Haloarenes

2. Polyhaloarenes

Polyhaloarenes contain two or more halogen atoms attached to the aromatic ring. They are further classified based on the number of halogens and their positioning on the ring:

• Dihaloarenes: Two halogen atoms attached to the ring.
• Trihaloarenes: Three halogen atoms attached to the ring.
• Tetrahaloarenes: Four halogen atoms attached to the ring, and so forth.

Characteristics:

• General Formula: C₆​H_(6−n)​X_n, where n represents the number of halogens.
• Isomerism: Polyhaloarenes exhibit isomerism depending on the position of the halogens (ortho, meta, or para).
  • Ortho (o-) Isomers: Halogens are attached to adjacent carbons.
  • Meta (m-) Isomers: Halogens are attached to carbons with one carbon between them.
  • Para (p-) Isomers: Halogens are attached to opposite carbons.

Example – Dichlorobenzene:

1. 1,2-Dichlorobenzene (o-Dichlorobenzene): Halogens at adjacent carbons.
2. 1,3-Dichlorobenzene (m-Dichlorobenzene): Halogens with one carbon between them.
3. 1,4-Dichlorobenzene (p-Dichlorobenzene): Halogens on opposite carbons.

Chemical Properties:

• Increased Stability: More halogens enhance the resonance stabilization of the molecule but also make it more resistant to certain reactions.
• Electron-Withdrawing Effect: Halogens are electron-withdrawing, making the aromatic ring slightly deactivated toward further substitution reactions.
• Reduced Reactivity: Polyhaloarenes are generally more resistant to nucleophilic substitution reactions.

Environmental Concerns:

Some polyhaloarenes, such as polychlorinated biphenyls (PCBs), are hazardous environmental pollutants due to their persistence and bioaccumulation in ecosystems. PCBs were once used in electrical insulators and coolants but were banned in many countries due to their toxic effects.

Uses:

Polyhaloarenes are used in the synthesis of pesticides, flame retardants, and other industrial chemicals. However, due to their environmental impact, the use of certain polyhaloarenes is restricted.

Preparation of Haloarenes

Haloarenes, or aryl halides, can be prepared through several methods that involve introducing a halogen atom onto an aromatic ring. Here are the primary methods of preparing haloarenes:

1. Direct Halogenation of Aromatic Compounds

This method involves the direct addition of a halogen (like chlorine or bromine) to an aromatic compound (typically benzene) in the presence of a catalyst.

Reaction:

• Benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst like ferric chloride (FeCl₃) or ferric bromide (FeBr₃).
• This produces chlorobenzene or bromobenzene, respectively.

Example:

C₆​H₆​+Cl₂​^FeCl3​​→ C₆​H₅​Cl+HCl

Notes:

• This method is used mainly for preparing chlorobenzene and bromobenzene.
• Fluorination and iodination of benzene by this method are less common due to reactivity issues.
  • Fluorination is highly exothermic and can be dangerous.
  • Iodination is less feasible as it requires oxidizing agents like nitric acid (HNO₃) to proceed efficiently.

2. Sandmeyer Reaction

The Sandmeyer reaction is an important method for preparing haloarenes, particularly when a specific position on the ring is required. This reaction involves converting an aromatic amine to an aryl halide.

Steps:

1. Diazotization: Aniline (C₆H₅NH₂) is treated with nitrous acid (generated in situ by reacting sodium nitrite with hydrochloric acid) at low temperature to form a diazonium salt.
2. Substitution: The diazonium salt is then treated with copper(I) chloride (CuCl) or copper(I) bromide (CuBr), which facilitates the replacement of the diazonium group with a halogen.

Reactions:

Formation of Benzene Diazonium Chloride:

C₆​H₅​NH₂​+HNO_2​+HCl→C₆​H₅​N₂⁺​Cl⁻+2H₂​O

Formation of Chlorobenzene: 

C₆​H₅​N₂⁺​Cl^{− CuCl}_→ ^​C₆​H₅​Cl+N₂​

Notes:

• This method is widely used for introducing chlorine or bromine atoms onto an aromatic ring.
• It is particularly valuable because it allows for high selectivity and control over the position of substitution.

3. Gattermann Reaction

The Gattermann reaction is similar to the Sandmeyer reaction but uses different reagents. In this reaction, a diazonium salt is prepared from aniline and then reacted with copper powder in the presence of a halogen acid (HCl or HBr).

Reaction:

C₆​H₅​N₂⁺​Cl^{− Cu,HCl} _→ ^​C₆​H₅​Cl+N₂​

Notes:

• The Gattermann reaction can be used to prepare both chlorobenzene and bromobenzene.
• It is less commonly used than the Sandmeyer reaction but is helpful when mild conditions are required.

4. Balz-Schiemann Reaction

The Balz-Schiemann reaction is a method specifically used for the preparation of fluoroarenes (aryl fluorides), which are challenging to produce by other means.

Steps:

1. Aniline is first diazotized to form a diazonium salt.
2. The diazonium salt is then treated with fluoroboric acid (HBF₄) to produce a diazonium tetrafluoroborate salt.
3. This salt is then heated, decomposing it to form the fluoroarene with the release of nitrogen gas and boron trifluoride (BF₃).

Reaction:

C₆​H₅​N₂⁺​BF₄^{−  Δ}​→C₆​H₅​F+N₂​+BF₃​

Notes:

• The Balz-Schiemann reaction is a valuable method for preparing fluorobenzene and other aryl fluorides.
• It is limited to fluorine as a substituent and is not suitable for preparing chloro- or bromoarenes.

5. Ulmann Reaction

The Ulmann reaction is used for preparing dihaloarenes, particularly when two halogen atoms need to be introduced into the aromatic system. It is typically used to synthesize polyhalogenated aromatic compounds.

Reaction:

• In this reaction, an aryl halide reacts in the presence of copper at high temperatures to form polyhaloarenes.

Example:

C₆​H₅​I+C₆​H₅​I ^Cu,Δ​→ C₆​H₅​−C₆​H₅

Notes:

• The Ulmann reaction is often used for coupling reactions and synthesis of biaryls rather than for direct halogenation.
• It requires high temperatures and the presence of copper, so it is not commonly used for simple haloarene synthesis.

Physical Properties of Haloarenes

• Boiling and Melting Points: Haloarenes generally have higher boiling points than their parent aromatic compounds due to increased molecular weight and stronger intermolecular van der Waals forces.
• Solubility: Haloarenes are typically insoluble in water but are soluble in organic solvents like ether, benzene, and chloroform.
• Density: Haloarenes are denser than water due to the presence of heavy halogen atoms.

Chemical Properties of Haloarenes

Haloarenes exhibit unique chemical properties due to the influence of both the halogen and the aromatic ring.

Electrophilic Substitution Reactions

Due to resonance, the halogen is ortho- and para-directing, but its electron-withdrawing nature also deactivates the ring toward further substitution. The resonating structures of haloarene are as follows:

From the resonating structures, it is clear that ortho and para positions are relatively rich in electron density and hence incoming electrophile attacks at these positions give ortho and para-substituted products. Hence, haloarenes (halogens) are o/p – directing towards electrophilic substitution reaction.

Note : Halogen shows –I effect. It withdraws electrons from benzene ring and ring gets somewhat deactivated as compared to benzene.

1. Halogenation: Halogenation takes place in the presence of iron or FeCl₃ or anhydrous AlCl₃ as a catalyst. Eg.

2. Nitration: Eg.

3. Sulphonation: Eg.

Note : p-product is major product and o-product is minor product.

4. Friedel-Craft alkylation: eg.

5. Friedel – Craft acylation: Eg.

Nucleophilic Aromatic Substitution Reactions

Haloarenes are generally less reactive toward nucleophilic substitution than alkyl halides due to the partial double-bond character between the carbon and halogen atoms, caused by resonance.

1. Reduction of haloarenes 

Haloarenes can be converted into the corresponding arenes by reduction with Ni-Al alloy in the presence of alkali.

2. Fitting And Wurtz-Fittig reaction

Haloarenes when treated with sodium in the presence of dry ether, diaryls are produced. This reaction is known as the Fitting reaction.

Haloarenes when treated with sodium and alkyl halide in the presence of dry ether gives toluene. This reaction is known as the Wurtz-Fittig reaction.

 

Some of the examples of nucleophilic substitution reactions are:

1. Reaction with aq. NaOH: When chlorobenzene is heated with an aqueous solution of NaOH at 300⁰C and 200 atm gives sodium phenoxide which on acidification gives phenol.

2. Reaction with aq. NH₃: Chlorobenzene when heated with aq. ammonia in the presence of cuprous oxide (Cu₂O) at 200⁰C under a pressure of 60 atm gives aniline.

3. Action with Cuprous Cyanide: Chlorobenzene when heated with cuprous cyanide (CuCN) at 200⁰Cin the presence of pyridine gives phenyl cyanide.

Phenyl Cyanide can be used to prepare different important compounds such as:

Applications of Haloarenes

Haloarenes are utilized in a wide range of applications due to their stability and unique chemical reactivity.

• Pharmaceuticals: Haloarenes are often used as intermediates in the synthesis of various pharmaceuticals, including antihistamines, antibiotics, and antidepressants.
• Agrochemicals: Many herbicides, insecticides, and fungicides are based on haloarene structures due to their bioactivity.
• Polymers and Synthetic Materials: Haloarenes serve as starting materials for the synthesis of various polymers, such as PVC, and in the production of dyes and pigments.
• Organic Synthesis: As intermediates in organic synthesis, haloarenes are valuable in forming carbon-carbon and carbon-heteroatom bonds.

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