Physical Properties of Haloalkanes and Haloarenes: Haloalkanes and haloarenes are formed when a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms. The compound formed when a hydrogen atom from an aliphatic hydrocarbon is replaced by a halogen atom is known as haloalkane. Alkyl halide and halogenoalkane are other names for it. Examples of haloalkanes are ethyl bromide \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\right)\), methyl bromide \(\left(\mathrm{CH}_{3} \mathrm{Br}\right)\) etc.

However, if a hydrogen atom is replaced from an aromatic hydrocarbon by a halogen atom, the resulting compound formed is known as haloarene. It is also known as aryl halide or halogenoarene. In a haloalkane \((\text {R}-\text {X}), \text {X}\) represents the halogen group. It is attached to an \(\mathrm{sp}^{3}\) hybridised atom of an alkyl group, whereas, in haloarene \((\mathrm{Ar}-\mathrm{X})\), the halogen is attached to an \(\text {sp}^{2}\) hybridised atom of an aryl group. Examples of haloarenes are bromobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\right)\), chlorobenzene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\right)\) etc. Haloalkanes and haloarenes are distinguished by the fact that haloalkanes are made from open-chain hydrocarbons (alkanes), whereas haloarenes are made from aromatic hydrocarbons. In this article, we will study about the physical properties of haloalkanes and haloarenes.

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Physical Properties of Haloalkanes and Haloarenes

Physical State

In their natural condition, alkyl halides are colourless. Bromides and iodides, on the other hand, take on colour when exposed to light. The breakdown of halogens in the presence of light is the cause of colour development. A reaction represents this phenomenon:

\(2 \text {R}-\text {I} \rightarrow \text {R}-\text {R}+\text {I}_{2}\)

Many halogen compounds that are volatile have a pleasant aroma. Haloarenes are found in colourless liquid form or crystalline solid form. Haloarenes have a characteristic smell.

Boiling Point

We know that the electronegativity of the carbon and halogen atoms in any given compound differs significantly, resulting in the formation of highly polarised molecules. The polarity of the \(\text {C}-\text {X}\) bond, as well as the larger molecular mass of the derivatives of halogen, results in the production of extremely strong intermolecular forces of attraction.

The dipole-dipole and van der Waals interactions are responsible for the stronger intermolecular forces of attraction. Intermolecular attraction forces determine the boiling point of haloalkanes and haloarenes. As a result, derivatives of chlorides, bromides, and iodides have greater boiling points than hydrocarbons with equal molecular masses. When we advance down the homologous series, the size and molecular mass of halogen members grow, producing stronger forces of attraction. As a result, as we travel down the homologous series, the boiling point rises. The boiling points of alkyl halides are in order \(\text {RI} > \text {RBr} > \text {RCl} > \text {RF}\). Additionally, the boiling point also increases for isomeric haloalkanes.

The boiling point, on the other hand, decreases as the compound branches. This is due to the fact that the branching of haloalkanes results in a smaller surface area, which reduces the interaction of van der Waal forces. Furthermore, as branching grows, the molecule takes on a spherical shape, reducing the area of contact and resulting in reduced intermolecular interactions.

At room temperature, derivatives such as methyl chloride, ethyl chloride, methyl bromide, and a few chlorofluoromethanes are gases. The higher members of the category, on the other hand, are frequently solids or liquids. Boiling points of haloarenes follow the order: Iodoarene > Bromoarene > Chloroarene. Furthermore, the boiling points of isomeric dihaloarenes are nearly identical.

Melting Point

The melting point of a chemical is determined by the strength of its lattice structure. Although the boiling points of isomeric dihalobenzenes are nearly identical, the melting points differ. In comparison to the ortho-isomer and meta-isomer of the identical molecule, the para-isomer has a greater melting point. It’s because, in comparison to ortho- and meta-isomers, para-isomers have a very compact crystal structure. As a result, the crystal structure can accommodate a greater number of molecules. As a result, it takes more energy to break the lattice structure, raising the melting point temperature of the compound.

Density

The mass of any compound is directly proportional to its density. As a result, the density rises as the mass increases in the homologous series. As a result, fluorine derivatives are less dense than chlorine derivatives, while chlorine derivatives are less dense than bromine derivatives. Furthermore, as the number of carbon and halogen atoms grows, so does density. Furthermore, it is determined by the halogen atom’s atomic mass. Take a look at the diagram below for an example.

The quantity of carbon atoms remains the same in the previous case, but the mass of halogen atoms differs. As a result, the density of derivatives increases. As a result, the relative densities are arranged in a certain way.

Solubility

Despite the fact that haloalkanes and haloarenes are polar compounds, they are water-insoluble. Dissolution of a compound and breaking the attractive interactions between halogen and the carbon atom demand a greater quantity of energy. When a bond is formed between a dissolution ion and water, however, less energy is released. Furthermore, the \(\text {R}-\text {X}\) bond has poor stability compared to the bond formed in water molecules due to polarity differences. As a result, haloalkanes and haloarenes neither form new \(\text {H}\)-bonds nor break old ones. As a result, \(\text {R}-\text {X}\) has a low solubility. However, because organic solvents have low polarity, these molecules are soluble in them. In the case of haloarenes, para-isomer is less soluble than ortho-isomer.

Stereochemical Aspects of Nucleophilic Substitution Reaction

Racemisation occurs during an \(\mathrm{SN}^{1}\) reaction, whereas inversion of configuration occurs with an \(\mathrm{SN}^{2}\). The following is a common nucleophilic substitution reaction:

If the reaction conditions are favourable for first-order kinetics, the process follows the \(\mathrm{SN}^{1}\) reaction mechanism.

SN¹ Reaction Mechanism

There are two steps to this mechanism.

Step 1: This is the first and is a slow step. The ionisation of the alkyl halide results in the formation of a stable carbocation intermediate. This intermediate has a flat geometry and is sp² hybridised.

Step 2: The nucleophile can attack the planar carbocation intermediate from either face equally effectively. This results in the creation of a mixture of products in equal quantities. This process is known as racemisation.

\(50 \%\) retention and \(50 \%\) inversion products are generated in the \(\mathrm{SN}^{1}\) mechanism. In the \(\mathrm{SN}^{1}\) reaction, racemisation occurs.

The alkyl halides will follow the \(\mathrm{SN}^{2}\) reaction if the reaction circumstances are extremely favourable for second-order kinetics.

SN² Reaction Mechanism

It’s a concerted process, which means it happens all at once. In this case, the nucleophile approaches from the opposite direction as the departing group. The leaving group is displaced from the substrate’s carbon atom, and a new bond with the nucleophile is formed at the same time. This produces a five-bond unstable transition state.

The tetrahedral geometry of an alkyl halide is not followed by these bonds in the transition state. This is a transitory state. The leaving group is then expelled, resulting in a product with a full configuration inversion. The name used for this type of inversion is Umbrella inversion. As a result, the \(\mathrm{SN}^{2}\) mechanism is accompanied by a complete configuration inversion.

Summary

1. Haloalkanes and haloarenes are formed when a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms.
2. In their natural condition, alkyl halides are colourless. Haloarenes are found in colourless liquid form or crystalline solid form.
3. The boiling points of alkyl halides are in order \(\text {RI} > \text {RBr} > \text {RCl} > \text {RF}\). Boiling points of haloarenes follow the order: Iodoarene > Bromoarene > Chloroarene.
4. Despite the fact that haloalkanes and haloarenes are polar compounds, they are water-insoluble.
5. Racemisation occurs during an \(\mathrm{SN}^{1}\) reaction, whereas inversion of configuration occurs with an \(\mathrm{SN}^{2}\).

FAQs on Haloalkanes and Haloarenes

Q.1. What are haloalkanes and haloarenes?
Ans: Haloalkanes and haloarenes are formed when a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms. The compound formed when a hydrogen atom from an aliphatic hydrocarbon is replaced by a halogen atom is known as haloalkane. However, if a hydrogen atom is replaced from an aromatic hydrocarbon by a halogen atom, the resulting compound formed is known as haloarene. It is also known as aryl halide or halogenoarene.

Q.2. Discuss the solubility of haloalkanes and haloarenes.
Ans: Haloalkanes and haloarenes are water-insoluble. Dissolution of a compound and breaking the attractive interactions between halogen and the carbon atom demand a greater quantity of energy. When a bond is formed between a dissolution ion and water, however, less energy is released. Furthermore, the \(\text {R}-\text {X}\) bond has poor stability when compared to the bond formed in water molecules due to polarity differences. As a result, haloalkanes and haloarenes neither form new \(\text {H}\)-bonds nor break old ones.

Q.3. Describe the density of haloalkanes and haloarenes briefly.
Ans: The mass of any compound is directly proportional to its density. As a result, the density rises as the mass increases in the homologous series. As a result, fluorine derivatives are less dense than chlorine derivatives, while chlorine derivatives are less dense than bromine derivatives.

Q.4. How many steps are present in SN¹ and SN² mechanisms?
Ans: There are two steps in \(\mathrm{SN}^{1}\) mechanism. \(\mathrm{SN}^{2}\) mechanism is a concerted process, which means it happens in a single step.

Q.5. What are the stereochemical aspects of the SN¹ and SN² mechanisms?
Ans: Racemisation occurs during an \(\mathrm{SN}^{1}\) reaction, whereas inversion of configuration occurs with an \(\mathrm{SN}^{2}\).

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