Alkyl Halides: Nomenclature, Classification, Isomerism, and Important Uses

Alkyl halides are compounds obtained by replacing a hydrogen atom in an alkane with a halogen atom (fluorine, chlorine, bromine, or iodine). Alkyl halides are also known as haloalkanes or halogenoalkanes.

Depending upon the number of halogen atoms, haloalkanes can be classified into monohaloalkanes, where one hydrogen atom is replaced by one halogen atom, and polyhaloalkanes, which replace two or more hydrogen atoms. The general formula of haloalkanes is C_nH_2n+1X (or RX), where R is an alkyl group and X is a halogen atom (F, Cl, Br, or I).

For examples,

Nomenclature of Haloalkane

There are two names associated with haloalkanes, i.e., the common name and the IUPAC name.

Common system

The common names of the monohalogen derivatives of alkanes are two-word: the name of the alkyl group is followed by the word fluoride, chloride, bromide, or iodide, depending upon the nature of the halogen.

IUPAC system

In the IUPAC system, the monohalogen derivatives of alkanes are named haloalkanes, i.e., the name can be obtained by prefixing the word halo- to the name of the alkane corresponding to the longest continuous carbon chain holding the halogen atom.

In the case of branched-chain alkanes, the following rules are followed;

• The longest continuous carbon chain containing the carbon attached to halogen is selected as the parent chain.
• The carbon atoms of the parent chain are numbered in such a way that the carbon atom carrying the halogen atom gets the lowest number.
• The position of the halogen atom and other substituents (alkyl groups) is indicated by the number of carbon atoms to which they are attached.

For example,

Classification of Alkyl halides

Depending upon the class of carbon to which the halogen is attached, alkyl halides are classified into three types;

1. Primary or 1^o-alkyl halides
2. Secondary or 2^o-alkyl halides
3. Tertiary or 3^o-alkyl halides

Primary or 1^o-alkyl halides

These are the haloalkane compounds in which the halogen group is attached to the primary carbon atom. For example,

Secondary or 2^o-alkyl halides

These are the haloalkane compounds in which the halogen group is attached to the secondary carbon atom. For example,

Tertiary or 3^o-alkyl halides

In these compounds, the halogen group of haloalkane is attached to a tertiary carbon atom. For example,

Isomerism in Alkyl halide

Haloalkanes are mainly shown two types of isomerisms;

1. Chain isomerism
2. Position isomerism

Chain isomerism

The haloalkanes containing four or more carbon atoms exhibit chain isomerism, in which the isomers differ in the chain of carbon atoms.

For example, C₄H₉Br has three, and C₅H₁₁Cl has four chain isomers.

Position isomerism

The haloalkanes containing three or more carbon atoms exhibit position isomerism, in which the isomerism differs in the position of the halogen atom within the same carbon chain.

For example, C₃H₇Cl has two position isomers.

Uses of Haloalkanes (Alkyl Halides)

• Haloalkanes are the most useful aliphatic compounds due to their strong reactivity.
• These are frequently used for introducing the hydrocarbon units (alkyl groups) in other molecules and form the basis for synthesizing innumerable organic compounds.
• Some of the haloalkanes are excellent solvents.
• Haloalkanes are included in fire extinguishers.
• These are frequently used to make cleaning products.
• These are also used for paint removal.

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