Haloalkanes and haloarenes are hydrocarbons that have 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).
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.
Classification of Haloalkanes and Haloarenes
Haloalkanes and haloarenes can be classified as the following:
1. 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:
- DiHaloalkanes and DiHaloarenes: These compounds contain two halogens.
- Trihaloalkanes and Trihaloarenes: These compounds contain three halogens.
- Tetra Haloalkanes and Tetra Haloarenes: These compounds contain four halogens.
2. Based on the Hybridization of Carbon
Haloalkanes and haloarenes are categorized into two categories based on the hybridization of the carbon atom to which the halogen is bonded.
Attachment of a halogen to an sp³ hybridized carbon: On this basis, alkyl halides and aryl halides 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: A halogen linked to the benzene ring via the sp³3 hybridized carbon.
Attachment of a halogen to an sp-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.
- An aryl halide is an aromatic ring with a halogen linked to an sp2 hybridized carbon.
3. 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 naming system is the most widely used way to name 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.
Common Names: Common names are an alternative nomenclature system that is often used for simple haloalkanes and haloarenes. In this system, the halogen is a suffix that follows 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.
From Hydrocarbons
Alkanes and alkenes are used to make haloalkanes; the process is explained below:
From Alkanes
Haloalkanes are formed when alkanes go through free radical halogenation in the presence of light.
From Alkenes
Haloalkanes can be formed using alkenes by electrophilic addition reactions, 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 Finkelstein Reaction describes how an alkyl halide reacts with sodium iodide to form an alkyl iodide in the presence of dry acetone.
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
Haloalkanes and haloarenes exhibit a range of physical and chemical properties, including the following:
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
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
Nucleophilic Substitution Reactions
Haloalkanes react with nucleophilic reagents due to the partially positively charged carbon atoms in the Cδ+-Xδ- 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 a 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-Fittig 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.