Is there a difference between the hardness of a small rock and a larger rock? Shockingly there isn’t. A small rock will be comparable in its hardness to the larger rock of the same type. This quality is because of the physical property of the rock. Similarly, everything in nature including compounds like haloalkane and haloarene has some physical properties as well as chemical properties. In this topic, we will understand more about the physical properties of haloalkanes and haloarenes.
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What are the Physical Properties?
Physical properties of a compound are those properties that are related to the physical aspects of the compound. For eg. shape, size, color, mass, boiling point, etc. The physical properties of any compound primarily depend upon,
- The mass of the compound
- Different forces of attraction including intermolecular and intramolecular forces of attraction.
When a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms then the compounds are known as haloalkanes and haloarenes. Halogens are the less reactive functional group in comparison to carboxyl or aldehyde group. Therefore, halogen group when attached as a functional group do not bring a drastic difference in the overall physical properties of a compound. However, some differences can be seen as we move down the group in the homologous series of haloalkanes and haloarenes due to the difference in the atomic masses.
Physical Properties of Haloalkanes & Haloarenes
1) Physical State
Alkyl halides are colourless in nature in its pure state. However, bromides and iodides develop colour on exposure to light. The reason for the development of colour is the decomposition of halogens in presence of light. The reaction representing the phenomenon is
2R−I  →  R−R + I2
Many of the halogen compounds having volatile nature have a sweet smell. Haloarenes are also colourless liquids or crystalline solids that have a characteristic smell.
2) Boiling Point
We know there is a large difference in electronegativity between the carbon and halogen atom of any given compound resulting in the development of highly polarized molecules. The polarity of the C-X bond and higher molecular mass in comparison to the actual hydrocarbon results in the development of very strong intermolecular forces of attraction in the derivatives of halogen.
The stronger intermolecular forces of attraction are due to dipole-dipole and van der Waals interaction. The boiling point of haloalkanes and haloarenes depends upon the intermolecular forces of attraction. Hence, the boiling points of derivatives of chlorides, bromides, and iodides are comparatively higher the hydrocarbons of the similar molecular mass.
The size and molecular mass of halogen members increase when we move down the group in the homologous series thereby forming stronger forces of attraction. Hence the boiling point increases as we move down the group in the homologous series.
The order of the boiling point of alkyl halides are RI > RBr > RCl > RF
Additionally, the boiling point also increases for isomeric haloalkanes. However, the boiling point decreases with the branching of the compound. This is because branching of haloalkanes results in the lesser surface area, thus decreasing the van der Waal’s forces interaction.
Moreover, as the branching increases the molecule forms sort of a spherical shape resulting in the decrease in the area of contact and forming weaker intermolecular forces. Derivatives such as methyl chloride, ethyl chloride, methyl bromide and few chlorofluoromethanes are gases at room temperature. However, the higher members of the group are usually solids or liquids.
Boiling points of haloarenes follow the order: Iodoarene > Bromoarene > Chloroarene. Moreover, the boiling point of isomeric dihaloarenes is almost similar.
3) Melting Point
The melting point is based on the strength of the lattice structure of a compound. Isomeric dihalobenzenes have almost similar boiling points but the difference can be seen in the melting points. Para-isomer have a higher melting point in comparison to ortho-isomer and meta-isomer of the same compound.
It is because para-isomers have highly compact crystal lattice in comparison to ortho-isomer and meta-isomer. Therefore, higher numbers of molecules are packed compactly in the crystal lattice. Hence, higher energy is required to break the lattice structure thereby increasing the melting point temperature of the compound.
4) Density
Density is directly proportional to the mass of any compound. Therefore, as the mass increases down the homologous series, the density increases. Thus, the derivatives of fluorine are less dense than derivatives of chlorine and derivatives of chlorine are less dense than derivatives of bromine.
Additionally, density increases with the increase in the number of carbon and halogen atoms. Furthermore, it depends upon the atomic mass of the halogen atom. For example, refer to the below diagram
In the above example, the number of carbon atoms remains same but the mass of halogen atoms is different from one another. This, in turn, increases the density of the derivatives. Therefore, the arrangement of relative densities are
RI > RBr > RCl
5) Solubility
Alkyl halides are slightly soluble in water. Even though haloalkanes and haloarenes are polar compounds, they are immiscible with water. Relatively larger amount of energy is required for dissolution of a compound and to break the attractive forces between halogen and the carbon atom.
However, less energy is released when a bond is formed after dissolution ion and water. Moreover, the stability of R-X bond is very low in comparison to the bond formed in water molecules polarity difference.
Thus, haloalkanes and haloarenes neither develop new H-bond nor it breaks the old H-bonds. Hence, the solubility of R-X is low. However, these compounds are soluble in organic solvents because of the low polarity of organic solvents such as ether, benzene etc.
Therefore, the strength of the intermolecular forces between organic solvents and carbon-halogen compounds is similar to carbon-halogen molecules and solvent molecules. In the case of haloarenes, para-isomer is less soluble than ortho-isomer.
Here’s a Solved Question for You
Q: Why does p-Dichlorobenzene have a higher melting point than the corresponding ortho-isomer and meta-isomers?
Solution: The para-isomer of Dichlorobenzene is symmetrically more compact and forms a more compact crystal lattice structure. This helps in the development of stronger intermolecular forces of attraction in comparison to of ortho-isomer and meta-isomers. Hence para-isomer of Dichlorobenzene requires a higher amount of energy to melt than its corresponding ortho-isomer and meta-isomers.
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