Why was the SN2 reaction we have just shown you carried out in DMF? You will generally find SN2 reactions are carried out in aprotic, and often less polar, solvents. SN1 reactions are typically carried out in polar, protic solvents. A common solvent for an SN2 reaction is acetone—just polar enough to dissolve the ionic reagents, but not as polar as, say, acetic acid, a common solvent for the SN1 reaction. It is fairly obvious why the SN1 reaction needs a polar solvent: the rate-determining step involves the formation of ions (usually a negatively charged leaving group and a positively charged carbocation) and the rate of this process will be increased by a polar solvent that can solvate these ions. More precisely, the transition state is more polar than the starting materials (note the charges in brackets in the scheme above) and so is stabilized by the polar solvent. Hence solvents like water or carboxylic acids (RCO₂H) are ideal. It is less obvious why a less polar solvent is better for the SN2 reaction. The most common SN2 reactions use an anion as the nucleophile. The transition state is then less polar than the localized anion as the charge is spread between two atoms. Here’s an example: the formation of an alkyl iodide from an alkyl bromide. Acetone fails to solvate the iodide well, making it more reactive; the transition state is less in need of solvation, so overall the reaction is faster.

DMF and DMSO, the polar aprotic solvents we discussed in Chapter 12 (p. 255) are also good solvents for SN2 reactions because they dissolve ionic compounds well but fail to solvate anions well, making them more reactive. The choice of Bu4N⁺—a large, non-coordinating cat ion—as the counterion for the reaction on p. 344 was also made with this in mind.

مواضيع ذات صلة

A very bad nucleophile in a good SN1 reaction: the Ritter reaction

The nucleophile in SN1 reactions

Epoxides as electrophiles

Ethers as electrophiles

Substituting alcohols with the Mitsunobu reaction

Nucleophilic substitutions on alcohols: how to get an OH group to leave

A closer look at electronic effects

A closer look at steric effects

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

Shape and stability of carbocations