Preparations from Haloarenes: Introduction, Electrophilic Substitution Reactions, Reduction, Reaction with aqueous Ammonia and Cuprous Cyanide, Practice problems, Frequently asked questions(FAQs)

Are you conscious of the presence of haloalkanes and haloarenes in the environment?

During the Vietnam War, haloarenes were employed as herbicides to clear jungles of foliage.

These haloarenes are resistant to degradation by bacteria and other microbes. As a result, it has continued to thrive unchanged in the soils of the rainforest to this day.

Hydrocarbons called haloalkanes and haloarenes have one or more hydrogen atoms replaced with halogen atoms. Haloalkanes are formed from open-chain hydrocarbons (alkanes), whereas haloarenes are derived from aromatic hydrocarbons. This is the distinction between the two compounds.

Table of content:

Introduction of Haloarenes
Electrophilic substitution reactions
Reduction reactions of haloarenes
Reaction with aqueous ammonia
Reaction with cuprous cyanide
Practice problems
Frequently asked questions(FAQs)

Introduction of Haloarenes:

When a carbon atom from the aromatic ring is directly bonded to a halogen atom, the result is an aromatic hydrocarbon halogen derivative known as a haloarene. They're referred to as aryl halides. Haloarenes are created when a hydrogen atom that is linked to an aromatic ring is swapped out for a halogen atom.

The systematic names of haloarenes or aryl halides are created by prefixing the name of the aromatic hydrocarbon with fluoro, chloro, bromo, or iodo.

As an example,

Arabic numerals are used to indicate the relative positions of the substituent groups in disubstituted or trisubstituted compounds. In order to produce the lowest number sequence, the series is numbered. The prefixes ortho (o), meta (m), and para (p) can be used to designate the relative locations 1,2, 1,3, and 1,4 in the context of disubstituted derivatives (p).

As an example,

Electrophilic substitution reactions:

An electrophilic substitution reaction is a chemical process in which an electrophile replaces a group on a molecule. An electrophile is a species that seeks for electrons. The displaced group typically consists of an atom of hydrogen. Haloarenes on the benzene ring experience electrophilic substitution through processes like nitration, halogenation, sulphonation, and Friedel-Crafts reactions.

Due to the -I effect (electron-withdrawing nature) of the halogen atom, the benzene ring in haloarenes is slightly deactivated for electrophilic substitution process. Electrophilic substitution occurs at the ortho and para locations with regard to the halogen atom because it is ortho, para-directing.

The benzene ring's ortho and para positions are more electron-rich than the meta position as a result of the delocalization of the halogen atom's electrons. Haloarenes consequently operate as ortho- and para-directing substituents in electrophilic substitution processes.

The -I effect causes the halogen atom to take electrons away from the benzene ring as well. Electrophilic substitution reactions in haloarenes proceed more slowly and necessitate more severe conditions than they do in benzene as a result of this partial ring deactivation.

1. Halogenation:

When haloarenes react with chlorine in the presence of a Lewis acid, they become halogenated (Ferric chloride). As an electrophile, the chlorine molecule will attack the compound's ortho and para positions, which are rich in electrons.

The reaction produces molecules that are both ortho and para. However, the ortho isomer will be the minor product and the para isomer will be the reaction's main byproduct.

2. Nitration:

The NO₂⁺ is the electrophile in the nitration of haloarenes. Nitric acid and sulfuric acid react, resulting in the creation of NO₂⁺ electrophile. Ortho and para compounds are created as a result of the electrophile attacking the electron-rich ortho and para locations. However, the ortho isomer will be the minor product and the para isomer will be the reaction's main byproduct.

3. Sulphonation:

SO3 acts as an electrophile in sulphonation. At Ortho and Para positions, it attacks the electron-rich haloarene. The reaction produces para chlorobenzenesulfonic acid and ortho chlorobenzenesulfonic acid, with the para isomer being the major product and the ortho isomer being the minor product.

4. Friedel-crafts alkylation:

The ortho and para locations of the haloarene attack the alkyl carbocation, which serves as the electrophile in Friedel-Crafts alkylation. Ortho and para compounds are both produced by the process. However, the ortho isomer will be the minor product and the para isomer will be the reaction's main byproduct.

5. Fridel-crafts acylation:

The ortho and para locations of the haloarene attack the acyl carbocation, which serves as the electrophile in Friedel-Crafts alkylation. Ortho and para compounds are both produced by the process. However, the ortho isomer will be the minor product and the para isomer will be the reaction's main byproduct.

Reduction reaction of haloarenes:

By reducing haloarenes with Ni-Al alloy in the presence of alkali, they are converted into their corresponding arenes.

Reaction with aqueous ammonia:

Aniline is created when chlorobenzene is heated with liquid ammonia and cuprous oxide (Cu₂O) at 200⁰C and 60 atmospheres of pressure.

Reaction with Cuprous cyanide:

Benzonitrile is created when chlorobenzene is burned with cupric cyanide (CuCN) at 200⁰C when pyridine is present.

Practice problems:

Q.1. Because of this, the C-Cl bond length in chlorobenzene is shorter than the C-Cl bond length in chloromethane.

(A) partial double bond character in chloromethane
(B) difference in hybridisation of carbon in C-Cl bond
(C) instability of phenyl cation
(D) possible repulsion between nucleophile and chlorobenzene

Answer: (B)

Solution: The C of C-Cl bond in chlorobenzene is SP2 hybridized, whereas the C of C-Cl bond in chloromethane is SP3 hybridized. As a result, chlorobenzene has more s character in the carbon of the C-Cl bond, which holds the electron pair of the C-Cl bond more tightly.

Q.2. When heated with aqueous NaOH solution, which of the following compounds is most easily converted to a phenol?

(A) Chlorobenzene
(B) 4-Chloronitrobenzene
(C) 2,4-Dinitrochlorobenzene
(D) 2,4,6-Trinitrochlorobenzene

Answer: (D)

Solution: The haloarene's reactivity is increased in the nucleophilic substitution reactions by the presence of electron withdrawing groups like -NO2 in ortho and para locations with respect to the halogen. As -NO2 group is not present in chlorobenzene , hence it transforms into phenol at high temperatures and pressures.

Q.3. A _______ reaction occurs when an electron-rich species stronger than the halide approaches the partly positive carbon atom of a haloalkane and establishes a new bond with the carbon atom, displacing the halogen in the process.

(A) displacement
(B) electrophilic substitution
(C) nucleophilic substitution
(D) elimination

Answer: (C)

Solution: Nucleophiles are electron-rich species, and a nucleophilic substitution reaction occurs when a stronger nucleophile replaces an existing one.

Q.4. What is the correct boiling point order for isomeric dichlorobenzenes?

(A) ortho>meta>para
(B) para>meta>ortho
(C) para>ortho>meta
(D) meta>ortho>para

Answer: (C)

Solution: The main criterion for determining melting point order is molecular packing. P-Dichlorobenzene is symmetrical and fits easily into a crystal lattice. As a result, intermolecular forces of attraction are stronger than in the ortho and meta isomers. Because the ortho and meta isomers structures are not as symmetrical and do not form a close lattice structure, the intermolecular forces of attraction are weaker. Because the para isomer has strong intermolecular forces of attraction, it takes more energy to melt and thus has a higher melting point than the ortho and meta isomers.

Frequently asked questions(FAQs):

1. Why do haloarenes undergo electrophilic substitution reactions?
Answer: The halogen atom in haloarenes uses the resonance effect to transfer electrons to the benzene nucleus, which increases the electron density at ortho and para positions. As a result, the electrophile attacks both the ortho and para positions, resulting in electrophilic substitution reactions in haloarenes.

2. Why is nucleophilic substitution in haloarenes difficult?
Answer: The C-X bond in haloarenes acquires partial double bond character due to the resonance effect of halogens attached to the ring and undergoes C-X bond length shortening. It strengthens the C-X bond and makes haloarenes more stable. Because of this, cleaving C-X bonds in haloarenes is much more difficult than cleaving C-X bonds in haloalkanes.

3. Why is para product major compared to ortho product during halogenation of chlorobenzene?
Answer: The electron-donating groups are the ortho and para directing groups. They direct the attachment of substituents at the ortho and para positions. They are also referred to as deactivating groups. In the resonance structures of chlorobenzene the ortho and para positions get negatively charged, i.e. the electron density is relatively more at ortho and para positions. The incoming electrophile is more likely to attack these positions.

The ortho product is known as the minor product and the para product is known as the major product. when an electrophilic substitution reaction takes place and produces the ortho and para products. The steric hindrance is the reason behind this. The presence of the groups near to each other in the ortho product creates an hindrance and the group tries to attach at the para position. As a result, the para product predominates the ortho product.

4. Why do haloarenes repel nucleophiles?
Answer: In haloarenes, cleaving the bond and replacing it with the nucleophile is difficult due to sp2 hybridisation and the partial double bond character of the C–X bond. Arenes are electron-rich compounds, and as a result, they repel nucleophiles attacking them.