Question 27.3: Recognizing SN1 and SN2 Reactions and Predicting the Product...
Recognizing S_N1 and S_N2 Reactions and Predicting the Products
Consider the following combinations of reactants. In each case, predict whether a substitution reaction will occur. If so, identify the products and suggest the likely mechanism.
(a) CN^- + CH_3CH_2CH_2Cl \longrightarrow
(b) Br^- + CH_3CH_2OH \longrightarrow
(c) CH_3OH + (CH_3)_3CCl \text{ (in methanol)} \longrightarrow
Analyze
To decide whether a reaction of this type will take place, we first identify the electrophile, nucleophile, and leaving group. Keep in mind that the order of reactivity for S_N2 is methyl > primary > secondary > tertiary and that the order is the opposite for S_N1.
(a) The nucleophile is the CN^- ion, the electrophile is the chloropropane, and the leaving group is the Cl^- ion. The CN^- is a much stronger base than the Cl^- ion, and so the equilibrium constant for the reaction should be large. The chloropropane is a primary haloalkane, so the likely mechanism is S_N2 and the product will be 1-cyanopropane, CH_3CH_2CH_2CN.
(b) The nucleophile is the Br^- ion, the electrophile is the ethanol, and the potential leaving group is the OH^- ion. The OH^- is a much stronger base than the Br^- ion, so the equilibrium constant for the reaction is much less than one. No reaction is expected.
(c) The nucleophile is the CH_3OH molecule, the electrophile is the t-butyl chloride, and the leaving group is the Cl^- ion. In this case we have to know the relative basicities of CH_3OH and Cl^- to decide in which direction the reaction will proceed. If we assume that the basicities of methanol and chloride ion are about the same (neither likes to accept a proton), we expect an equilibrium to be established. However, the fact that we are using a large excess of methanol (methanol is the solvent) shifts the equilibrium toward the product (Le Châtelier’s principle). The product is tert-butyl methyl ether, (CH_3)_3COCH_3, and the likely mechanism is S_N1 since the electrophile is a tertiary haloalkane.
Assess
In Section 27-3 we will see that other products, not just substitution products, are possible in some of these reactions.