General rules for naming organic compounds

 

In the common system, all the isomeric alkanes (having same molecular formula) have the same parent name. The names of various isomers are distinguished by prefixes. The prefix indicates the type of branching in the molecule. For example,

(1)     Prefix n-(normal) is used for those alkanes in which all the carbon atoms form a continuous chain with no branching.

                             ;        

(2)     Prefix iso is used for those alkanes in which one methyl group is attached to the next-to-end carbon atom (second last) of the continuous chain.

                               

                              

(3)     Prefix neo is used for those alkanes which have two methyl groups attached to the second last carbon atom of the continuous chain.

                               

 

IUPAC system of nomenclature of complex compounds

 

The naming of any organic compound depends on the name of normal parent hydrocarbon from which it has been derived. IUPAC system has framed a set of rules for various types of organic compounds.

(1)     Rules for Naming complex aliphatic compounds when no functional group is present (saturated hydrocarbon or paraffins or Alkanes)

(i)      Longest chain rule: The first step in naming an organic compound is to select the longest continuous chain of carbon atoms which may or may not be horizontal (straight). This continuous chain is called parent chain or main chain and other carbon chains attached to it are known as side chains (substituents). Examples :

 
 

 


                    

 

 

If two different chains of equal length are possible, the chain with maximum number of side chains or alkyl groups is selected.

(ii)     Position of the substituent: Number of the carbon atoms in the parent chain as 1, 2, 3,....... etc. starting from the end which gives lower number to the carbon atoms carrying the substituents. For examples,

                                                 

The number that indicates the position of the substituent or side chain is called locant.

                             

                             

(iii)    Lowest set of locants: When two or more substituents are present, then end of the parent chain which gives the lowest set of the locants is preferred for numbering.

 

This rule is called lowest set of locants. This means that when two or more different sets of locants are possible, that set of locants which when compared term by term with other sets, each in order of increasing magnitude, has the lowest term at the first point of difference. This rule is used irrespective of the nature of the substituent. For example,

                             

                              

The correct set of locants is 2, 3, 5 and not 2, 4, 5. The first set is lower than the second set because at the first difference 3 is less than 4. (Note that first locant is same in both sets 2; 2 and the first difference is with the second locant 3, 4. We can compare term by term as 2-2, 3-4 (first difference), 5-5. Only first point of difference is considered for preference. Similarly for the compounds,

Set of locants: 2, 7, 8 (Correct)

Set of locants: 3, 4, 9 (Wrong)

First set of locants 2, 7, 8 is lower than second set 3, 4, 9 because at the first point of difference 2 is lower than 3.

 

Lowest sum rule: It may be noted that earlier, the numbering of the parent chain containing two or more substituents was done in such a way that sum of the locants is the lowest. This rule is called lowest sum rule. For example, the carbon chain of alkanes given below should be numbered as indicated in structures A and not according to structure B.

                             ;  

A (correct) Sum of locants =3+4=7

                             

B (wrong) Sum of locants =4+5=9

 

According to latest IUPAC system of nomenclature, the lowest set of locants is preferred even if it violates the lowest sum rule. For example,

 

 

 

This compound is numbered as 2, 7, 8 and not as 3, 4, 9 in accordance with latest lowest set of locants rule, even though it violates lowest sum rule.

(iv)    Presence of more than one same substituent: If the same substituent or side chain occurs more than once, the prefixes di, tri, tetra ..........etc., are attached to the names of the substituents. For example,

                             

(v)     Naming different substituents: If two or more different substituents or side chains are present in the molecule, they are named in the alphabetical order along with their appropriate positions.

 

 

(vi)    Naming different substituents at equivalent position: In case, there are different alkyl substituents at equivalent positions, then numbering of the parent chain is done in such a way that the alkyl group which comes first in the alphabetical order gets the lower number.

                                   

 

(vii)   Naming the complex substituents (or substituted substituents): If the substituent on the parent chain is complex (i.e. it is branched) it is named as substituted alkyl group by numbering the carbon atom of this group attached to the parent chain as 1. The name of such substituent is given in brackets in order to avoid confusion with the numbering of the parent chain. For example,

 

                    The name of the complex substituent is always written in brackets.

 

While deciding the alphabetical order of the various substituents, the name of the complex substituent is considered to begin with the first letter of the complete name. It may be remembered that in case of simple substituents, however, the multiplying prefixes are not considered. The names of simple substituents are first alphabetized and then multiplying prefixes are inserted. For example,

 

It may be noted that dimethyl propyl (a complex substituent) is alphabetized under d and not under m. Therefore, it is cited before ethyl (e).     

 

 

 

The substituent dimethyl is cited first because it is alphabetized under d. Similarly,

 

 

 

When the names of two or more complex substituents are composed of identical words, priority for citation is given to the substituent which has lowest locant at the first cited point of difference within the complex substituent. For example,

 

 
 

 

 


 

 

 

– 5(1-methyl butyl)-7-(2-methyl butyl) dodecane

The substituent (1-methylbutyl) is written first because it has lower locant than the substituent (2-methylbutyl).

 

When the same complex substituent (substituted in the same way) occurs more than once, it is indicated by the multiplying prefix bis (for two), tris (for three), tetra kis (for four) etc.

 
 

 

 


                   

 

 

 

 

 

(viii)   Cyclic hydrocarbons: These compounds contain carbon chain skeletons which are closed to form rings. The saturated hydrocarbons with ring of carbon atoms in the molecule are called cycloalkanes. These have the general formula .

 

The cyclic compound is named by prefixing cyclo to the name of the corresponding straight chain alkane. For example,

 

 
 

 

 

 

 


 

 

 

 

If side chains are present, then the rules given in the previous section are applied. For example,

         

 

 

 

 

When more than one side chains are present, the numbering is done beginning with one side chain so that the next side chain gets the lower possible number. For example,

 

 

 

 

 

 

 
 

 

 

 

 

 

 

 

 

 

 


When a single ring system is attached to a single chain with a greater number of carbon atoms or when more than one ring system is attached to a single chain, then it is named as cycloalkylalkanes. For example,

 
 

 


 

 

 

 

 
 

 

 

 

 

 

 

 


In case of substituted cycloalkenes, the double bond is given the lowest possible number and numbering is done in such a way that the substituents get the lowest number. 

 

According to the IUPAC system of Nomenclature, certain trivial or semi- systematic names may be used for unsubstituted radicals. For example, the following names may be used,

 

Isopropyl

Sec- Butyl

 

Isopentyl

tert-Pentyl

Isobutyl

tert-Butyl

Neopentyl

Isohexyl

 

However, when these are substituted, these names cannot be used as such. For example,

                              

It may be noted that while writing the substituent's name in alphabetical order, the prefixes iso-and neo-are considered to be part of the fundamental name. However, the prefixes sec-and tert-are not considered to be the part of the fundamental name.

 

          (2)     Rules for IUPAC names of polyfunctional organic compounds

Organic compounds which contain two or more functional groups are called polyfunctional compounds. Their IUPAC names are obtained as follows,

(i)      Principal functional group: If the organic compound contains two or more functional groups, one of the functional groups is selected as the principal functional group while all the remaining functional groups (also called the secondary functional groups) are treated as substituents. The following order of preference is used while selecting the principal functional group.

 

Sulphonic acids > carboxylic acids > anhydrides > esters > acid chlorides > acid amides > nitriles > aldehydes > ketones > thiols > alcohols >alkenes > alkynes.

 

All the remaining functional groups such as halo (fluoro, chloro, bromo, iodo), nitroso (–NO), –nitro (–NO2), amino (–NH2) and alkoxy (–OR)  are treated as substituents.

 

Table: 22.12

 

Order of preference

Preflx

Suffix (ending)

– SO3H

Sulpho

Sulphonic acid

– COOH

Carboxy

– oic acid

– COOR

Alkoxy carbonyl

Alkyl alkanoate

– COX

Haloformyl

Oyl halide

– CONH2

Carbamoyl

 – amide

– C º N

Cyano

– nitrile

– CHO

Formyl

– al

> C = O

Keto

– one

– OH

Hydroxy

– ol

– NH2

Amine

– amine

 C =  C

– ene

– C º C –

– yne

– O –

Epoxy

– X

Halo

– NO2

Nitro

 

(ii)     Selecting the principal chain: Select the longest continuous chain of carbon atoms containing the principal functional group and maximum number of secondary functional groups and multiple bonds, if any.

(iii)    Numbering the principal chain: Number the principal chain in such a way that the principal functional group gets the lowest possible number followed by double bond and triple bond and the substituents, i.e.

                   Principal functional group > double bond > triple bond > substituents

(iv)    Alphabetical order: Identify the prefixes and the positional numbers (also called locants) for the secondary functional groups and other substituents and place them in alphabetical order before the word root.

                  

                             

 

(3)     Polyfunctional compounds containing more than two like functional groups : According to latest convention (1993 recommendations for IUPAC nomenclature), if an unbranched carbon chain is directly linked to more than two like functional groups, the organic compound is named as a derivative of the parent alkane which does not include carbon atoms of the functional groups. For example,

                               

 

                    Following the above rule, citric acid may be named as,

                             

2-Hydroxypropane-1, 2, 3-tricarboxylic acid

(formerly 3-carboxy-3-hydroxypentane-1, 5-dioic acid )

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