Physical Properties of Haloalkanes and Haloarenes: Introduction and Explanation

You might have heard about the depletion of the ozone layer. But do you know the compound responsible for the depletion of ozone layer is a haloalkane compound i.e. chlorofluorocarbon. These halogen compounds have many uses in our everyday life. For Example- chloroquine is used for the treatment of malaria, halothane is used as an anaesthetic during surgery, etc.

Key Terms: Organic chemistry, hydrogen atom, reactions, Alkyl halides, chloroquine, Aliphatic hydrocarbon, halogen, atom, Chlorine, Iodine, Bromine, Fluorine

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Haloalkanes or Alkyl Halides

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Haloalkanes or Alkyl Halides are formed by replacement of Hydrogen atom(s) in an Aliphatic hydrocarbon by halogen atom(s) like Chlorine, Iodine, Bromine and Fluorine.

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Haloarenes or Aryl Halides

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Haloarenes or Aryl Halides are formed by the replacement of Hydrogen atom(s) in an Aromatic hydrocarbon by halogen atom(s) like Chlorine, Iodine, Bromine and Fluorine.

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Physical Properties of Haloalkanes and Haloarenes

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Physical properties depend upon mass of the compounds and the type of intermolecular and intramolecular forces of attraction. Both the Alkyl Halides and Aryl Halides have some physical properties that create the difference between them. Some of the properties are:

1. Physical State-

Haloalkanes are colourless in nature in their pure state. However, bromides and iodides develop colour on exposure to light due to which alkyl-bromide and alkyl-iodide have colour. The reason for the development of colour is that halogens get decomposed in the presence of light. Many of the halogen compounds having a volatile nature have a sweet smell. 

Haloarenes are also colourless liquids or crystalline solids with a characteristic smell.

2. Boiling Point-

Boiling Point of a Molecule depends on the amount of energy provided in the form of heat or temperature to break the force of attraction between any two molecules in any state. The energy required to break this intermolecular force decides the level of Boiling of that molecule in its liquid state.

Therefore, we can say that

Boiling Point ∝  Intermolecular forces

There are two types of Intermolecular Forces:

(a) Hydrogen Bond (exists rarely) and

(b) Van der Waals Forces (exists generally): It exists between two molecules of alkyl halides ( R-Br-R-Br) as Bromine does not form hydrogen bonds. There are different types of Van der Waals Forces such as Dipole-Dipole force, ion- Dipole force, London force etc.

Between these two, Hydrogen bonds are stronger than the Van der Waals Forces as Van der Waals Forces are the weakest forces of all.

Van der Waals forces are important for establishing the bonds between the molecules as it is connected to the size of the molecules. 

Van der Waals Force ∝ Surface Area ∝ Size ∝ Molecular mass

Some of the cases within Alkyl Halides to establish the relation with the boiling point are:

• Order of Boiling point in different Alkyl Halides:

CH₃-F , CH₃-Cl , CH₃- Br , CH₃-I

As the molecule having a bigger size has a greater Boiling point. And we studied above that the Boiling Point depends upon the Intermolecular forces (Van der Waals Force) and Van der Waals Force depends on the size of the molecule.

So, we know that the molecular size of Halides is in the order: 

 F < Cl < Br < I 

So the order of Boiling point of Alkyl Halide can be given as-

CH₃-F < CH₃-Cl < CH₃- Br < CH₃-I

• Order of Boiling Point between a Alcohol and a Alkyl Halide:

CH₃- OH and CH₃-Cl

As the force of attraction between the molecules of alcohols is Hydrogen bond and between the molecules of Alkyl Halides it is Van der Waals forces. Since, the Hydrogen Bond is stronger than the Van der Waals forces therefore, the Boiling point of Alcohol is more than the Alkyl Halide.

CH₃- OH > CH₃-Cl

• Order of Boiling Point between Alkane and Alkyl Halide:

CH₃-CH₂-CH₃ and CH₃-CH₂-CH₂-Cl

Both of them have the Van der Waals forces and no hydrogen bonds. But in Alcohol molecules, there is a London force (a type of Van der Waals forces) as these are also Non-Polar molecules. On the other hand, in Alkyl Halide molecules experience a Dipole- Dipole force of attraction (another type of Van der Waals forces) as Chlorine is more electronegative than Carbon or they are Polar molecules.

Since, Dipole- Dipole force is stronger than London forces therefore the boiling point of Alkyl Halide is stronger than Alkane as it depends on the Van der Waals forces.

CH₃-CH₂-CH₃ < CH₃-CH₂-CH₂-Cl

(Alkane)            (Alkyl Halide)

• Order of Boiling Points between isomeric Alkyl Halide:

In the case of Isomeric Alkyl Halide, as the branching increases the boiling point decreases. So, we can write as: Boiling Point ∝ Branching

This happens because as the branching increases, the molecule turns to take a spherical shape and we know that spherical shape tends to have the minimum surface area and therefore it leads to weak Van der Waals forces between the molecules or weak boiling points.

Therefore the order of Boiling Point will be:

Order of Boiling Point between isomeric Alkyl Halide

• Order of Boiling Point of Haloarenes:

Boiling points of haloarenes follow the order: 

Iodoarene > Bromoarene > Chloroarene. 

The order of boiling point of monohalogen derivatives of benzene is in the order as- iodo > bromo > chloro > fluoro. Moreover, the boiling point of isomeric di-haloarenes is almost similar.

3. Melting Point-

Melting Point of a structure depends on the lattice structure of the compound. And the molecules having efficient packaging have high melting points as it requires larger force of attraction to break the structure.

• Order of Melting Point between the different positions of Halobenzenes ( Aryl Halide):

Melting Point between the different positions of Halobenzenes:

In Halobenzenes, the Para isomers have a higher melting point than ortho and Meta isomers because in Para isomers there is symmetry in the crystal lattice of the structure.

4. Density-

We know that Density = MassVolume

Now the relation between the halogens in terms of their volume will be:

CH₃-F < CH₃- Cl < CH₃-Br < CH₃-I 

Volume increases as the size of the molecule increases and due to this the mass of the molecule also increases. On moving down the group of halides both the volume and mass increases.

Therefore, if there is an increase in Mass as compared to volume then there will be an increase in Density as the Mass & Density are directly proportional to each other. 

But if there is an increase in Volume as compared to Mass and we know that Volume & Density are indirectly proportional to each other, then there will be a decrease in the Density.

Here is the order of density of alkyl halides with the density-

Density will be:

n-C₃H₇Cl < n-C₃H₇Br < n-C₃H₇I ( as the size increases)

Order of density of alkyl halides when number of halogen increases-

Density will be:

CH₂Cl₂ < CHCl₃ < CHCl₄ (as the number of halogen group increases)

5. Solubility-

In case of Alkyl Halide, it does not form hydrogen bonds with water molecules as the halogen group is not bonded with the hydrogen part of the alkyl group. Therefore it is not possible that the alkyl halide gets soluble in water. Although it will form a new bond whenever alkyl halide is mixed with water after breaking the earlier bonds between the molecules. 

Solubility of alkyl halides

Hence the Alkyl Halides are not soluble or less soluble in water as it will not form the bond with water due to lack of sufficient energy. And they are soluble in Non-polar solvents (organic solvents like- ether, benzene, etc.).

In case of Haloarenes- They are the non-polar compound which cannot form the hydrogen bond with water molecules. As a result, Haloarenes are insoluble in water but they are soluble in organic solvents.

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