Study Nomenclature of Haloalkanes and Haloarenes

There are so many sculptures worldwide, but when we speak of Eiffel Tower, we get a clear idea about its structure and location. When we speak of sunflower, a visualisation appears in our mind of how it changes its direction along with the Sun. Similarly, organic compounds are also named based on their structure and position of functional groups. The organic compounds which we will be dealing with in this article are the compounds that contain halogens as substituents. Let’s learn how these compounds are named.

Halogens are the group \(17\,({\rm{VIIA}})\) elements of the Modern Periodic Table that comprise of five chemically related elements: Fluorine \(({\rm{F}})\), Chlorine \(({\rm{Cl}})\), Bromine \(({\rm{Br}})\), Iodine \(({\rm{I}})\), and Astatine \(({\rm{At}})\). Both haloalkanes (aliphatic halides) and haloarenes (aromatic halides) have halogen substituents. These compounds have their trivial or common names as well as scientific names. However, their scientific names are based on a set of rules defined by IUPAC (International Union of Pure and Applied Chemistry).

Haloalkane

The alkanes that have one or more halogens bonded to the \({\rm{s}}{{\rm{p}}^3}\) hybridised carbon atoms of the alkyl group are known as haloalkanes. These compounds are generally represented as \({\rm{R}} – {\rm{X}}\), where \({\rm{R}}\) represents the alkyl group, and \({\rm{X}}\) denotes halogen atom, which can be fluorine \(({\rm{F}})\), chlorine \(({\rm{Cl}})\), bromine \(({\rm{Br}})\), or iodine \(({\rm{I}})\).

Common Names of Haloalkane

Alkyl halides with simple alkyl groups (up to four carbon atoms) are often called by common names. Alkyl halides are commonly named as the following-

1. The name of the alkyl group \(+\) the stem of the name of the halogen, with the ending -ide.

Common Name Format: alkyl name \(+\) halide name

For example- \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named Ethyl bromide.

2. Prefix \({\rm{n}}\)- is used in the case of the unbranched chain. For example: \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named as \({\rm{n}}\)-Propyl bromide.

3. The prefix iso- is used when except one carbon atom, all others are in a continuous chain. For example: \({\rm{C}}{{\rm{H}}_3}{\rm{CH}}\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named isobutyl bromide.

4. The prefix neo- is used when two carbon atoms form a continuous chain, and these two carbons are part of the terminals. For example: \({\rm{C}}{{\rm{H}}_3}{\rm{C}}\left( {{\rm{C}}{{\rm{H}}_3}} \right)\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named neo-pentyl bromide.

Rules of Nomenclature

1. The longest carbon chain (LCC) consisting of halogen substituent is chosen as the parent chain.
2. The longest carbon chain (LCC) is numbered such that the carbon atom(s) bearing the halogen(s) get the lowest number(s).

3. The name of the parent alkane is preceded by the locant number of the halogen with a prefix indicating the halogen substituents. The prefixes are fluoro-, chloro-, bromo-, and iodo-.

Locant number-halo prefix \(+\) parent alkane

Example- \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named as \(1\)-Bromopropane.

\({\rm{C}}{{\rm{H}}_3}{\rm{CH}}\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}}{{\rm{H}}_2}{\rm{Br}}\) is named \(1\)-Bromo-\(2\)-methylpropane.

4. If multiple halogen atoms are attached to the LCC, then the halogen atoms are labelled using Greek numerical prefixes such as di, tri, tetra, indicating the number of identical halogen atoms attached to the longest carbon chain.
For example:

5. The numeral is repeated many times if more than one halogen atom is attached to the same carbon atom.
For example:

6. In the case of different types of halogens attached to the LCC, they are named alphabetically. For example:

Naming Dihaloalkanes

Haloalkanes that have only two halogen groups in their molecules are called dihaloalkanes. Dihaloalkanes can be categorised into Geminal dihalides and Vicinal dihalides.  

1. Geminal dihalides: These dihaloalkanes have the same halogen atom attached to a particular carbon atom.

In the common system, geminal dihalides are named alkylidene halides.
For example, Ethylidene chloride

2. Vicinal dihalides: These dihaloalkanes have the same halogen atom on the neighbouring carbon atoms

In the common system, vicinal dihalides are named alkylene dihalides. For example, Ethylene dichloride

In the IUPAC system, Geminal and Vicinal halides are named dihaloalkanes, preceded by the locant number. Numbering is given based on the position of the halogen atoms.

Naming Halides with Multiple Bonds

1. If multiple bonds (double or triple bond) are present in the longest carbon chain containing the halogen substituent, then the multiple bonds occupy a higher priority than the halogens. This means the numbering of the carbon chain is done to give the multiple bonds the lowest locant.

For example:

The common name and IUPAC name of some halogen derivatives are listed below

Haloarenes

Haloarenes are the aromatic compounds in which one or more halogen atoms are bonded to the \({\rm{s}}{{\rm{p}}^2}\) hybridised carbon atom of the aryl group. The general structural formula of haloarenes is \({\rm{Ar – X}}\), where \({\rm{Ar}}\) represents the aryl group, and \({\rm{X}}\) denotes halogen atom, which can be fluorine \({\rm{(F)}}\), chlorine \({\rm{(Cl)}}\), bromine \({\rm{(Br)}}\), or iodine \({\rm{(I)}}\).

Naming Monohaloarenes

1. The common name and IUPAC name of Aryl halides are the same. These are named by prefixing “halo” to the parent name of the aromatic hydrocarbon.

For example:

Naming Substituted Haloarenes

1. If there is more than one halogen substituent on the aromatic ring, then, according to IUPAC nomenclature, the relative positions of the substituents are indicated by mathematical numerals.

2. In the common system, the relative position of two groups is shown by prefixes \({\rm{o}} – ,{\rm{m}} – \) and \({\rm{p}} – \) that indicate ortho, meta, or para positions.

The halogen atom acts as a substituent rather than as a functional group. Hence, they do not give characteristic suffix to the hydrocarbon. They rank the lowest in the priority list of functional groups.

Solved Examples on Nomenclature of Haloalkanes and Haloarenes

Q.1. Give the common and IUPAC names of the following compound.
\({\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2}{\rm{CHCl}}\)
Ans: The chlorine atom is attached to the middle carbon atom of the alkyl group \(\left[ {{{\left( {{\rm{C}}{{\rm{H}}_3}} \right)}_2}{\rm{CH}} – } \right]\) that has three carbon atoms. Therefore, the alkyl group is isopropyl, and the common name of the compound is isopropyl chloride. For the IUPAC name, the prefix chloro- is used for the \({\rm{Cl}}\) atom attached to the second carbon atom of the propane chain. This results in \(2\)-Chloropropane.

Q.2. Give the IUPAC names of the following compound.

Ans: The longest chain that forms the parent chain consists of six carbon atoms; hence it is hexane. Methyl \(\left( {{\rm{C}}{{\rm{H}}_3}} \right)\) and bromo \({\rm{(Br)}}\) groups are attached to the parent chain. Halogens are the least priority substituents; hence, the methyl group occupies the lowest locant over the bromo group. Therefore, the methyl and bromo groups are attached to second and fourth carbon atoms, respectively. Listing the substituents in alphabetical order gives the name \(4\)-bromo-\(2\)-methyl hexane.

Summary

Haloalkanes or alkyl halides and haloarenes or aryl halides are important derivatives of open chain hydrocarbons. These are widely used in our day-to-day life, primarily as insecticides, pesticides, and preservatives. Hence, it is essential to learn their names along with their structures. In this article, we learnt the different ways to name alkyl halides and aryl halides. We also learned how substituted alkyl halides and aryl halides are named.

FAQs on Nomenclature of Haloalkanes and Haloarenes

Q.1. What is the common name of Haloarenes?
Ans: The common name of haloarenes is aryl halides. In the IUPAC system of nomenclature, the prefix ‘halo” (bromo or chloro or iodo or fluoro) is placed before the name of the aromatic hydrocarbon.

Q.2. What is the common name of Haloalkanes?
Ans: The common name of Haloalkanes is alkyl halides. Haloalkanes with simple alkyl groups (up to four carbon atoms) are often called by common names. The names of haloalkanes comprised two parts- name of the alkyl group \(+\) name of the halogen atom, with the ending -ide, for example-Ethyl bromide.

Q.3. What is the difference between haloalkanes and Haloarenes?
Ans: Haloalkanes or alkyl halides and haloarenes or aryl halides are the hydrocarbons in which one or more hydrogen atoms have been replaced with halogen atoms. The primary difference between haloalkanes and haloarenes is that haloalkanes are derivatives of open-chain hydrocarbons (alkanes), whereas haloarenes are derivatives of aromatic hydrocarbons.

Q.4. What is the haloalkanes formula?
Ans: The general formula for non-cyclic halogenoalkanes is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}} + 1}}{\rm{X}}\), where \({\rm{X}} = {\rm{F}},{\rm{Cl}},{\rm{Br}}\) or \({\rm{I}}\). The halogenoalkane general formulas are similar to alkanes, where one \({\rm{H}}\) atom is replaced with a single halogen atom to form a monosubstituted alkane.

Q.5. Are haloalkanes and alkyl halides the same?
Ans: Haloalkanes are also known as alkyl halides. These are the halogen derivatives of open chain hydrocarbons (alkanes). In these compounds, one or more hydrogen atoms are replaced by a halogen atom (fluorine, chlorine, bromine, or iodine).

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