Question

Which of the following have the highest boiling point?
A. $C{H_3}Cl$
B. $C{H_3}C{H_2}C{H_2}Cl$
C. $C{H_3}C{H_2}C{H_2}Br$
D. $C{H_3}C{H_2}C{H_2}I$

Answer 1

Boiling point of a compound depends upon the type of force of attraction present. Also, for the same type of force of attraction boiling point depends on the molecular weight of the compounds. Now, by using this you can easily find the correct option from the given ones.

Complete step by step answer:
1.) It is known to you that for organic compounds, you should always refer to the force of attraction present.
2.) We can compare the boiling point in organic molecules by considering the effect of different intermolecular forces like hydrogen bonding, dipole-dipole, dipole-induced dipole and London dispersion forces.
3.) The order of strength of different types of bonding that affects boiling points: hydrogen bonding>dipole-dipole>dipole-induced dipole>London dispersion forces.
4.) If the compounds have the same intermolecular forces then their boiling point is compared on the basis of their molecular weight.
5.) Now, coming to the question we can see that the compounds given in the options are polar molecules and they all possess dipole-dipole interaction. So, to compare their boiling point we should compare their molecular weight.
6.) By comparing the molecular weight of the compounds given in options, we can easily say that $C{H_3}C{H_2}C{H_2}I$ has the highest molecular weight among all. So, it has the highest boiling point.

Therefore, from above we can easily conclude that option D is the correct option.

Note: a) It should be remembered that amongst isomeric alkanes, the branched chain isomer has invariably a lower boiling point than the corresponding n-alkane.
b) Also, you should remember that melting points of alkanes also increase with increase in carbon content but the variation is not regular.