Q.Ethanol (b.p. $78\,°\text{C}$) and methoxymethane / dimethyl ether (b.p. $-24\,°\text{C}$) are constitutional isomers, both $\text{C}_2\text{H}_6\text{O}$. Explain the large difference in their boiling points.

Ethanol and dimethyl ether are constitutional isomers with the identical molecular formula $\text{C}_2\text{H}_6\text{O}$ and essentially identical molar mass, so any large difference in physical properties between them must come from a difference in intermolecular forces rather than from size or weight. Ethanol's structure, $\text{CH}_3\text{CH}_2\text{--O--H}$, retains a hydrogen directly bonded to oxygen, which lets ethanol molecules hydrogen-bond extensively to one another (each molecule both donating, through its O--H, and accepting, through a lone pair on O). Dimethyl ether's structure, $\text{CH}_3\text{--O--CH}_3$, has both of oxygen's other bonds to carbon, with no O--H hydrogen left at all -- so while its oxygen lone pairs could accept a hydrogen bond from a suitable donor (such as water), dimethyl ether molecules cannot hydrogen-bond with one another, leaving only much weaker dipole-dipole and dispersion forces holding them together. This is why ethanol boil …

Note that the two compounds share identical formula/mass, then locate the structural difference (an O--H hydrogen present in one, absent in the other) th …

Attributing the boiling-point gap to a difference in molecular weight or polarity magnitude rather than to the presence or absence of a hydroge …