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Reactions of Haloarenes

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Are you aware that haloarenes can be produced by marine organisms? Haloarenes can be produced by marine organisms that can utilize the chloride and bromide found in ocean waters. They’ve been recognized to offer a variety of therapeutic qualities. As a result, both artificially and naturally, haloarenes undergo a variety of reactions. Let’s look at the concept Reactions of Haloarenes in more detail !

Haloarenes

Haloarenes are aromatic hydrocarbon halogen derivatives with the halogen atom bonded directly to a carbon atom of the aromatic ring. Aryl halides is another name for them. When a hydrogen atom linked to an aromatic ring is replaced with a halogen atom, haloarenes are formed.

Ar – H + X     ⇢    Ar – X + H

Haloarene has the general formula Ar–X, where Ar denotes an aryl group and X denotes a halogen atom.

Reactions of Haloarenes

Haloarenes or aryl halide reactions can be classified into three categories :

  1. Electrophilic Substitution Reactions
  2. Nucleophilic Substitution Reactions
  3. Reaction with metals

Electrophilic Substitution Reactions of Haloarenes

A species that seek electrons is known as an electrophile. In an organic compound, an electrophile replaces another electrophile in an electrophilic substitution process. The electrophilic processes of the benzene ring, such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions, are all carried out on haloarenes. We’ll go through each one individually, but first, let’s look at how Haloarenes react to an electrophile’s attack :

  1. The benzene ring becomes slightly deactivated towards electrophilic substitution reactions due to the halogen’s electron-withdrawing tendency.
  2. There is more electron or negative charge in the ortho- and para- positions of the ring than in the meta-position due to its varied resonant patterns. As a result, haloarenes operate as both an o- and p- directive for electrophilic substitution reactions.

Because of the aforementioned factors, haloarenes are less active in electrophilic substitution reactions than regular benzene rings. As a result, compared to benzene, these reactions are slower and necessitate more extreme conditions.

  • Halogenation: Haloarenes react with chlorine in the presence of a solvent to produce haloarenes (say ferric chloride). The chlorine molecule has a small positive charge and tends to become polar. As a result, chlorine works as an electrophile, attacking the compound’s electron-rich Ortho and para positions.

Halogenation

Both ortho and para compounds will be formed as a result of this reaction. The reaction’s principal result will be the para isomer, while the minor product will be the ortho isomer. Chlorobenzene interacts with Lewis acid to produce ortho and para replacements of dihalo-benzenes in the following example.

  • Nitration: Because of the existence of two electronegative oxygen atoms in the molecule, NO2 is generated first from nitric acid, which is begun by the presence of sulphuric acid; NO2 has an electrophilic centre over N. The electron-rich ortho and para locations are attacked by NO2, yielding the para isomer as the main product and the ortho isomer as the minor product.

Nitration

Sulphonation: SO3 is an electrophile during sulphonation. At the ortho and para locations, it targets the electron-rich haloarene. Para and Ortho Chloro-benzene sulphonic acids are formed as a result of the reaction, with the para isomer being the predominant product and the ortho isomer being the minor.

Sulphonation

  • Friedel-Crafts Reactions: Because of the positive charge present in the carbon atom, the electrophile in this circumstance is the alkyl and acetonic group. There are two types of Friedel-Crafts Reactions :

Friedel-Crafts Alkylation Reactions:

Friedel-Crafts Alkylation

Friedel-Crafts Acylation Reactions:

Friedel-Crafts Acylation

Nucleophilic Substitution Reactions of Haloarenes

When it comes to haloarenes, nucleophilic substitution reactions are tricky. However, under some conditions, haloarenes undergo a nucleophilic substitution process. The following are the main causes for haloarenes reduced or nonreactive character in nucleophilic substitution reactions :

  • The hydroxyl group acts as a substitute: The halogen atom is replaced by a hydroxyl group when a haloarene is heated to 623K under 300 atmosphere with an aqueous sodium hydroxide solution, generating phenoxide. When dilute hydrochloric acid is used to acidify phenoxide, phenol is produced.

  • Effect of Resonance: The benzene ring’s π – electrons conjugate with the halogen atom in the haloarene structure in the case of haloarenes. The C-X bond develops partial double bonds as a result of this resonance. The haloarene has a more difficult partial double bond cleavage than the haloalkane. As a result, haloarenes are difficult to cleave by a nucleophile and have a lower reactivity in nucleophilic substitution reactions.

Example of Resonance

In a C-X bond, the difference in carbon atom hybridization. In the case of haloarenes, the member of the halogen group is bound to an sp2 hybridized carbon atom. In haloalkanes, on the other hand, the halogen is bound to the sp3 hybridized carbon atom. Compared to sp3C, sp2C has a stronger s character. sp2C is, therefore, more electronegative than sp3C.

As a result, sp2C is better at removing electrons from the C-X bond and retaining them close to itself. As a result, the bond length between haloarenes and haloalkanes is shorter. Bonds that are shorter in length are stronger. In haloalkane, for example, the C—Cl bond length is 177 pm, while in haloarene it is 169 pm.

Reaction of Haloarenes with Metals

Metals react with haloarenes in a limited way. There are two primary reactions :

  • Fitting Reaction: The aryl group substitutes for the halogen atom in haloarene. When sodium metal is heated in the presence of dry ether, a diaryl is produced. A fitting reaction is what we call this.

Fitting Reaction

In this reaction, dry ether is used to react a mixture of haloarenes with sodium. Diaryl is the end result.

  • Wurtz-Fitting Reaction: When a halogen atom of a haloarene is heated in the presence of sodium in an ethereal solution of an alkyl halide, the halogen atoms of the haloarene are replaced by the alkyl group, resulting in the formation of a higher arene.

Wurtz-Fitting Reaction

In the presence of dry ether and sodium, a combination of alkyl halides interacts with an aryl halide. Alkyl arene is the finished product.

Sample Questions

Question 1: Define Haloarenes.

Answer:

Haloarenes are aromatic hydrocarbon halogen derivatives with the halogen atom bonded directly to a carbon atom of the aromatic ring. Aryl halides is another name for them. When a hydrogen atom linked to an aromatic ring is replaced with a halogen atom, haloarenes are formed.

Question 2: What causes electrophilic substitution of haloarenes? 

Answer:

Through the resonance effect, the halogen atom in haloarenes transfers electrons to the benzene nucleus, which is electron-deficient in comparison to the halogen atom. Electrophilic substitution reactions occur in haloarenes as a result of the electrophile attacking at both the ortho and para positions.

Question 3: Why is nucleophilic substitution in haloarenes so difficult?

Answer:

In haloarenes, the C–X bond takes on a partial double bond character and shortens. It boosts the C–X bond’s strength and gives haloarenes more stability. Cleavage of C–X bonds in haloarenes is substantially more difficult than in haloalkanes because of this.

Question 4: Explain the Mechanism of Aromatic Nucleophilic Substitution Reaction.

Answer:

The π – electrons migrate in such a way that the electron density in the benzene ring delocalizes at Ortho- and Para- positions as a nucleophile approaches and attacks the C-X bond. If an electron withdrawing group is present at the Ortho and Para locations of the benzene ring in this situation, it will withdraw the negative charge on the carbon atom, stabilizing the negative charge.

As a result, the presence of electron withdrawing groups like NO2 at the Ortho and Para positions aids the nucleophile’s attack. Furthermore, resonance as well as an electron withdrawing group like NO2 help to stabilize the carbocation.

The reaction will proceed in a sequence of fast and slow steps, resulting in the production of a high resonance stabilized sigma complex with a high resonance. Finally, when the negative charge delocalizes due to the elimination of the Cl bond, the π – electrons will be recovered. The final step is to form the product.

Question 5: Why do haloarenes have an ortho para directional effect?

Answer:

Ortho para directing is haloarenes. Because the halogen atoms on the benzene ring in haloarenes are ortho and para directing groups on the benzene ring, this is the case. The electron density at the o– and p– positions of the ring increases slightly due to resonance. The o- and p-positions in haloarenes have greater electron density centers than the m-positions due to resonance.

Question 6: What types of Friedel-Crafts reactions are there?

Answer:

There are Two types of Friedel-Crafts Reactions :

  1. Friedel-Crafts Alkylation Reactions
  2. Friedel-Crafts Acylation Reactions

Because of the positive charge present in the carbon atom, the electrophile in this circumstance is the alkyl and acetonic group.

Friedel-Crafts Alkylation Reactions:

Friedel-Crafts Alkylation

Friedel-Crafts Acylation Reactions:

Friedel-Crafts Acylation


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Last Updated : 14 Mar, 2022
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