Physical Properties of Haloalkanes & Haloarenes
The physical properties of alkyl halides (haloalkanes) and aryl halides (haloarenes) differ significantly from their parent hydrocarbons due to the polarity of the Carbon-Halogen bond and the higher molecular mass of halogen atoms.
1. Physical State & Color
- Lower members ($CH_3Cl, CH_3Br, C_2H_5Cl$) are colorless gases at room temperature.
- Higher members are liquids or solids.
- Color: Alkyl halides are generally colorless when pure. However, Bromides and Iodides develop color upon exposure to light due to decomposition (releasing free halogen).
- Smell: Many volatile halogen compounds have a sweet, pleasant smell.
2. Boiling Points
Boiling points are determined by intermolecular Van der Waals forces (dipole-dipole interactions and London dispersion forces).
A. Trend with Halogen Atom
For the same alkyl group, boiling point increases with the size and mass of the halogen atom.
Reason: Magnitude of Van der Waals forces increases with size/mass.
B. Trend with Carbon Chain
For the same halogen, boiling point increases with the size of the alkyl group.
C. Effect of Branching
For isomeric haloalkanes, boiling point decreases with branching.
Reason: Branching makes the molecule more spherical, reducing the surface area for contact. This weakens the Van der Waals forces.
$n\text{-Butyl Bromide} > sec\text{-Butyl Bromide} > tert\text{-Butyl Bromide}$
3. Melting Points (Symmetry Effect)
For Haloarenes, a special trend is observed among isomers.
The Para-Isomer Anomaly
Para-isomers generally have much higher melting points than their ortho and meta counterparts.
Reason: Symmetry. The para-isomer is more symmetrical, allowing it to pack more closely and efficiently in the crystal lattice. This leads to stronger intermolecular forces and a higher melting point.
Order: $Para > Ortho \approx Meta$
4. Density
Haloalkanes are generally heavier than water (except some fluorides and chlorides).
- Density increases with increasing atomic mass of the halogen ($R-I > R-Br > R-Cl$).
- Density increases with the number of halogen atoms (e.g., $CCl_4 > CHCl_3 > CH_2Cl_2$).
5. Solubility
Despite being polar, haloalkanes are very slightly soluble in water.
Why?
To dissolve, the haloalkane must break the strong Hydrogen Bonds between water molecules. The energy released by forming new attractions between haloalkane and water is not sufficient to overcome the energy required to break the H-bonds of water.
They are highly soluble in organic solvents (like alcohol, ether, benzene) because the new intermolecular forces formed are similar in strength to the ones broken.
6. Dipole Moment (Important Exception)
The C-X bond is polar due to the electronegativity difference.
Expected Order: $CH_3F > CH_3Cl > CH_3Br > CH_3I$ (Based on Electronegativity).
Actual Order
$$ CH_3Cl > CH_3F > CH_3Br > CH_3I $$
Explanation: Dipole moment ($\mu$) = Charge ($q$) $\times$ Distance ($d$).
Although Fluorine has higher electronegativity ($q$ is high), the C-F bond length ($d$) is very short. For Chlorine, the larger bond length compensates for the slightly lower charge, resulting in the highest dipole moment for Methyl Chloride.
Properties Quiz
Test your concepts on Physical trends. 10 MCQs with explanations.
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