Nomenclature of Haloalkanes

In the IUPAC system, the mono halogen-substituted alkanes are named haloalkanes. Their names are derived by prefixing the words fluoro, chloro, bromo or iodo (according to the halogen atom present) to the name of the longest straight chain present in the compound. In naming haloalkanes, the numbering of carbon atoms in the longest chain is done from the end from which the halogen atom is nearer. A suitable Arabic numeral indicates the halogen atom position. If side chains are present, their names are also mentioned in alphabetical orders, and suitable Arabic numerals also mention their positions. Bromoethane, chloroethane, 2 -bromobutane are some of the examples.

What Are Haloalkanes?

Hydrocarbons are regarded as the parent compounds of all other organic compounds. The replacement of hydrogen atoms in saturated aliphatic hydrocarbons by a halogen atom gives rise to haloalkanes. The halogen derivatives of alkanes are usually termed haloalkanes or alkyl halides. They are obtained by replacing a hydrogen atom present in an alkane with a halogen atom. Depending upon the number of hydrogen atoms replaced by halogen atoms, they are termed mono-, di-, tri-, tetra- or poly haloalkanes.

R-H  R-X

The general formula of haloalkanes is, where is a halogen atom. They can also be represented as an alkyl group. Some typical haloalkanes are, etc.

Process for the Nomenclature of Haloalkanes

The nomenclature of haloalkanes can be done in the following ways:

• The trivial or common system: In the trivial system, haloalkanes are termed as alkyl halides. The trivial name is obtained by adding the word halide to the name of the corresponding alkyl group. The derived name is always written as two separate individual words.

For example, methyl chloride is ethyl bromide.

In the trivial system, the prefixes n-, iso-, and neo- are generally used to describe different types of alkyl groups.

The prefix n- stands for normal. It is used to describe an alkyl group that contains straight chains of carbon atoms.

For example, is named as an n-propyl group.

The prefix iso is used to describe an alkyl group that contains a methyl branch at the end of the chain.

The prefix neo- is used to describe those alkyl groups which contain two methyl  branches at the same carbon atom at the end of the chain.

Classification of Haloalkanes

Depending upon the nature of the carbon atom to which the halogen atom is attached, the alkyl halides are further classified as follows:

• Primary alkyl halides: These are the haloalkanes in which the halogen atom is attached to a primary carbon. For instance, ethyl bromide.

• Secondary alkyl halides: These are the haloalkanes in which the halogen atom is attached to a secondary carbon. For instance, 2-bromopropane.

• Tertiary alkyl halides: These are the haloalkanes in which the halogen atom is attached to a tertiary carbon. For instance, tert. butyl chloride
  
  

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Nomenclature of Haloarenes
The systematic names of halarenes or aryl halides are derived by adding prefixes fluoro, chloro, bromo, iodo (according to the halogen present ) before the name of the aromatic hydrocarbon. For instance, chlorobenzene, bromobenzene.

In the case of disubstituted or trisubstituted compounds, the relative positions of the substituent groups are indicated by the Arabic numerals. The numbering of the ring is done in such a way that the sequence gives the lowest sequence of numbers. In the case of disubstituted derivatives, the relative positions 1,  and 1,4 can also be indicated by prefixes ortho (o-), meta and para respectively.

Solved Examples

1. Write the IUPAC nomenclature and the structural formula of two such compounds containing five carbon atoms each in their molecule and which are optically active.
   Solution:
   The structural formula and the IUPAC names of such compounds are 2-chloropentane and 2-chloro-2-methylbutane, respectively.

2. Which one among the following pairs does have higher boiling points in these nomenclatures of haloalkanes?
   a. 1-bromopropane or 1-iodopropane
   b. 1-bromopropane or 1-bromobutane
   c. isobutyl bromide or t-butyl bromide

The Significance of Organic Compound Nomenclature

Organic chemistry covers the major portion of the syllabus in the chemistry of class 11 and 12 in CBSE. in the introduction chapters of chemistry in class 11 we get to know about different types of organic compounds or hydrocarbons which are formed by the combination of carbon ( C ), Oxygen (O)and hydrogen (H). The valency of a carbon atom is 4 which combines with 4 atoms of hydrogen to form the basic structure of methane (CH4) in organic chemistry. oxygen atoms with the valency of 2 often replace two hydrogen atoms to combine with the carbon atom. There are a vast number of compounds formed by the combinations of atoms of these 3 elements. And also the atoms of other elements like nitrogen (N), sulfur (S), and many more are also found in nature in a state combined with the hydrocarbons.

So it is essential for a definite structure of naming these compounds for easy recognition and remembering them. There was a time when any organic compounds found were given the local name for day-to-day use. Later it was found difficult to use different names in different places. With respect to general concern, it was decided to name all the compounds which would be acceptable all over the world. The body named as International Union of Pure and Applied Chemistry (IUPAC) was bestowed with the responsibility of identifying and naming all hydrocarbons available and developing a particular system of nomenclature for the naming of any hydrocarbons that would come up in the future. 

Haloalkanes are such a category of hydrocarbons named by IUPAC. Halogen atoms are the atoms of Fluorine, Chlorine, Bromine, Iodine. These halogen atoms are the deciding factor in the nomenclature of the compounds. The prefix used for the naming of the Organic compound is as follows: 'floro', 'chloro', 'Bromo', and 'iodo'. The presence of such halogens imparts a certain aromatic nature to the substance. That is why such organic compounds are also known as aromatic hydrocarbons.