One family of ethers reacts in nucleophilic substitution even without protic or Lewis acids. They are the three-membered cyclic ethers called epoxides (or oxiranes). The leaving group is genuinely an alkoxide anion RO^₋, so obviously some special feature must be present in these ethers making them unstable. This feature is ring strain, which comes from the angle between the bonds in the three-membered ring that has to be 60° instead of the ideal tetrahedral angle of 109°. You could subtract these numbers and say that there is ‘49° of strain’ at each carbon atom, making about 150° of strain in the molecule. This is a lot: the molecule would be much more stable if the strain were released by opening up to restore the ideal tetrahedral angle at all atoms. This can be done by one nucleophilic attack.

Epoxides react cleanly with amines to give amino alcohols. We have not so far featured amines as nucleophiles because their reactions with alkyl halides are often bedevilled by overreaction (see the next section), but with epoxides they give good results.

It is easy to see that inversion occurs in these SN2 reactions when the epoxide is attached to (or ‘fused with’) another ring. With this five-membered ring nucleophilic attack with inversion gives the trans product. As the epoxide in the starting material is up, attack has to come from underneath. The new C–N bond is down and inversion has occurred.

مواضيع ذات صلة

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

The nucleophile in SN1 reactions

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

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