Question

The boiling points of methanol, water and dimethyl ether are 338 K, 373 K and 307.5 K respectively.Which one of the following statements gives the best explanation for these wide variations in b.p.?

• A. The molecular mass increases from water $\left(18\right)$ to methanol $\left(32\right)$ to diethyl ether $\left(74\right)$.
• B. The extent of intermolecular H-bonding decreases from water to methanol and it is absent in ether.
• C. The extent of intramolecular H-bonding decreases from ether to methanol to water.
• D. The density of water is greater than that of methanol which is in turn less than dimethyl ether.

Answer 1

Answer: (B)

The extent of intermolecular H-bonding decreases from water to methanol and it is absent in ether.

The presence of $OH$ groups leads to H-bonding in methanol and water whereas no $H$ bonding takes place in dimethyl. Water has more H-bonding than methanol. In methanol, the $+I$ effect of methyl group increases the negative charge intensity on $O$ and thus, reduces the extent of Hydrogen bonding.