Conventions Used In IUPAC System Of Nomenclature

Here, we are going to discuss on "

Conventions Used In IUPAC System Of Nomenclature". By understanding this topic, we would be able to name organic compounds according to IUPAC System.

1. Position of Locants: A Locant is a numeral (1,2,3…) or a letter(o, m, p- etc.) used to locate the position of a substituent or a carbon-carbon multiple bond. These locants are placed immediately before the part of the name to which they relate. For example, according to the latest rules, for but-2-ene names like or 2-butene are not correct.                                                                                                                            
2. Numerical Prefixes: Numeral prefixes are used to describe a multiplicity of identical feature of a structure in chemical nomenclature. These are derived from Greek or Latin number names. Some Common numerical prefixes are given below:

Number	Numerical Term	Number	Numerical Term
1	mono-	7	hepta-
2	di-	8	octa-
3	tri-	9	nona-
4	tetra-	10	Deca-
5	penta-	11	undeca-
6	hexa-	12	dodeca-

Numerical prefixes such as 'bis', 'tris', 'tetrakis' etc., are used to indicate a multiplicity of a substituted substituent i.e., when the substituent is itself substituted.

Number                            2                          3                         4                         5

Numerical term              bis                         tris                  tetrakis               pentakis                

  3. Brackets or Parenthesis. In chemical nomenclature brackets or parenthesis are used around prefixes defining substituted substituents after the numerical multiplicative prefixes, if any.

  4. Use of Hyphen: The use of hyphen, brackets and numerical prefixes are illustrated in the following examples:

(a) A hyphen separates a locant from the words or syllables of the name. For Example,

                     

                                 

(b) When a substituent has been put in parenthesis a hyphen appears (after the final parenthesis) only if parenthesis is followed by a locant.

5. Elision of 'e': The terminal 'e' in the name of primary suffix is elided or omitted when followed by a secondary suffix beginning with  'a', 'i', 'o', 'u' or 'y' (Please note that 'y' is there in the list and 'e' is not there).

Here, terminal 'e' of primary suffix - 'ane' is removed.

Here, terminal 'e' of primary suffix - 'ane' is not removed as it is followed by 'd' of secondary suffix - 'diol'. For this purpose, the secondary suffix which follows must be considered along with the numerical affix (if any).

6. Alphabetical order of the prefixes :  When two or more prefixes are attached to the parent or principal chain, then these prefixes are arranged in the alphabetical order. While arranging prefixes in alphabetical order, multiplicative affixes like di-, tri-, tetra- etc., (if any) are not considered except for in case of substituted substituents.

Important: When two or more prefixes consist of identical words priority for citation is given to that group which contain the lowest locant in the first point of difference.

For Example: 

In this example we have two prefixes that consists of identical words i.e., chloroethyl. Here, priority for citation is given to that group which contains the lowest locant at the first point of difference. Because 1-chloroethyl has a lower locant than that of 2-chloroethyl.

In this example both the prefixes start consists of identical words. Here, priority for citation is given to 1, 1, 2-trichloroethyl because locant set 1, 1, 2 is lower that the locant set 1, 2, 2.

7. Complete name of the organic compounds must start with a capital letter. There are certain exceptions to this rule which are given below.

(i)

(ii)

8. Lowest set of locant rule: (a) if there is only one substituent the carbon atoms in the principal chain is numbered from the end which gives smaller number (Locant) to the carbon atom carrying the subsutituent. e.g.

In this example the correct locant for methyl group in 2 and not 3. This is because 2 is smaller than 3.

(b) In case there are two or more than two substituents, then the principal or parent chain is numbered from the end which gives the lowest set of locants. The lowest set of locants is decided  as follows:

When series of locants containing the same number of terms are compared term by term, that series is 'lowest' which contains the lowest number on the occasion of the first difference. This is clear from the following examples.

Example 1.

Correct name : 2,7,8-Trimethyldecane(sum of locants : 2 + 7 + 8 =  17)

Incorrect name : 3, 4, 9-Trimethyldecane( sum of locants : 3 + 4 + 9 =16)

Out of the two sets of locants i.e., 2,7,8 and 3,4,9 the first point of difference is the first locant, which is 2 in the first set and 3 in the second set. As 2 is a lower locant than 3, the first set i.e., 2,7,8 is the lower set of locants.

Example 2. 

Correct name: 1,6,7-Trichlorooctane (Sum of Locants : 1 + 6 + 7 = 14)

Incorrect name : 2, 3, 8-Trichlorooctane ( Sum of Locants 2 + 3 + 8 = 13)

Out of the two sets of locants i.e., 1,6,7 and 2,3,8 the first point of difference is the first locants itself, which is 1 in case of first set and 2 in case of second set.

9. If two or more different substituents are present at equivalent positions from the two ends of the principal chain (i.e., same set of locants is obtained, when numbered from either end), then the numbering of chain is done in such a way that the group which comes first in the alphabetic order gets a lower locant. For example,