Chloromethane, nucleophilic substitution reaction

The reaction of hydroxide ion with chloromethane to yield methanol and chloride ion is an example of a general reaction type called a nucleophilic substitution reaction ... 

Chemical research, starting in the 1890s, has shown that nucleophilic substitution reactions can involve two types of mechanisms. The reaction between chloromethane and the hydroxide ion (equation 27.1) has a rate law that is first order in both the nucleophile and the electrophile. That is,... 

We can use the overall reaction order to distinguish between the two possible mechanisms, A and B. Experimentally, the rate of formation of methanol is found to be proportional to the concentrations both of chloromethane and of hydroxide ion. Therefore the reaction rate is second order overall and is expressed correctly by Equation 8-2. This means that the mechanism of the reaction is the single-step process B. Such reactions generally are classified as bimolecular nucleophilic substitutions, often designated SN2, S for substitution, N for nucleophilic, and 2 for bimolecular, because there are two reactant molecules in the transition state. To summarize For an SN2 reaction,... 

Let us now consider another mechanism for nucleophilic substitution the SnI reaction. When tert-butyl chloride reacts with sodium hydroxide in a mixture of water and acetone, the kinetic results are quite different than for the reaction of chloromethane with hydroxide. The rate of formation of tert-butyl alcohol is dependent on the concentration of tert-butyl chloride, but it is independent of the concentration of hydroxide ion. Doubling the tert-butyl chloride concentration doubles the rate of the substitution reaction, but changing the hydroxide ion concentta-tion (within limits) has no appreciable effect tert-Butyl chloride reacts by substitution at virtually the same rate in pure water (where the hydroxide ion is 10 M) as it does in 0.05M aqueous sodium hydroxide (where the hydroxide ion concentration is 500,000 times larger). (We shall see in Section 6.10 that the important nucleophile in this reaction is a molecule of water.)... 

Hayami, J., Koyanagi, T., Hihara, N., Kaji, A. Substrate-nucleophile association in the Finkelstein reaction system in a dipolar aprotic solvent. Formation of complex between substituted chloromethanes and halide ion in acetonitrile. Bull. Chem. Soc. Jpn. 1978, 51, 891-896. 

This reaction is said to be second older overall. It is reasonable to conclude, therefore, that r the reaction to take place a hydroxide ion and a chloromethane molecule must collide. We also say that the reaction is bimolecular. (By bimolecular we mean that two species are involved in the step whose rate is being measured. In general the number of species involved in a reaction step is called the molecularity of the reaction.) We call this kind of reaction an Sjv2 teaction, meaning substitution, nucleophilic, bimolecular. 

The transition state is a fleeting arrangement of the atoms in which the nucleophile and the leaving group are both partially bonded to the carbon atom undergoing substitution. Because the transition state involves both the nucleophile (e.g., a hydroxide ion) and the substrate (e.g., a molecule of chloromethane), this mechanism accounts for the second-order reaction kinetics that we observe. 

This mechanism is designated Sn2, with the S indicating substitution, the N indicating nucleophilic, and the 2 indicating that the rate-determining step is bimolecular. The reaction profile for an Sn2 reaction is shown in Figure 27-1, in which the transition state formed by the HO ion and the chloromethane is shown. The hydroxide attacks on the side opposite the Cl atom, and the C—Cl bond starts to break as the C—O bond simiiltaneously starts to form, producing the transition state shown. In the Sn2 mechanism, the nucleophile donates a pair of electrons to the electrophile to form a covalent bond. 

---