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.
Consider the following statements about the physical properties of haloalkanes and haloarenes.
(A) The boiling points of isomeric haloalkanes increase with increase in branching.
(B) The boiling points of isomeric dihalobenzenes are very nearly the same.
(C) Alkyl bromides and iodides are colourless when pure, but they develop colour when exposed to light.
(D) Haloalkanes dissolve in organic solvents because the new attractions between haloalkane and solvent molecules have much the same strength as the attractions that are broken.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B), (C) and (D) only
The physical properties of halogen compounds depend mainly on the intermolecular attractions between their molecules: how strong they are, and how well the molecules pack together.
(A) The boiling points of isomeric haloalkanes increase with increase in branching. — False. The boiling points of isomeric haloalkanes decrease as branching increases. For example, among the isomeric bromobutanes, the most branched one, 2-bromo-2-methylpropane, boils lowest.
(B) The boiling points of isomeric dihalobenzenes are very nearly the same. — True. The o-, m- and p-isomers have almost equal boiling points; they differ in their melting points, where the symmetrical para-isomer melts highest.
(C) Alkyl bromides and iodides are colourless when pure, but they develop colour when exposed to light. — True. Pure alkyl halides are colourless; bromides and iodides become coloured on standing in light.
(D) Haloalkanes dissolve in organic solvents because the new attractions between haloalkane and solvent molecules have much the same strength as the attractions that are broken. — True. The energy given out by the new attractions makes up for the energy used, so they dissolve. In water it is different: the new attractions are weaker than the hydrogen bonds broken in water, so haloalkanes are only very slightly soluble.
NCERT (2026-27 edition):
6.6 Physical Properties – “The boiling points of isomeric haloalkanes decrease with increase in branching.”
“Boiling points of isomeric dihalobenzenes are very nearly the same. However, the para-isomers are high melting as compared to their ortho- and meta-isomers.”
“Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light.”
“…However, haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.”