Physical Properties
of Haloalkanes and Haloarenes

Colour

Alkyl halides are colourless when pure but bromides and iodides develop colour when exposed to light because they decompose into halogens.

2R−I → R−R + I₂

Melting and Boiling Points

The melting points and boiling points of haloalkanes are higher than those of corresponding hydrocarbons due to higher molecular mass of haloalkanes .
The melting and boiling points increase with increase in size due to higher vander waal's forces. Hence the order is : R−I > R−Br > R−Cl > R−F
The boiling points of isomeric haloalkanes decrease with increase in branching as surface area decreases on branching and Vander waal's forces decrease.

Boiling point decreases with branching

CH₃CH₂CH₂CH₂Cl > (CH₃)₂CHCH₂Cl > (CH₃)₃CCl
The boiling points of isomeric dihalobenzene are nearly the same.
The melting point of p-dihalobenzene is always higher than those of the o- and p- isomers. Because p-isomers are more symmetrical and hence their molecules pack closely in the crystal lattice. This leads to stronger molecular forces of attraction.

o-dichlorobenzene

Unsymmetrical

m-dichlorobenzene

Unsymmetrical

p-dichlorobenzene

Symmetrical

Arrange 'propane, 1-chloropropane and isopropyl chloride' in order of increasing boiling points.

Solubility

Haloalkanes and haloarenes, though polar, are immiscible with water because they neither form H-bond nor break the already existing H-bonds (because new attraction of water and haloalkanes is weaker).
Haloalkanes are soluble in organic solvents of low polarity like ether, benzene etc., because new intermolecular forces are similar to those in the low polarity solvents.

Density

Alkyl fluorides and alkyl chlorides are generally lighter than water whereas alkyl bromides and alkyl iodides are heavier. The relative densities follow the order :RI > RBr > RCl.
The densities of the alkyl halides decreases as the size of the alkyl group increases.

Which alkyl halide has the highest density ?

Stability

The stability of the haloalkanes having the same alkyl group decreases in the order : R−F > R−Cl > R−Br > R−I since the strength of the C−X bond decreases in the order : C−F > C−Cl > C−Br > C−I

Dipole Moment

The dipole moment of methyl halides follows the order: CH₃Cl > CH₃F > CH₃Br > CH₃I. Due to the very small size of F, fluorides have lower dipole moments than chlorides.

Questions

Question : Which one of the following has the highest dipole moment?

CH₂Cl₂
CHCl₃
CCl₄

Answer : The three dimensional structures of the above compounds along with the direction of dipole moment in each of their bonds are given below:

CCl₄ being symmetrical has zero dipole moment. In CHCl₃, the two C-Cl dipole moments being equal and opposite cancel the effect of each other. The other two dipoles (C-H and C-Cl) are opposite but not equal because the dipole moment of C-H bond is smaller than that of C-Cl bond. As a result, CHCl₃ has a finite dipole moment — 1.03 D to be precise.

In CH₂Cl₂, the resultant of two C-Cl dipole moments is in the same direction as the resultant of two C-H dipoles. Therefore, CH₂Cl₂ has a dipole moment higher than that of CHCl₃. Thus, CH₂Cl₂ has the highest dipole moment among the three.

Question : The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride. Why?
Answer : The chlorine atom in chlorobenzene is attached to an sp² hybridised carbon whereas in cyclohexyl choride, it is attached to an sp³ hybridised carbon.

Comparison of dipole moment between chlorobenzene and cyclohexyl chloride

An sp² hybridised carbon is more electronegative than an sp³ hybridised carbon due to greater s character. Due to this reason, the difference of electronegativiy between carbon and chlorine is less in chlorobenzene. Further, due to resonance in chlorobenzene, the C-Cl bond in chlorobenzene acquires some double bond character while the C-Cl bond in cyclohexyl chloride is a pure single bond. In other words, the C-Cl bond in chlorobenzene is shorter than in cyclohexyl chloride. Therefore, the dipole moment — which is defined as the product of charge and distance — of chlorobenzene is lower than that of cyclohexyl chloride.

Question : p-Dichlorobenzene has higher melting point than those of o- and m-isomers. Discuss.
Answer : The p-isomer being more symmetrical fits closely in the crystal lattice. This leads to stronger intermolecular forces of attraction than those of o- and m-isomers. Thus, a larger amount of energy is required to melt the p-isomer than the corresponding o- and m-isomers.

For the same alkyl group, the order of boiling points of alkyl halides is: