Recognizing SN1 and SN2 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^- + CH3CH2CH2Cl →

Question

Recognizing [latex]S_N1[/latex] and [latex]S_N2[/latex] 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) [latex]CN^- + CH_3CH_2CH_2Cl \longrightarrow[/latex]

(b) [latex]Br^- + CH_3CH_2OH \longrightarrow[/latex]

(c) [latex]CH_3OH + (CH_3)_3CCl ext{ (in methanol)} \longrightarrow[/latex]

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 [latex]S_N2[/latex] is methyl > primary > secondary > tertiary and that the order is the opposite for [latex]S_N1[/latex].

Accepted Answer

(a) The nucleophile is the [latex]CN^-[/latex] ion, the electrophile is the chloropropane, and the leaving group is the [latex]Cl^-[/latex] ion. The [latex]CN^-[/latex] is a much stronger base than the [latex]Cl^-[/latex] ion, and so the equilibrium constant for the reaction should be large. The chloropropane is a primary haloalkane, so the likely mechanism is [latex]S_N2[/latex] and the product will be 1-cyanopropane, [latex]CH_3CH_2CH_2CN[/latex].

(b) The nucleophile is the [latex]Br^-[/latex] ion, the electrophile is the ethanol, and the potential leaving group is the [latex]OH^-[/latex] ion. The [latex]OH^-[/latex] is a much stronger base than the [latex]Br^-[/latex] ion, so the equilibrium constant for the reaction is much less than one. No reaction is expected.

(c) The nucleophile is the [latex]CH_3OH[/latex] molecule, the electrophile is the t-butyl chloride, and the leaving group is the [latex]Cl^-[/latex] ion. In this case we have to know the relative basicities of [latex]CH_3OH[/latex] and [latex]Cl^-[/latex] 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, [latex](CH_3)_3COCH_3[/latex], and the likely mechanism is [latex]S_N1[/latex] 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.

Question 27.3: Recognizing SN1 and SN2 Reactions and Predicting the Product...

Related Answered Questions

Verified Answer:

Verified Answer:

Predicting the Products of an Aromatic Substitution Reaction Predict the products of the mononitration of Analyze The groups are different, and so the o-, p-directing group (—OH) guides the reaction. ...

Verified Answer:

Predicting the Products of a Reaction Involving an Alcohol Predict the products of the following reactions. If appropriate, suggest a mechanism by which the reaction occurs by using arrows to show the movement of electrons. (a) 1-propanol + sodium hydroxide → (b) (R)-2-bromo-3-methylbutane + ...

Verified Answer:

Verified Answer:

Verified Answer:

Identifying Types of Organic Reactions State whether each of the following reactions is a substitution, an addition, an elimination, or a rearrangement reaction. (In the equations below, H^+ and Pb are catalysts.) ...

Verified Answer: