only one molecule is involved in the transition state of the rate-determining step. The mechanism for an S l reaction is based on the following experimental evidence ... 

This mechanism is designated S l. Again, the S indicates substitution and the N nucleophilic in this case, the 1 indicates that the rate-determining step is unimolecular. The reaction profile for an S l reaction is shown in Figure 27, which includes three transition states and two intermediates. 

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