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.

For a particular alkyl group R, what is the correct order of boiling points of the following compounds?

 RBr > RCl > RF

The correct answer is option 1.  RBr > RCl > RF.

To explain the order of boiling points for alkyl halides, we need to consider how various factors such as molecular mass, polarizability, and intermolecular forces affect the boiling point.

Factors Affecting Boiling Points:

Molecular Mass:

Higher molecular mass generally leads to higher boiling points because larger molecules have more electrons, leading to stronger dispersion forces (a type of van der Waals force).

Polarizability:

Larger atoms with more electrons are more polarizable, meaning their electron clouds can be more easily distorted. This increases the strength of dispersion forces. In alkyl halides, the halogen's polarizability increases in the order: F < Cl < Br < I.

Intermolecular Forces:

Dispersion Forces: All molecules experience these, but they are stronger in larger, more polarizable molecules.

Dipole-Dipole Interactions: Present in polar molecules, but these are typically less significant than dispersion forces in determining boiling points for alkyl halides.

Given Alkyl Halides:

\(R-F\) (alkyl fluoride):

Fluorine is the smallest halogen with the lowest molecular mass and lowest polarizability. Dispersion forces are weakest among the given halides.

Boiling Point: Lowest

\(R-Cl\) (alkyl chloride):

Chlorine is larger and more polarizable than fluorine but smaller and less polarizable than bromine. Dispersion forces are stronger than in alkyl fluorides but weaker than in alkyl bromides.

Boiling Point: Moderate

\(R-Br\) (alkyl bromide):

Bromine is larger and more polarizable than chlorine and fluorine. Dispersion forces are the strongest among the given halides. Boiling Point: Highest

Order of Boiling Points: Given the increasing molecular mass and polarizability from fluorine to chlorine to bromine, the boiling points will increase in the same order:

\(R-F\) (alkyl fluoride): Lowest boiling point due to the smallest size and weakest dispersion forces.

\(R-Cl\) (alkyl chloride): Higher boiling point than \(R-F\) but lower than \(R-Br\).

\(R-Br\) (alkyl bromide): Highest boiling point due to the largest size and strongest dispersion forces.

Conclusion: The correct order of boiling points for the given alkyl halides is: RBr > RCl > RF

This order reflects the increasing strength of dispersion forces, which dominate the boiling point trends in alkyl halides.