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Physical Properties
of Haloalkanes and Haloarenes

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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

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

Solubility

Density

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?

  1. CH2Cl2
  2. CHCl3
  3. 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:

Dipole moment of CH2Cl2 is 1.62 D Dipole moment of CHCl3 is 1.03 D Dipole moment of CCl4 is 0 D

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:

Boiling point of alkyl halides

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The dipole moment of CH3X, where X is a halogen, follows the order:

The order of dipole momemnt in CH3X

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Which of the following orders regarding the boiling points of the given alkyl chloride is correct?

Comparison of boiling point in alkyl chloride

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