Question

Arrange the following compounds in increasing order of boiling point.
Propan - 1 - ol, butan - 1 - ol, butan - 2 - ol, pentan - 1 - ol
(A) Propan - 1 - ol, butan - 2 - ol, butan - 1 - ol, pentan - 1 - ol
(B) Propan - 1 - ol, butan - 1 - ol, butan - 2 - ol, pentan - 1 - ol
(C) Pentan - 1 - ol, butan - 2 - ol, butan - 1 - ol, Propan - 1 - ol
(D) Pentan - 1 - ol, butan - 1 - ol, butan - 2 - ol, Propan - 1 - ol

Answer 1

The boiling point of an alkane or its subsequent derivatives like alkanols etc, depend mainly on the structure of the compound. As the number of branches to the main chain increases, the boiling point goes on decreasing. Draw the structures of the given alkanols and then identify the structure having a smaller chain as that will have the lowest boiling point.

Complete Solution :
- Boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure present around the liquid and the liquid thus changes into a vapor.
- It is important to note that as we go on increasing the number of carbon atoms in the organic compound, the boiling point goes on increasing. In short, boiling point is directly proportional to the molecular mass of the compound.
- In case of isomeric carbon atoms, the group attached to the less substituted carbon will have a relatively higher boiling point temperature.
Based on this we can conclude that the increasing order of the above alkanols is:

Propan - 1 - ol < butan - 2 - ol < butan - 1 - ol < pentan - 1 - ol
So, the correct answer is “Option A”.

Note: It is important to know that boiling point decreases on branching of groups. This is because the organic compound changes from straight line to a curve. It is observed that as a compound structure gets closer to a sphere, the boiling point reduces accordingly.