Written By Sahana Soma Kodarkar
Last Modified 22-06-2023

Haloarenes: Definition, Properties, Uses, Preparation

Are you aware that haloalkanes and Haloarenes can be found in the environment? During the Vietnam War, haloarenes were employed as herbicides to defoliate the jungles. Microorganisms such as bacteria are unable to break down these haloarenes. As a result, it has remained unaltered in the rainforest soils to this day.

Hydrocarbons having one or more hydrogen atoms substituted by halogen atoms are known as haloalkanes and haloarenes. Haloalkanes and haloarenes are distinguished by the fact that haloalkanes are formed from open-chain hydrocarbons (alkanes), whereas haloarenes are derived from aromatic hydrocarbons. To know more about haloarenes, read the below article.

Define Haloarenes

The halogen derivatives of aromatic hydrocarbons having the halogen atom directly attached to a carbon atom of the aromatic ring are called haloarenes. They are also referred to as aryl halide. Haloarenes are obtained when a halogen atom replaces a hydrogen atom attached to an aromatic ring.

\({\text{Ar}} – {\text{H}} + {\text{X}} \to {\text{Ar}} – {\text{X}} + {\text{H}}\)

For example,

The General formula for haloarene is \({\text{Ar}} – {\text{X,}}\) where \({\text{Ar}}\) represents an aryl group and \({\text{X}}\) is a halogen atom.

Some examples of Haloarenes are as follows:

Learn Haloarenes and Phenols Here

Nomenclature of Haloarenes

1. The systematic names of haloarenes or aryl halides are derived by adding prefixes fluoro, chloro, bromo, or iodo before the name of the aromatic hydrocarbon. For example,

2. The relative positions of the substituent groups are denoted by Arabic numerals in disubstituted or trisubstituted compounds. The numbering is done in such a way that the series yields the lowest number sequence. The relative positions \(1,2;1,3;\) and \(1,4\) can alternatively be represented by the prefixes ortho \(\left({O – } \right),\) meta \(\left({{\text{m}} – } \right),\) and para \(\left({{\text{p}} – } \right),\) in the case of disubstituted derivatives

For example,

Preparation of Haloarenes

The \( – {\text{OH}}\) group of phenol cannot be easily substituted because the \({\text{C}} – {\text{O}}\) bond in phenols has a partial double bond character due to resonance and is thus stronger and more difficult to cleave. As a result, haloarenes are not synthesized from phenols.

The following are some methods of haloarenes preparation:

1. From Arenes: Direct halogenation of benzene in the presence of halogen carrier or Lewis acids such as \({\text{FeC}}{{\text{l}}_3},{\text{AlC}}{{\text{l}}_3},\) and others can produce haloarenes. Iodine and iron filings, in addition to these, can be utilised as halogen carriers. The reaction is catalyzed by resonance stabilized carbocation and involves electrophilic substitution.

2. From diazonium Compounds (By Sandmeyer’s Reaction): The method involves treating primary aromatic amines with nitrous acid at low temperatures \(\left( {0^\circ – 5\,^\circ {\rm{C}}} \right)\) until a diazonium salt is formed. The process is referred to as diazotization.

\({\text{NaN}}{{\text{O}}_2} + {\text{HCl}} \to {\text{HN}}{{\text{O}}_2} + {\text{NaCl}}\)

The resulting diazonium compound is treated with \({\text{CuCl}}\) and \({\text{HCl}}\) or \({\text{CuBr}}\) to give the corresponding haloarenes.

The reaction of diazonium salts with \({\text{CuCl}}\) and \({\text{CuBr}}\) in the presence of corresponding halogen acids is known as Sandmeyer’s reaction.

3. From Benzene (Commercial Method): Chlorobenzene is manufactured by passing a mixture of benzene, hydrogen chloride and oxygen over \({\text{CuC}}{{\text{l}}_2}\) as a catalyst at \(525\,{\text{K}}.\) This process is known as the Raschig Process.

Properties of Haloarenes

The physical and chemical properties of haloarenes are explained below:

Physical Properties of Haloarenes

Physical state and colour: Haloarenes are either colourless liquid or crystalline solids.
Solubility: They are insoluble in water but soluble in organic solvents such as ethyl alcohol, diethyl ether, etc. Haloarenes have low polarity. Therefore, they are insoluble in water. Due to low polarity, they cannot form hydrogen bonds with the water molecule and cannot break the hydrogen bonds present in water molecules.
Density: They are heavier than water. The density of haloarenes increases with an increase in the atomic number of the halogen atom.

For example,

Haloarene	\({{\text{C}}_2}{{\text{H}}_5}{\text{F}}\)	\({{\text{C}}_2}{{\text{H}}_5}{\text{Cl}}\)	\({{\text{C}}_2}{{\text{H}}_5}{\text{Br}}\)	\({{\text{C}}_2}{{\text{H}}_5}{\text{I}}\)
Density \(\left({{\text{g/mL}}\,{\text{at}}\,293\,{\text{K}}} \right)\)	\(1.031\)	\(1.112\)	\(1.495\)	\(1.838\)

3. Melting Point and Boiling Point: The melting points and boiling points of mono haloarene increases with an increase in the size of the halogen atom, provided the aryl group is the same.

Chemical Properties of Haloarenes

Haloarenes are chemically less reactive than haloalkanes. The low reactivity of haloarenes and their important chemical reactions are as follows.

1. Reactivity of Haloarenes

Haloarenes contains \({\text{C}} – {\text{X}}\) bond and possess polarity but are less reactive towards nucleophilic substitution reactions than the haloalkanes.

Following are the factors which are responsible for the low reactivity of haloarenes:

I. Resonance stabilisation: Haloarenes undergo resonance stabilisation. The \({\text{C}} – {\text{X}}\) bond present in them acquires partial double bond character and undergo shortening of \({\text{C}} – {\text{X}}\) bond length. It increases the strength of \({\text{C}} – {\text{X}}\) bond and imparts stability of haloarenes. This is why cleavage of \({\text{C}} – {\text{X}}\) bonds in haloarenes is much more difficult than the cleavage of \({\text{C}} – {\text{X}}\) bonds in haloalkanes. This is why haloarenes are less reactive towards nucleophilic substitution reactions.

II. Hybridisation of the carbon of \({\text{C}} – {\text{X}}\) bond: The carbon atoms of the \({\text{C}} – {\text{X}}\) bond in haloarenes are in a state of \({\text{s}}{{\text{p}}^2}\) hybridised. Because the \({\text{s}}{{\text{p}}^2}\) hybridised carbon atom has a more \({\text{s}}\)-character, it is more electronegative. The shared electron pair is held more securely in haloarenes due to the more electronegative \({\text{s}}{{\text{p}}^2}\) hybridised carbon. This shortens the \({\text{C}} – {\text{X}}\) bond length and strengthens the \({\text{C}} – {\text{X}}\) bond in haloarenes once again. As a result, the cleavage of \({\text{C}} – {\text{X}}\) bond in haloarenes becomes more difficult, and haloarenes show lesser reactivity towards nucleophilic substitution reaction.

III. The polarity of \({\text{C}} – {\text{X}}\) bond: The low polarity of \({\text{C}} – {\text{X}}\) bond in haloarenes is responsible for the lesser reactivity. The \({\text{s}}{{\text{p}}^2}\) hybridised carbon in haloarenes, being more electronegative, has a lesser tendency to release electrons to the halogen atom. As a result, the \({\text{C}} – {\text{X}}\) bond in haloarenes is less polar. The lesser polarity of \({\text{C}} – {\text{X}}\) bond in haloarenes makes the release of halide ions more difficult. Thus, the attack of the nucleophile on the \({\text{C}} – {\text{X}}\) bond becomes less pronounced. This is why haloarenes are less reactive towards nucleophilic substitution reactions.

Chemical Reactions of Haloarenes

1. Nucleophilic substitution reaction: The halogen atom in haloarenes is firmly linked to the benzene ring and is resonance stabilized. As a result, nucleophiles like \({\text{O}}{{\text{H}}^ – },{\text{C}}{{\text{N}}^ – }\) etc. cannot easily displace the halogen atom. In comparison to haloalkanes, haloarenes are substantially less reactive towards nucleophilic substitution reactions. However, they can be compelled to undergo nucleophilic substitution reactions under drastic conditions.
Some examples are given below.

Substitution by hydroxyl group: When a haloarene is heated at \(623\,{\text{K}}\) under \(300\) atmosphere with an aqueous sodium hydroxide solution, the halogen atom is replaced by a hydroxyl group, yielding phenoxide. When phenoxide is acidified with dilute hydrochloric acid, phenol is formed.

2. Electrophilic substitution reaction: Haloarenes undergo electrophilic substitution reactions in the benzene ring. Electrophiles target the electron-rich benzene ring of haloarenes because it acts as a source of electrons. In haloarenes, electrophilic substitution occurs at the \({\text{o}}\)- and \({\text{p}}\)-positions relative to the halogen atom. Halogenation, nitration, sulphonation, and Friedel-Crafts reactions are some of the most typical electrophilic substitution reactions revealed by haloarenes.

a. Halogenation: When haloarenes are treated with a halogen in the presence of a halogen carrier, \({\text{o}}\)- and \({\text{p}}\)- di haloarenes are obtained.
For example, when chlorobenzene is treated with chlorine, in the presence of ferric chloride, ortho-dichlorobenzene and para-dichlorobenzene (major product) are formed.

b. Friedel-Crafts reactions: Haloarenes undergo Friedel-crafts alkylation and acylation in the presence of anhydrous aluminium chloride. The substitution reaction takes place at ortho and para positions.

Friedel-craft Alkylation

Friedel-craft Acylation

3. Reaction with Metals: Haloarenes reacts with certain metals and forms different types of compounds. Some of the important reactions of haloarenes with metals are given below;

a. Fitting reaction: The halogen atom of haloarene is replaced by the aryl group. A diaryl is generated when it is heated with sodium metal in the presence of dry ether. This reaction is known as a fitting reaction.
For example, When chlorobenzene is heated with sodium metal, diphenyl is formed.

b. Wurtz-Fitting reaction: When a halogen atom of haloarenes 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, forming a higher arene.

For example,

4. Reduction reactions: Haloarenes can be converted to hydrocarbons by reducing them using a nickel-aluminium alloy in the presence of an alkali.
For example, Chlorobenzene is converted to benzene when it combines with alkali in the presence of nickel-lithium alloy.

Uses of Haloarenes

Haloarenes are used in the manufacture of several compounds. For example, chlorobenzene is used in the manufacture of phenol, picric acid, DDT, etc.
Picric acid is used as an antiseptic and as a dye.
DDT is used as an insecticide for mosquitoes, flies, moths and agricultural pests.

Summary

Haloarenes are obtained when a halogen atom replaces a hydrogen atom attached to an aromatic ring. They cannot be easily substituted for phenols as the \({\text{C}} – {\text{O}}\) bond is stronger and more difficult to leave. They are used to manufacture several compounds, including picric acid, DDT, chlorobenzene, and picric Acid.

FAQs

Q.1. What are Haloarenes examples?
Ans: Examples of haloarenes are chlorobenzene, iodobenzene, bromobenzene.

Q.2. What are Haloarenes and Haloalkanes?
Ans: The halogen derivatives of aromatic hydrocarbons having the halogen atom directly attached to a carbon atom of the aromatic ring are called haloarenes.
Haloalkanes are a class of chemical compounds that comprise of an alkane with one or more hydrogen atoms replaced by halogen atoms.

Q.3. What is the classification of Haloalkanes?
Ans: Haloalkanes are classified into monohaloalkanes (one halogen atom), dihaloalkanes (two halogen atoms), trihaloalkanes (three halogen atoms), and polyhaloalkanes based on the number of halogens present. They are also classified as primary, secondary, and tertiary haloalkanes based on the nature of the \({\text{C}} – {\text{X}}\) bond.

Q.4. Why are Haloarenes more stable than Haloalkanes?
Ans: Haloarenes are more stable because they can donate their lone pair of electrons inside the rings for resonance.

Q.5. Which is more reactive, Haloalkanes and Haloarenes?
Ans: Haloalkanes are more reactive than haloarenes.

Q.6. Why are haloarenes less reactive?
Ans: The \({\text{s}}{{\text{p}}^2}\) hybridised carbon in haloarenes being more electronegative has a lesser tendency to release electrons to the halogen atom. As a result, the \({\text{C}} – {\text{X}}\) bond in haloarenes is less polar. The lesser polarity of \({\text{C}} – {\text{X}}\) bond in haloarenes makes the release of halide ions more difficult. The low polarity of \({\text{C}} – {\text{X}}\) bond in haloarenes is responsible for the lesser reactivity.

Q.7. Why do haloarenes undergo electrophilic substitution?
Ans: The halogen atom in haloarenes releases electrons to the benzene nucleus through the resonance effect, which is relatively electron-deficient compared to the halogen atom. As a result, the electrophile attacks in both the ortho and para positions, making electrophilic substitution reactions occur in haloarenes.

Q.8. Why are haloarenes insoluble in water?
Ans: Haloarenes have low polarity. Therefore, they are insoluble in water. Due to low polarity, they cannot form hydrogen bonds with the water molecule and cannot break the hydrogen bonds present in water molecules.

Q.9. Why are haloarenes not prepared from phenol?
Ans: The \( – {\text{OH}}\) group of phenol cannot be easily substituted because the \({\text{C}} – {\text{O}}\) bond in phenols has partial double bond character due to resonance and is thus stronger and more difficult to cleave. As a result, haloarenes are not synthesised from phenols.

Q.10. Why are haloarenes ortho para directing?
Ans: Haloarenes are ortho para directing. It is because the halogen atoms present on the benzene ring in haloarenes are ortho and para directing groups. Due to resonance, it increases the electron density slightly at the ring’s \({\text{o}}\)– and \({\text{p}}\)- positions. Thus, due to resonance, the \({\text{o}}\)- and \({\text{p}}\)- positions in haloarenes are relatively higher electron density centres than \({\text{m}}\)-positions.

Q.11. Why is nucleophilic substitution difficult in haloarenes?
Ans: The \({\text{C}} – {\text{X}}\) bond present in haloarenes acquires partial double bond character and undergo shortening of \({\text{C}} – {\text{X}}\) bond length. It increases the strength of \({\text{C}} – {\text{X}}\) bond and imparts stability of haloarenes. This is why cleavage of \({\text{C}} – {\text{X}}\) bonds in haloarenes is much more difficult than the cleavage of \({\text{C}} – {\text{X}}\) bonds in haloalkanes.

Students can take haloalkanes and haloarenes notes from the above-discussed questions to revise the concept quickly.