Question

Which is the correct increasing order of boiling points of the following compounds?
1- Iodobutane, 1- Bromobutane, 1- Chlorobutane, Butane
a.) Butane < 1- Chlorobutane < 1- Bromobutane < 1- Iodobutane
b.) 1- Iodobutane < 1- Bromobutane < 1- Chlorobutane < Butane
c.) Butane < 1- Iodobutane < 1- Bromobutane < 1- Chlorobutane
d.) Butane < 1- Chlorobutane < 1- Iodobutane < 1- Bromobutane

Answer 1

Hint: The boiling point of a molecule depends on the molecular mass. It increases with increase in the molecular mass. Thus, higher is the molecular mass of the molecule, greater will be its boiling point. So, the molecule with lower molecular mass will come first and then the next and so on till the highest one.

Complete answer:
First, let us understand the boiling point and various factors on which it depends.
> The boiling point is defined as the point on the temperature scale at which the vapour pressure of a liquid is equal to the atmospheric pressure surrounding the liquid. The boiling point of a molecule depends upon various factors as-
> The first one is atmospheric pressure. On decrease in atmospheric pressure surrounding the liquid, the boiling point of the liquid also decreases.
> The other one is the forces of attraction between molecules. If the forces of attractions are strong and the molecule is tightly held, then it has high boiling point. On weakening of forces of attraction also, the boiling point decreases.
> These factors work when we have the same molecules. In case with different types of molecule; the boiling point depends upon the molecular mass of molecules. The molecule with higher molecular mass will have greater boiling point.
> So, now utilizing the last factor, we know that the butane is a simple linear chain with 58g as its molecular mass. It is composed of carbon and hydrogen atoms only.
> And if we see the other molecules given then that one hydrogen is replaced by a halogen atom which is heavier than hydrogen. Thus, butane will have the lowest molecular mass.
> And in case of halogens, we know that molecular mass increases when we move from chlorine to bromine and then to iodine.
So, the 1- Iodobutane will have the greatest molecular mass and thus highest boiling point.
Thus, we can write the order of increasing molecular mass as-
Butane < 1- Chlorobutane < 1- Bromobutane < 1- Iodobutane
This will be the same order for increase in boiling points also.
So, the increasing order of boiling points will be as-
Butane < 1- Chlorobutane < 1- Bromobutane < 1- Iodobutane
Thus, option a.) is the correct answer.

Note: It must be noted that the large molecules have more number of electrons and nuclei. So, they create more van der Waals forces of attractions and as a result, they have high boiling points.
One more thing, it is said that on branching of a molecule, the boiling point decreases because the packing is disturbed. So, according to that, butane with a linear chain should have the highest boiling point. The fact is true but is applicable to molecules with almost the same molecular mass. Here, the factor molecular mass dominates the branching. So, butane will have the lowest boiling point.