6.7:
SN2 Reaction: Kinetics
A nucleophilic substitution reaction of an alkyl halide can proceed in two possible mechanisms: either in a single step where bond breakage and formation occur simultaneously or in a stepwise process, including the formation of a carbocation intermediate. But how is this determined?
In the late 1930s, Sir Christopher Ingold and Edward D. Hughes studied the kinetics of various nucleophilic substitution reactions to elucidate which mechanism is favored.
The reaction rate was experimentally determined by measuring the initial rate of reactant’s disappearance or the product’s appearance. Further, the effect of reactant concentration on the rate equation was studied by varying the concentration of one reactant while keeping the other constant.
Ingold and Hughes observed that the reaction rate for primary alkyl halides, like chloromethane with hydroxide ions, is linearly dependent on the concentration of both reactants for a given solvent and temperature condition. Doubling the concentration of either reactant at a time doubled the rate. When the concentration of both reactants was doubled, the rate quadrupled, indicating that the reaction between the primary alkyl halide and the nucleophile follows second-order kinetics.
This suggests that both the nucleophile and the alkyl halide collide in a single step to form the product during this substitution reaction, meaning the molecularity of this reaction is bimolecular, as two species participate in the rate-limiting step.
Thus, reactions following this mechanism are classified as Substitution, Nucleophilic, 2nd order, or, in short, as SN2 reactions.
Since both reactants influence the reaction rate in SN2 reactions, the reaction speed and yield can be improved by choosing stronger nucleophiles and carbon electrophiles containing better leaving groups.
In addition, the rate of an SN2 reaction is influenced by the reactants’ concentrations, temperature, and solvent.
6.7:
SN2 Reaction: Kinetics
Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a reaction.
Kinetic Studies: How to Measure the Rate?
A chemical reaction proceeds at a certain rate at a given temperature, pressure, and solvent. The rate of a reaction can only be identified experimentally. To do so, the rate at which the reactants disappear or the rate at which the products appear in the reaction mixture is measured. In specific time intervals, aliquots are withdrawn from the reaction mixture and analyzed for the concentration of the reactants or the products formed. Further, while keeping the temperature and solvent conditions the same, the reaction's initial concentration is varied to observe the effect on the rate of the reaction.
Nucleophilic Substitution Reactions and Kinetics
In the 1930s, British chemists Sir Christopher Ingold and Edward D. Hughes studied rate kinetics of various substitution reactions to understand the probable mechanism of nucleophilic substitution reactions. They observed that a nucleophilic substitution reaction of an alkyl halide proceeded by two possible mechanisms – either in single-step or via two steps.
For specific reactions, such as chloromethane and sodium hydroxide, the reaction rate depended on the concentration of both the nucleophile and the alkyl halide. Specifically, they observed that when the concentration of either of the reactants was doubled, the reaction rate doubled. Furthermore, when the concentration of both reactants was doubled, the rate increased four times, as shown in Table 1.
Table 1. Rate study of the reaction between chloromethane and hydroxide ions at 60 °C
| Experiment Number | Initial [CH3Cl] |
Initial [HO−] |
Initial Rate (mol L−1s−1) |
| 1 | 0.0010 | 1.0 | 4.9 × 10⁻7 |
| 2 | 0.0020 | 1.0 | 9.8 × 10⁻7 |
| 3 | 0.0010 | 2.0 | 9.8 × 10⁻7 |
| 4 | 0.0020 | 2.0 | 19.6 × 10⁻7 |
Thus, the rate equation was found to be:
Rate ∝ [CH3Cl][HO−]
This shows that the rate is first order with respect to each reactants' concentration and second-order overall.
SN2 Reactions and Kinetics
- • The kinetics of the above reaction suggest that for the substitution to occur, the nucleophile and the alkyl halide collide in a single step to form the product.
- • The molecularity of such reactions is said to be bimolecular as two species are involved in the rate-limiting step.
- • Thus, the reaction between chloromethane and sodium hydroxide is an SN2 reaction, where S stands for Substitution, N for Nucleophilic, and 2 for a bimolecular reaction.
Suggested Reading
- Brown, W.H., & Iverson, B.L., & Anslyn, V.E., & Foote S.C. (2014). Organic Chemistry. Mason, Ohio: Cengage Learning, 349.
- Solomons, G., & Fryhle, C. & Snyder, S. (2015). Organic Chemistry. New Jersey, NJ: Wiley, 245-246.
- Loudon, M., & Parise, J. (2016). Organic Chemistry. New York, NY: Macmillan Publishers,391-392.
- Klein, D. (2017). Organic Chemistry. New Jersey, NJ: Wiley, 277.
- Clayden, J., & Greeves, N., & Warren, S. (2012). Organic Chemistry. Oxford: Oxford University Press, 330.