Nomenclature Of Haloalkanes And Haloarenes

Initially, there was no proper system for the naming of compounds. Mostly there were trivial names that were used depending upon the country and region. These trivial names were based on the discoverer or the nature of the compound or its place of discovery.

The system of trivial names was not standard and led to much confusion, thus raising the need for a standard system for the naming of organic compounds. IUPAC came up with a set of rules that are used universally for the naming of organic compounds.

There are two names associated with every compound:

  1. Common name – It is different from a trivial name in the sense that it also follows a rule for its nomenclature.
  2. IUPAC name – The IUPAC (International Union of Pure and Applied Chemistry) naming system is the standard naming system that chemists generally use.

Rules of Nomenclature

  1. Find the longest carbon chain.
  2. Number the longest carbon chain such that the carbon atom(s) to which the halogen(s) is/are attached get the lowest number(s).
  3. Multiple halogen atoms are labelled with the Greek numerical prefixes such as di, tri, tetra, to denote the number of identical halogen atoms attached to a carbon atom. If more than one halogen atoms attached to the same carbon atom, the numeral is repeated that much time.
  4. In case, different types of halogens are attached, they are named alphabetically.
  5. The position of the halogen atom is indicated by writing the position and name of the halogen just before the name of the parent hydrocarbon.

The Methodology of Writing Name

  1. First, write the root word for the parent hydrocarbon (depending upon the no. of carbon atoms in the longest carbon chain).
  2. Secondly, calculate the number of halogen atoms present. If there are multiple halogen atoms present, then arrange the halogens alphabetically in the prefix, labelling them with their respective positions. But, if the same halogen atom is present more than once then use the prefixes di, tri, tetra, etc.

Nomenclature of Haloalkanes

Alkyl halides are named in two ways. In the common system, the alkyl group is named first followed by an appropriate word chloride, bromide, etc. The common name of an alkyl halide is always written as two separate words. In the IUPAC system, alkyl halides are named as haloalkanes. The other rules followed in naming compounds is that

  • Select the longest chain of carbon atoms containing the halogen atom.
  • Number the chain to give the minimum number to the carbon carrying halogen atom.
  • If multiple bonds (double or triple bond) is present, then it is given the preference in numbering the carbon chain.
  • The IUPAC name of any halogen derivative is always written as one word.
Compound Common Name IUPAC Name
CH3-Cl Methyl Chloride Chloromethane
CH3-CH2-Br Ethyl bromide Bromoethane
CH3-C(CH3)2-Br tert-Butyl bromide 2-Bromo-2-methylpropane
CHCl3 Chloroform Trichloromethane
CH3-CH(Br)2 Ethylidene bromide 1,1-Dibromoethane
CH2=CH-CH2-I Allyl iodide 3-Iodoprop-1-ene

Nomenclature of Haloarenes

  • Aryl halides are named by prefixing “halo” to the name of the parent aromatic hydrocarbon.
  • If there is more than one substituent on the ring then the relative positions of the substituents are indicated by mathematical numerals.
  • In the common system, the relative position of two groups is shown by prefixes ortho, meta or para.

The common and IUPAC names of some representative haloarenes are given below.

IUPAC Names of Some Haloarenes

IUPAC Names of Some Haloarenes

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