5-Bromopentan-1-ol is treated with $\mathrm{NaH}$ in an inert solvent to give a cyclic ether Z, which is then heated with excess conc. $\mathrm{HI}$. The final organic product is

$\mathrm{NaH}$ forms the alkoxide, which displaces bromide intramolecularly (intramolecular Williamson) to give the six-membered cyclic ether oxane (tetrahydropyran). Excess hot $\mathrm{HI}$ cleaves both C-O bonds by SN2, and the intermediate iodo alcohol is converted into the iodide, giving $\mathrm{I}-(\mathrm{CH}{}_2){}_5-\mathrm{I}$.