Although ethers are relatively inert toward reaction, they usually show good solvent properties for many nonpolar organic compounds. This strong dissolving power coupled with low reactivity makes ethers good solvents in which to run reactions.
An acid‐catalyzed cleavage that occurs when hydriodic acid (HI) mixes with ethers is the most significant reaction that ethers experience. This reaction proceeds via a nucleophilic substitution mechanism. Primary and secondary alkyl ethers react by an S N2 mechanism, while tertiary, benzylic, and alcylic ethers cleave by an S N1 mechanism. A typical S N2 reaction would be the reaction of ethylisopropyl ether with HI. The mechanism for this reaction is:
Notice that for S N2 substitution, the alkyl halide came from the less sterically hindered group. For S N1 type reactions, the alkyl halide forms from the fragment of the original molecule that forms the more stable cation. Thus, the reaction of t‐butyl ethyl ether with HI gives t‐butyl iodide and ethyl alcohol. The following mechanism occurs:
Notice that if the original ionization of the t‐butyl ethyl ether formed a t‐butoxide ion and an ethyl carbocation, this would be a less stable arrangement. (Remember, the order of stability of carbocations is 3° > 2° > 1°.)