Haloalkanes and Haloarenes

Haloalkanes and haloarenes are hydrocarbons that have had one or more hydrogen atoms replaced with halogen atoms. The major distinction between haloalkanes and haloarenes is that the former are formed from open-chain hydrocarbons (alkanes), whilst the latter are derived from aromatic hydrocarbons.

Haloalkanes are typically known as alkyl halides, whereas haloarenes are known as aryl halides. Multiple halogen atoms can be found in these substances. In general, halogen atoms are connected to sp³ hybridized carbon atoms in haloalkanes, whereas sp² hybridized carbon atoms are attached to haloarenes. The variation in the hybridization state of the carbon atom in the C-X bond is responsible for the two families’ distinct properties. Haloalkanes and haloarenes are more chemically reactive than parent alkanes and aromatic compounds due to the presence of halogens. These chemicals have a variety of medical applications as well.

Classification of Haloalkanes and Haloarenes

Haloalkanes and Haloarenes can be classified as:

• Based on the Number of Halogens
• Based on the Hybridization of Carbon
• Based on the Nature of Carbon-Halogen Bond

Let’s understand these classifications in detail as follows:

Based on the Number of Halogens

Based on the number of halogens present in an organic compound, it can be classified as 

• Mono Haloalkanes and Mono Haloarenes: These are molecules with only one halogen atom.
• Poly Haloalkanes and Poly Haloarenes: These are molecules that include two or more halogen atoms. These are further classified into the three types listed below:
  • Di Haloalkanes and Di Haloarenes: These compounds contain two Halogens.
  • Tri Haloalkanes and Tri Haloarenes: These compounds contain three Halogens.
  • Tetra Haloalkanes and Tetra Haloarenes: These compounds contain four Halogens.
  • and so on, we can define these further as penta, hexa, septa, octa, nona, and deca haloalkanes and haloarenes.

Based on the Hybridization of Carbon

Haloalkanes and Haloarenes are categorized into two categories based on the hybridization of the Carbon atom to which Halogen is bonded.

• Attachment of halogen to sp3 hybridized carbon: On this basis, Alkyl Halide and Aryl Halide can be divided into three types:
  • Alkyl Halide: Halogen connected to an alkyl chain is referred to as an alkyl halide.
  • Allylic Halide: A halogen linked to the sp³ hybridized carbon next to C=C.
  • Benzylic Halide: Halogen linked to the benzene ring via the sp³ hybridized carbon.

• Attachment of halogen to sp² hybridized carbon: On this basis, alkyl halide and aryl halide can be divided into three types:
  • Vinyl Halide [RCH=CHX] is a halogen that has been bonded to sp² hybridized carbon.
  • Aryl Halide is an aromatic ring with halogen linked to sp² hybridized carbon.

Based on the Nature of Carbon-Halogen Bond

Haloalkanes and haloarenes are categorized into two categories based on the number of halogen atoms on an alkyl or aryl halide molecule –

• Primary Alkyl Halide: a halogen atom is joined to a primary carbon atom.
• Secondary Alkyl Halide: a halogen atom linked to a secondary carbon atom.
• Tertiary Alkyl Halide: a halogen atom joined to a tertiary carbon atom.

Nomenclature of Haloalkanes and Haloarenes

To name haloalkanes and haloarenes, various nomenclature systems are used, including the IUPAC (International Union of Pure and Applied Chemistry) system and common names.

IUPAC Nomenclature

The IUPAC nomenclature system is the most widely accepted system for naming organic compounds. According to this system, the name of a haloalkane and haloarene consists of the prefix indicating the halogen and the name of the parent alkane, followed by the suffix -ane and -benzene in the case of arenes. 

For example, the IUPAC name of CH₃Cl is 1-chloromethane, the IUPAC name of CH₃CH₂Br is 1-bromoethane, and the IUPAC name of C₆H₅Cl is 1-chlorobenzene.

Learn more about the Nomenclature of Organic Compounds.

Common Names

Common names are an alternative nomenclature system that is often used for simple haloalkanes and haloarenes. In this system, the halogen is named as a suffix is used and preceded by the name of the parent alkane or arene. For example, CH₃Cl is commonly known as methyl chloride, and C₆H₅Cl is commonly known as phenyl chloride.

Preparation of Haloalkanes

Haloalkanes can be prepared using many methods, some of these methods are as follows:

From Alcohols

Alcohols can be converted to alkyl halides by replacing the hydroxyl group with a halogen using concentrated halogen acids, phosphorus halides, or thionyl chloride. Thionyl chloride is preferred because it produces pure alkyl halides and escapes as gases SO² and HCl. A zinc chloride catalyst is required for primary and secondary alcohols, while tertiary alcohols can react with HCl at room temperature. 

Constant boiling with HBr is used for preparing alkyl bromide, and heating with sodium or potassium iodide in orthophosphoric acid yields R—I. 

Note: Alcohol’s reactivity with haloacid follows the order of 3°>2°>1°.

From Hydrocarbons

Haloalkanes are formed using alkanes as well as alkenes, the reaction and explanation for the process is discussed as follows:

From Alkanes

Haloalkanes are formed when alkanes went through free radical halogenation in the presence of light.

From Alkenes

Haloalkanes can be formed using Alkenes by electrophilic addition reaction, where alkenes react with HX to form RX. The order of reactivity of alkenes with halides is HI > HBr > HCl > HF.

From Halogen Exchange (Finkelstein Reaction)

The reaction of Alkly halide with sodium iodide to form alkyl iodide in the presence of dry acetone is called the Finkelstein Reaction.

Preparation of Haloarenes

There are various methods of preparation of haloarenes, some of the most important ones are discussed as follows:

From Arenes

Haloarenes can be prepared by the substitution of one or more hydrogen atoms in an alkane or arene by a halogen atom using halogenating agents, such as chlorine, bromine, or iodine.

From Amines (Sandmeyer’s reaction)

The reaction between a primary aromatic amine and sodium nitrite in cold aqueous mineral acid produces a diazonium salt. Cuprous chloride or cuprous bromide can be used to replace the diazonium group with Cl or Br, respectively.

Properties of Haloalkanes and Haloarenes

There are various physical and chemical properties shown by the Haloalkanes and Haloarenes, some of those properties are as follows:

Physical Properties

• Haloalkanes are colorless, odorless, and hydrophobic, and are relatively heavier than alkanes.
• Haloarenes are normally colorless or crystalline solids and are heavier than water.
• Melting Point: The melting point of a compound depends on the strength of its lattice structure and follows a similar trend to the boiling point.
• Boiling Point: The boiling point and melting point of haloarenes are similar to those of alkyl halides containing the same number of carbon atoms.
  • Monohalogen derivatives of benzene have a boiling point in the order Iodo > Bromo > Chloro > Fluoro.
• Density: The density of haloalkanes increases down the homologous series and fluoro derivatives are less dense than chloro derivatives.
• Dipole Moment: The dipole moment of haloalkanes and haloarenes depends on the difference in electronegativity of carbon and halogens, and the electronegativity of halogens decreases down the group.
  • Electronegativity of Halides
    • F > Cl > Br > I
  • Bond Length
    • C-F < C-Cl < C-Be < C-I
  • Bond Dipole
    • C-Cl > C-F > C-Br > C-I
    • 1.56    1.51    1.48  1.29
• Haloarenes are less reactive than haloalkanes due to resonance effects and differences in the hybridization of the C-X bond.
• All halogen compounds are less inflammable than hydrocarbons, and the inflammability decreases with an increase in halogen content.
• The boiling points of chlorides, bromides, and iodides are higher than those of hydrocarbons of comparable molecular mass.
• The dipole moment of haloarenes increases as the number of halogen atoms increases.
• Haloalkanes and haloarenes are slightly soluble in water but dissolve in organic solvents.
• Boiling points of haloarenes increase as the number of halogen atoms increases, and density increases in the order 
  • Ar-I > Ar-Br > Ar-Cl > Ar-F.
• The para-isomers of haloalkanes have a higher melting point than their ortho and meta isomers due to their symmetry fitting better in the crystal lattice.

Chemical Properties

Haloalkanes are a highly reactive class of organic compounds due to the presence of a polar carbon-halogen bond. They can undergo several types of reactions, including 

• Nucleophilic Substitution
• Elimination Reaction
• Reaction with Metals
• Reduction

Let’s discuss this reaction in detail.

Nucleophilic Substitution Reactions

Haloalkanes react with nucleophilic reagents due to the partially positively charged carbon atoms in the C^δ+ – X^δ- bond. These reactions include:

Hydrolysis: Haloalkanes react with aqueous KOH or moist Ag₂O/H₂O to form alcohols.

Ambident Nucleophiles: Nucleophiles such as cyanide and nitrite ions can attack the nucleophilic center from two sides.

The following illustration shows important Nucleophilic Substitution Reactions of haloalkanes.

Elimination Reactions

Haloalkanes can undergo elimination reactions when treated with alcoholic KOH or alcoholic NaOH, resulting in the formation of alkenes.

Reaction with Metals

Haloalkanes react with metals such as Na, Mg, Zn, and Al in the presence of ether to form corresponding alkyl-substituted metals.

Reaction with Sodium Metal: When alkyl halides are treated with sodium in the presence of dry ether, higher alkanes are formed. If a mixture of two different alkyl halides is treated with sodium in the presence of dry ether, a mixture of alkanes is obtained. Self-coupling products are formed in preference to cross-coupling products.

Reaction with Magnesium Metal: When an alkyl halide is treated with pure and dry magnesium in the presence of pure and dry ether, an alkyl magnesium halide known as a Grignard reagent is formed. This reagent is used in the preparation of a large number of organic compounds.

Reduction

Haloalkanes can be reduced to form alkanes by treating them with reducing agents such as LiAlH₄ or NaBH₄.

Reactions of Haloalkanes and Haloarenes

Haloalkanes and Haloarenes are reactive in nature, so they give many reactions with different substances, some of these reactions are as follows:

Substitution Reactions

When one component of one compound is substituted in the process of reaction by a more reactive component, this reaction is called Substitution Reaction. The following reactions are some examples of Substitution Reactions,

Freidel – Crafts Reaction

Friedel-Craft’s reaction is the addition of an alkyl group or an acyl group into a benzene ring. In this reaction, chlorobenzene is treated with methyl chloride in the presence of anhydrous aluminum chloride, forming a mixture of 1-Chloro-2-methylbenzene and 1-Chloro-4-methylbenzene. Chlorobenzene also reacts with acetyl chloride in the presence of anhydrous aluminum chloride to form o- and p-Chloroacetophenones.

Williamson Ether Synthesis: Haloalkanes react with sodium alkoxide to form corresponding ethers.

Elimination Reactions

An elimination reaction is a type of organic chemical reaction where two atoms or groups are removed from an adjacent carbon atom in a molecule to form an unsaturated compound. Dehydrohalogenation is an example of an elimination reaction where alkyl halides undergo the removal of a hydrogen atom from the β-carbon and a halogen atom from the α-carbon to form an alkene.

Dehydrohalogenation Reaction

When alkyl halides are heated with an alcoholic solution of potassium or sodium hydroxide, they undergo dehydrohalogenation, and alkenes are formed. Saytzeff’s rule states that the preferred product in a dehydrohalogenation reaction is the alkene with the greater number of alkyl groups attached to the doubly bonded carbon atoms. This means that a more substituted double bond is formed.

Wurtz-Fittig Reaction

As Wilhelm Rudolph Fittig extended the work of Charles Adolphe Wurtz, hence the name Wurtz Fitting Reaction. In this reaction, aryl halides react with alkyl halides and sodium metal in the presence of dry ether to give substituted aromatic compounds.

Uses of Haloalkanes and Haloarenes

These chemicals have a variety of useful applications, which are described below.

• Because these organic compounds may dissolve non-polar substances, they are utilized as solvents.
• Many alkyl and aryl halide compounds are employed in medicine. One such example is the antibiotic chloramphenicol, which is used to treat typhoid fever.
• Another example is chloroquine, which is extremely effective in treating malaria.
• Dichlorodiphenyltrichloroethane (commonly known as DDT) is a pesticide.
• Some haloalkanes and haloarenes have negative environmental impacts and are classified as contaminants. One such example is chlorofluorocarbons (or CFCs), which contribute to the depletion of the ozone layer, which shields the Earth from dangerous solar radiation.

Environmental Effects

These chemicals are widely used in commercial applications. However, halocarbons have been connected to major pollutants and poisons that have a negative impact on the ecosystem. CFC (chlorofluorocarbon), for example, is a well-known contributor to ozone depletion in the atmosphere. Methyl bromide is another highly debated fumigant that has been connected to numerous negative environmental impacts. Because of their destructive effects, these chemicals have repeatedly been shown to be a severe hazard to the environment. However, other chemicals, such as methyl iodide, have no ozone-depleting impacts on the environment. Furthermore, the molecule has been designated as a non-ozone layer depleted by the USEPA (United States Environmental Protection Agency).

Read More,

• Organic Chemistry
• Purification of Organic Compounds
• Classification of Organic Compounds

Sample Questions on Haloalkanes and Haloarenes

Question 1: What is the difference between haloalkanes and Haloarenes?

Answer:

Haloalkanes are chemicals created when hydrogen atoms in aliphatic hydrocarbons (alkanes) are replaced by halogen atoms. The compounds generated when hydrogen atoms linked to benzene rings are replaced by halogen atoms are known as haloarenes.

Question 2: Which is the example of Haloarenes?

Answer: 

Haloarenes are chemicals that are generated when hydrogen atoms linked to benzene rings are replaced by halogen atoms. Chlorobenzene, bromobenzene, iodobenzene, 2-Chlorotoluene, and other haloarenes are examples.

Question 3: What are the uses of Haloarenes?

Answer:

DDT, Picric acid, Phenol, and other chemicals containing haloarenes were developed. DDT was employed as an insecticide to kill anopheles mosquitoes that spread malaria, but it was prohibited in 1973 due to its toxicity. Picric acid is used in the manufacture of explosives, matches, electric batteries, coloured glass making, and dye synthesis. Phenol is a chemical that is used to make nylon and other synthetic textiles.

Question 4: What are Haloalkane’s reactions?

Answer:

Grignard reagents are formed when haloalkanes react with magnesium metal in the presence of fully anhydrous ether to generate organomagnesium halide. In the presence of an aqueous alkali solution or a moist silver oxide solution, haloalkane undergoes a nucleophilic substitution process, forming alcohols.

FAQs on Haloalkanes and Haloarenes

Q1: What are Haloalkanes and Haloarenes?

Answer:

Haloalkanes are organic compounds that contain one or more halogen atoms (fluorine, chlorine, bromine, or iodine) attached to a carbon atom. Haloarenes, on the other hand, are aromatic compounds that contain one or more halogen atoms attached to a carbon atom in the aromatic ring.

Q2: What are the Physical Properties of Haloalkanes and Haloarenes?

Answer:

Haloalkanes and haloarenes have higher boiling points and melting points compared to their corresponding alkanes and arenes due to the presence of halogen atoms that increase the intermolecular forces. They are also less soluble in water and more soluble in organic solvents due to their nonpolar nature.

Q3: What are Some Common Uses of Haloalkanes and Haloarenes?

Answer:

Haloalkanes and haloarenes are used in a variety of industrial and pharmaceutical applications. They are used as solvents, refrigerants, flame retardants, and as intermediates in the production of plastics, pesticides, and pharmaceuticals.

Q4: What are Some Common Reactions of Haloalkanes and Haloarenes?

Answer:

Some common reactions of haloalkanes and haloarenes include nucleophilic substitution, elimination, and oxidative addition. Nucleophilic substitution involves the replacement of a halogen atom by a nucleophile, such as a hydroxide ion or an alkoxide ion. Elimination involves the removal of a halogen atom and a hydrogen atom from adjacent carbon atoms to form an alkene. Oxidative addition involves the addition of a metal atom or ion to the halogen atom, resulting in the formation of a new metal-halogen bond.

Q5: How are Haloalkanes and Haloarenes Prepared?

Answer:

Haloalkanes can be prepared by reacting an alkane with a halogen, such as chlorine or bromine, in the presence of light or heat. Haloarenes can be prepared by substituting a hydrogen atom on an aromatic ring with a halogen using a halogenating agent, such as chlorine or bromine.