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.

Which of the following has the highest boiling point?

Tetrachloromethane

The correct answer is option 4. Tetrachloromethane.

Let us delve deeper into why tetrachloromethane \((CCl_4)\) has the highest boiling point among the given compounds by examining the factors influencing boiling points:

Factors Influencing Boiling Points

Molecular Mass (or Molar Mass):  Higher molecular mass typically leads to higher boiling points due to stronger van der Waals forces (dispersion forces). Larger molecules have more electrons, which can create temporary dipoles more easily, leading to stronger intermolecular attractions.

Intermolecular Forces:

Dispersion Forces (London Forces): Present in all molecules, but stronger in larger and heavier molecules.

Dipole-Dipole Interactions: Present in polar molecules where partial charges attract each other.

Hydrogen Bonding: Strong dipole-dipole interaction occurring in molecules where hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine (not relevant here as none of the compounds have hydrogen bonded to these atoms).

Polarity:

Polar molecules have dipole-dipole interactions in addition to dispersion forces, leading to higher boiling points compared to non-polar molecules of similar size.

Analysis of Given Compounds

1. Chloromethane \((CH_3Cl)\)

Molecular Mass: 50.5 g/mol

Polarity: Moderately polar due to one chlorine atom.

Intermolecular Forces: Dipole-dipole interactions and dispersion forces.

Boiling Point: -24°C

2. Dichloromethane \((CH_2Cl_2)\)

Molecular Mass: 84.9 g/mol

Polarity: More polar than \(CH_3Cl\) due to two chlorine atoms.

Intermolecular Forces: Stronger dipole-dipole interactions and dispersion forces.

Boiling Point: 40°C

3. Trichloromethane \((CHCl_3)\)

Molecular Mass: 119.4 g/mol

Polarity: Highly polar due to three chlorine atoms.

Intermolecular Forces: Even stronger dipole-dipole interactions and dispersion forces.

Boiling Point: 61°C

4. Tetrachloromethane \((CCl_4)\)

Molecular Mass: 153.8 g/mol

Polarity: Non-polar (symmetrical molecule with four chlorine atoms, canceling out dipoles).

Intermolecular Forces: Strong dispersion forces due to high molecular mass.

Boiling Point: 77°C

Why \(CCl_4\) Has the Highest Boiling Point

Dispersion Forces:

The main contributor to the boiling point of \(CCl_4\) is its strong dispersion forces, which increase with molecular mass. \(CCl_4\) has the highest molecular mass among the four compounds, leading to the strongest dispersion forces.

Polarity:

Although \(CCl_4\) is non-polar and lacks dipole-dipole interactions, its higher molecular mass and the resulting stronger dispersion forces outweigh the dipole-dipole interactions present in the other chloromethanes.

Summary

Tetrachloromethane \((CCl_4)\) has the highest boiling point among the given compounds primarily due to its high molecular mass, leading to strong dispersion forces. These forces are sufficient to result in a higher boiling point compared to the other chloromethanes, despite CCl4 being non-polar.