Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. Molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass. The attractions get stronger as the molecules get bigger in size and have more electrons.
Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms.
For a particular alkyl group R, what is the correct order of the boiling points of the following compounds?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → RBr > RCl > RF
The passage states that the intermolecular attractions in haloalkanes get stronger as the molecules get bigger in size and have more electrons. With the same alkyl group R, only the halogen changes, and size, mass and number of electrons rise in the order F < Cl < Br.
RBr — highest boiling point: bromine is the largest and heaviest of the three, so the van der Waals forces are strongest.
RCl — middle: chlorine is smaller and lighter than bromine but larger and heavier than fluorine.
RF — lowest boiling point: fluorine is the smallest and lightest, so the attractions between the molecules are weakest.
NCERT (2026-27 edition):
6.6 Physical Properties – “… For the same alkyl group, the boiling points of alkyl halides decrease in the order: RI> RBr> RCl> RF. This is because with the increase in size and mass of halogen atom, the magnitude of van der Waal forces increases.”