Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have sweet smell.

Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. As we have already learnt, 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.

Solubility
The haloalkanes are very slightly soluble in water. In order to dissolve haloalkane in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. 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.

If chloromethane has a boiling point of 250K, what will be the probable boiling point of methane?

110K

The correct answer is option 1. \(110 K\).

To determine the probable boiling point of methane given the boiling point of chloromethane, we need to compare their properties in terms of molecular mass and intermolecular forces.

Molecular Structure and Intermolecular Forces:

Chloromethane \((CH_3Cl)\)

Molecular Mass: 50.5 g/mol

Structure: \(CH_3Cl\) has one chlorine atom, which makes it a polar molecule.

Intermolecular Forces: It experiences both dipole-dipole interactions due to its polarity and dispersion forces.

Boiling Point: 250K

Methane \((CH_4)\)

Molecular Mass: 16 g/mol

Structure: \(CH_4\) is a tetrahedral molecule with no polar bonds, making it a non-polar molecule.

Intermolecular Forces: It only experiences dispersion forces (also known as London dispersion forces), which are weaker than dipole-dipole interactions.

Comparison of Boiling Points:

Intermolecular Forces: Chloromethane, being polar, has stronger intermolecular forces (dipole-dipole interactions) in addition to dispersion forces. Methane, being non-polar, only has dispersion forces. As a result, methane will have a lower boiling point compared to chloromethane because dipole-dipole interactions in chloromethane provide additional energy required to overcome these forces during boiling.

Molecular Mass: Chloromethane has a significantly higher molecular mass compared to methane. Higher molecular mass usually leads to stronger dispersion forces, but this is secondary to the effect of polarity in this comparison.

The actual boiling point of methane is around 111.5K. This value reflects the weaker intermolecular forces (only dispersion forces) acting in methane, requiring less energy to transition from the liquid to the gas phase.

Conclusion: Given the boiling point of chloromethane (250K) and the comparative analysis of the molecular properties and intermolecular forces, methane's boiling point would be significantly lower due to its lower molecular mass and lack of polarity.

Therefore, the most probable boiling point of methane is: 110K.