Question

Arrange the following compounds in the increasing order of their boiling points :
I. $CH_3CH_2CH_2CHO$
II. $CH_3CH_2CH_2CH_2OH$
III. $C_2H_5—O—C_2H_5$
IV. $CH_3CH_2CH_2CH_2CH_3$

• A. I < III < II < IV
• B. III < IV < I < II
• C. III < II < IV < I
• D. IV < III < II < I

Answer 1

Answer: (B)

The molecular masses of these compounds are

$CH_3CH_2CH_2CHO (72), CH_3CH_2CH_2CH_2OH (74)$,

$C_2H_5OC_2H_5 (74), CH_3CH_2CH_2CH_2CH_3 (72)$.

Since, only butan-1-ol undergoes extensive intermolecular $H$-bonding therefore its boiling point is the highest. Butanal is more polar than ethoxy ethane, therefore, dipole-dipole interaction in butanal is stronger than that of ethoxy ethane. The van der Waal's forces of attraction between $C_2H_5OC_2H_5$ are slightly lower than those between molecules of $n$-pentane. Hence, boiling point of $n$-pentane is slightly
higher than that of $C_2H_5OC_2H_5$. Thus, the overall increasing order of boiling point is

$C_2H_5OC_2H_5 < CH_3CH_2CH_2CH_2CH_3$

$< CH_3CH_2CH_2CHO < CH_3CH_2CH_2CH_2OH$