11.5 : 醚生成卤代烷：酸性裂解

Ethers are generally unreactive towards direct nucleophilic substitutions, as the resulting alkoxide ion is a strong base, and thus, a poor leaving group.

However, when heated with strong acids, such as hydrobromic or hydriodic acids, ethers get converted into alkyl halides.

Acidic cleavage of ethers is a typical nucleophilic substitution reaction. The type of nucleophilic substitution and the conditions required for the reaction depend on the nature of the alkyl groups bonded to oxygen.

Ethers with primary alkyl groups undergo acidic cleavage in the presence of excess hot concentrated acids by an S_N2 mechanism.

The reaction mechanism begins with a proton transfer from the acid catalyst to the ether oxygen, forming an oxonium ion, a better leaving group.

Next, the halide ion acts as a nucleophile and attacks the less substituted carbon of the oxonium ion in an S_N2 reaction, displacing alcohol and forming the first molecule of alkyl halide.

Excess hydriodic acid subjects the alcohol molecule to another round of S_N2 reaction, forming a second molecule of alkyl halide.

Ethers with tertiary, allylic, or benzylic alkyl groups undergo acidic cleavage under mild conditions, such as moderate temperatures and dilute acids, by an S_N1 mechanism.

For example, when ethyl tert-butyl ether reacts with dilute hydriodic acid, protonation of ether forms a stable tertiary carbocation with the loss of primary alcohol.

Finally, the tertiary carbocation reacts with the iodide ion by an S_N1 mechanism, forming a tert-butyl iodide.

The reactivity of the halogen acids towards ether cleavage increases with the nucleophilicity of the halide ions.

Therefore, both hydriodic and hydrobromic acids readily cleave ethers. Hydrochloric acid is less efficient, and hydrofluoric acid does not cleave ethers.