Ethers are stable molecules that do not react with nucleophiles: THF and Et₂O are widely used as solvents for this reason. To make them react, we need to make the oxygen positively charged so that it can accept electrons more readily, and we also need to use a very good nucleophile. A good way of doing both is to treat with HBr or HI, which protonate the oxygen. Iodide and bromide are excellent nucleophiles in SN2 reactions (see below), and attack will occur preferentially at the carbon atom more susceptible to SN2 reactions (usually the less hindered one). Aryl alkyl ethers cleave only on the alkyl side—you cannot get attack through the benzene ring.

So far, we have used only protic acids to help oxygen atoms to leave. But Lewis acids—species other than H⁺ that also have an empty orbital capable of accepting a lone pair—work well too, and the cleavage of aryl alkyl ethers with BBr₃ is a good example. Trivalent boron compounds have an empty p orbital so they are very electrophilic and prefer to attack oxygen. The result ing oxonium ion can be attacked by Br− in an SN2 reaction.

مواضيع ذات صلة

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

The nucleophile in SN1 reactions

Epoxides 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

The effect of solvent

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