Predicting Whether a Substitution Reaction Proceeds by an SN1 or SN2 Mechanism Does the substitution reaction below follow an SN1 or an SN2 mechanism? What are the products? Write the steps of the mechanism and use arrows to show the movement of electrons. Analyze The haloalkane is a 2° haloalkane.

Question

Predicting Whether a Substitution Reaction Proceeds by an [latex]S_N1[/latex] or [latex]S_N2[/latex] Mechanism

Does the substitution reaction below follow an [latex]S_N1[/latex] or an [latex]S_N2[/latex] mechanism? What are the products? Write the steps of the mechanism and use arrows to show the movement of electrons.

Analyze
The haloalkane is a 2° haloalkane. Secondary haloalkanes undergo substitution reactions by either the [latex]S_N1[/latex] or [latex]S_N2[/latex] depending on the nucleophile and solvent. (See Table 27.2.)

Accepted Answer

Methanol is the nucleophile and the haloalkane is the electrophile. Because the nucleophile is uncharged, its nucleophilicity is determined primarily by the polarizability of the nucleophilic atom (O). The O atom is relatively small and not very polarizable; thus, [latex]CH_3OH[/latex] is a weak nucleophile. A weak nucleophile disfavors an [latex]S_N2[/latex] reaction. Also, the solvent is polar protic and will help to stabilize a carbocation. With a weak nucleophile and a polar protic solvent, we expect the substitution reaction to occur by an [latex]S_N1[/latex] mechanism. The carbocation that is formed reacts with a solvent molecule [latex]CH_3OH[/latex] (a solvolysis reaction) to form a protonated ether. The final product is an ether, which is obtained when a proton is transferred from the protonated ether to a [latex]CH_3OH[/latex] molecule from the solvent. We will obtain two products, the R and S stereoisomers, because [latex]CH_3OH[/latex] can attack the carbocation from either side. The steps are as follows:

Step 1: Formation of a carbocation

Step 2: Nucleophilic attack by [latex]CH_3OH[/latex]

Step 3: Loss of proton to solvent (ignoring stereochemistry)

Thus, the reaction will produce a racemic mixture consisting of the (R) and (S) stereoisomers of 2-methoxy-4-methylpentane.

Assess
To name the products, you may find it helpful to review the nomenclature rules given in Chapter 26. The reaction considered in this example is also called a solvolysis reaction, because the solvent acts as the nucleophile.

TABLE 27.2 Relative Reactivities of Haloalkanes
	$\begin{matrix} \ \quad \underset{\|}{CH_3} \\ H_3C-C-X \\ \ \quad \overset{\|}{CH_3} \end{matrix}$	$\begin{matrix} \ \quad \underset{\|}{H} \\ H_3C-C-X \\ \ \quad \overset{\|}{CH_3} \end{matrix}$	$\begin{matrix} \ \underset{\|}{H} \\ H-C-X \\ \ \overset{\|}{CH_3} \end{matrix}$	$\begin{matrix} \ \underset{\|}{H} \\ H-C-X \\ \ \overset{\|}{H} \end{matrix}$
Electrophile	3°	2°	1°	Methyl
Stability of Carbocation	Forms a relatively stable carbocation		Form relatively unstable carbocations
$S_N1$ Reactivity	$\xleftarrow{\text{ increasing }S_N1 \text{ reactivity}\text{ }}$		No $S_N1$
$S_N2$ Reactivity	No $S_N2$	$\xrightarrow{\text{ increasing }S_N2 \text{ reactivity}\text{ }}$
$\alpha$ Carbon	Sterically hindered		Not sterically hindered
Solvent	Use a polar protic solvent to promote the $S_N1$ reaction		Use a polar aprotic solvent to promote the $S_N2$ reaction

Question 27.4: Predicting Whether a Substitution Reaction Proceeds by an SN...

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