Question 27.4: Predicting Whether a Substitution Reaction Proceeds by an SN...
Predicting Whether a Substitution Reaction Proceeds by an S_N1 or S_N2 Mechanism
Does the substitution reaction below follow an S_N1 or an S_N2 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 S_N1 or S_N2 depending on the nucleophile and solvent. (See Table 27.2.)
| 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 | ||
The symbols 1°, 2°, and 3° stand for primary, secondary, and tertiary, respectively.
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, CH_3OH is a weak nucleophile. A weak nucleophile disfavors an S_N2 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 S_N1 mechanism. The carbocation that is formed reacts with a solvent molecule CH_3OH (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 CH_3OH molecule from the solvent. We will obtain two products, the R and S stereoisomers, because CH_3OH can attack the carbocation from either side. The steps are as follows:
Step 1: Formation of a carbocation
Step 2: Nucleophilic attack by CH_3OH
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.