Unit 12 Aldehydes, Ketones And Carboxylic Acids (Intext Questions-3)
Intext Question
12.3 Arrange the following compounds in increasing order of their boiling points.
$\mathrm{CH_3} \mathrm{CHO}, \mathrm{CH_3} \mathrm{CH_2} \mathrm{OH}, \mathrm{CH_3} \mathrm{OCH_3}, \mathrm{CH_3} \mathrm{CH_2} \mathrm{CH_3}$
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Answer
The molecular masses of the given compounds are in the range 44 to $46 . \mathrm{CH_3} \mathrm{CH_2} \mathrm{OH}$ undergoes extensive intermolecular $\mathrm{H}$-bonding, resulting in the association of molecules. Therefore, it has the highest boiling point. $\mathrm{CH_3} \mathrm{CHO}$ is more polar than $\mathrm{CH_3} \mathrm{OCH_3}$ and so $\mathrm{CH_3} \mathrm{CHO}$ has stronger intermolecular dipole - dipole attraction than $\mathrm{CH_3} \mathrm{OCH_3} \tilde{\mathrm{A}} \phi$ " ${ }^{\prime \prime}{ _1} \mathrm{CH_3} \mathrm{CH_2} \mathrm{CH_3}$ has only weak van der Waals force. Thus, the arrangement of the given compounds in the increasing order of their boiling points is given by:
$\mathrm{CH_3} \mathrm{CH_2} \mathrm{CH_3}<\mathrm{CH_3} \mathrm{OCH_3}<\mathrm{CH_3} \mathrm{CHO}<\mathrm{CH_3} \mathrm{CH_2} \mathrm{OH}$
