Introduction and Classification of Haloalkanes and Haloarenes

Do you know haloalkanes and haloarenes persist in the environment? Haloarenes were used as herbicides during Vietnam War for defoliating the jungles and making it easier to fight in the war. These haloarenes cannot be decomposed and broken down by microorganisms like bacteria. As a result of which it is still intact in the soils of the jungles to this day.

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Do you know haloalkanes containing halogens like chlorine, bromine, and iodine are a threat to the atmospheric ozone layer? One of the very common examples of the haloalkane compound that lead to ozone depletion is chlorofluorocarbon. We might have come across haloalkanes and haloarenes several times in our day to day life as well as industries for various applications. Let us know more about these organic compounds.

Browse more Topics under Haloalkanes And Haloarenes

• Nomenclature and Nature of C-X bond
• Physical properties
• Methods of Preparation
• Chemical Reactions – Nucleophilic Substitution Reactions
• Chemical Reactions and Stereochemistry
• Polyhalogen compounds
• Reactions of haloarenes

Haloalkanes & Haloarenes

When a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by halogen atoms then the compounds are termed as haloalkanes and haloarenes. If a hydrogen atom is replaced from an aliphatic hydrocarbon by a halogen atom the resulting compound formed is called as haloalkane. It is also known as alkyl halide and halogenalkane.

However, if a hydrogen atom is replaced from an aromatic hydrocarbon by a halogen atom the resulting compound formed is known as haloarene. It is also known as aryl halide or halogenoarene. In a haloalkene (R – X), X represents halogen group. It is attached to an sp³ hybridized atom of an alkyl group whereas in haloarene (Ar – X) the halogen is attached to an sp² hybridized atom of an aryl group.

Examples of Haloalkanes

Ethyl bromide − CH₃CH₂ − Br (sp³C)

Example of Haloarenes

Bromobenzene − C₆H₅Br (sp²C)

The key difference between haloalkanes and haloarenes is that haloalkanes are obtained from open-chain hydrocarbons (alkanes) whereas haloarenes are obtained from aromatic hydrocarbons (it is a type of hydrocarbon that forms a circular structure or ring-like structure due to sigma bonds and delocalized pi electrons between carbon atoms)

Learn the methods of preparation of Haloalkanes and Haloarenes here.

Definition of Haloalkanes

Haloalkanes are organic chemical compounds formed by replacement of one or more hydrogen atom from an alkane group by a halogen group (elements of group 17 such as chlorine, bromine, Fluorine, iodine, etc.).

Haloalkanes are saturated organic compounds that have all the chemical bonds attached to the carbon atom in single bonds and the halogen atom is attached to a single carbon atom.

Definition of Haloarenes

Aryl Halides/Haloarenes/Halogenoarene are aromatic compounds in which one or more hydrogen atoms attached to an aromatic ring is replaced by a halogen group. Haloarenes mainly differ from haloalkanes in the method of preparation and in its properties. This class compound and its derivatives are very broad and used for several purposes. One of the most important members of haloarene class is aryl chlorides.

Classification of Haloalkanes & Haloarenes

They can be classified on the basis of-

• Number of hydrogen atoms
• Compounds with sp³ C—X Bond
• Compounds having the sp² C-X Bond

1) Number of Hydrogen Atoms

On the basis of the number of hydrogen, they can be divided into mono, di or poly (tri, tetra, and so on) compounds of haloalkanes and haloarenes. It is named depending on the number of halogen atom these compounds contain in their structures. For example,

 

 

 

Monohalocompounds can again be classified into classes and subclasses on the basis of hybridization of the carbon atom to which the halogen atom is attached.

2) Compounds with sp³ C—X Bond [Here X (Halogen Group) = F, Cl, Br, I]

These compounds can be further divided into three types. They are:

i) Alkyl Halides/ Haloalkanes (R – X)

In this class, the halogen atom is attached to an alkyl group. The general homologous formula followed by this class is C_nH_2n+1 X. They are further classified into mainly three types on the basis of the carbon atom to which the carbon bearing halogen (X) atom is bonded- primary, secondary, and tertiary. This classification is based on the nature of carbon atom to which the halogen is attached.

ii) Allylic Halides

This classification of compounds is formed by bonding of halogen group having sp₃ hybridized carbon atom present next to a carbon-carbon double bond structure (C=C). The carbon-carbon double bond structure is also known as allylic carbon. Thus, the name allylic halides.

iii) Benzylic Halides

This type of compounds is formed when halogen atom is attached to an sp³ hybridized carbon atom. The sp₃ hybridized carbon atom should be present next to an aromatic ring in order to form benzyl halides.

3) Compounds Having the sp²C-X Bond

This class of compounds includes vinyl halides and aryl halides.

i) Vinyl Halides

These compounds are formed when halogen atom is attached to an sp² hybridized carbon atom present next to a carbon-carbon double bond (C=C).

ii) Aryl Halides

This class of compounds is formed when the halogen group is bonded to an sp² -hybridized atom of carbon in an aromatic ring.

Applications of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are used for many industrial and day to day purposes. They are used as flame retardants, propellants, solvents, pharmaceuticals, refrigerants, fire extinguishants, and many more.

• They are used as solvents for non-polar compounds.
• The derivatives of these compounds are used for medicinal purposes such as chloramphenicol is used for the treatment of typhoid fever.
• Synthetic halogen compounds such as chloroquine are used for the treatment of people suffering from malaria.
• DDT is utilized as insecticides

Environmental Effects

There is a widespread use of these compounds for commercial purposes. However, halocarbons are linked with serious pollutants and toxins that adversely affect the environment.  For example, a popular compound CFC (chlorofluorocarbon) is one of the significant reasons for ozone depletion in the atmosphere. Another very controversial fumigant linked to many harmful environmental effects is methyl bromide. These compounds have been time and again proved to be the major problem to the environment because of its harmful effect.

However, there are compounds such as Methyl iodide that does not cause any ozone-depleting effects to the environment. Furthermore, USEPA (the United States Environmental Protection Agency) has termed the compound as a non-ozone layer deplete.

A Solved Question for You

Q: Identify 1°, 2° and 3° haloalkanes from the structures given below.

Solution: 1)- Primary (1° haloalkane), 2)- Tertiary (3° haloalkane), 3)- Secondary (2° haloalkane)