Question

Arrange according to boiling point: $C{H_3}C{H_2}C{H_3}$, $C{H_3}CHO$, $C{H_3}C{H_2}OH$, $C{H_3}OC{H_3}$.

Answer 1

Hint: First identify the functional group associated with each compound and then check what are the intermolecular forces associated with each of them because boiling point increases with increasing strength of these intermolecular forces.

Complete step by step answer:
-First of all let us talk about boiling point.
The temperature at which the vapour pressure of a liquid becomes equal to the pressure surrounding the liquid is known as the boiling point of the liquid. At this temperature the liquid changes to vapour state.

-The boiling point of various compounds depends on 3 major factors:
 (1) Branching: Increase in branching decreases boiling point.
 (2) Boiling point increases with increase in the number of carbon atoms.
 (3) Intermolecular forces: The relative strength of intermolecular forces like ionic bond, hydrogen bond, dipole-dipole attraction and Van der Waals dispersion forces also affects the boiling point of compounds. The strength order of these bonds is as follows:
 Ionic bond > Hydrogen bonding > dipole-dipole attraction > Van der Waals dispersion forces
These attractive forces and their influence varies in compounds according to the functional groups present in them.

-Now let us look at the compounds given:
$C{H_3}C{H_2}OH$: This is ethyl alcohol and as the name says it has an alcohol functional group. This functional group has a very high ability to form hydrogen bonds since more than 1 hydroxyl group is present in it. These hydrogen bonds are responsible for the very high boiling point of alcohols. Ethyl alcohol undergoes extensive hydrogen bonding and thus has the highest boiling point.
$C{H_3}OC{H_3}$ is an ether named Dimethyl ether and $C{H_3}CHO$ is an aldehyde named ethanal. Both of them are polar but $C{H_3}CHO$ is more polar than $C{H_3}OC{H_3}$ and so has stronger intermolecular dipole-dipole interactions as compared to $C{H_3}OC{H_3}$. So, the boiling point of $C{H_3}CHO$ is more than $C{H_3}OC{H_3}$.
While $C{H_3}C{H_2}C{H_3}$ is not at all polar and has only weak Van der Waal forces of attraction, which makes its boiling point minimum.

-So, the sequence of boiling point will be as follows:
  $C{H_3}C{H_2}OH$ > $C{H_3}CHO$ > $C{H_3}OC{H_3}$ > $C{H_3}C{H_2}C{H_3}$

Note: Hydrogen bonding is a special form of dipole-dipole attraction. It is formed only when a hydrogen atom which is bonded to an electronegative atom (O, F, N) is present in the vicinity of some other electronegative atom which has a lone pair of electrons. Alcohols show H-bonding due to presence of H atom attached to electronegative O atom and it does H bonding with O atom of other alcohol molecules.