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Acidic ether cleavages are typical nucleophilic substitution reactions and take place by either ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$or${\mathbf{S}}_{\mathbf{N}}\mathbf{2}$mechanisms, depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an ${\text{S}}_{N}2$mechanism, in which ${\text{I}}_2$or${\text{Br}}_2$ attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether exclusively yields isopropyl alcohol and iodoethane on cleavage by HI.

Tetrahydrofuran reacts with hydrogen iodide HI, in the aqueous medium to form 4-iodobutan-1-ol and ethyliodide. Tetrahydrofuran is cyclic ether which is sensitive towards strong acid like hydrogen iodide (HI). In the presence of aqueous solution we get the desired product as shown below:

Tetrahydrofuran4-iodobutan-1-ol