Naming of Polyfunctional Compounds - JEE Important Topic

Polyfunctional compounds are compounds which contain two or more than two functional groups in an organic compound. Some of the examples of polyfunctional compounds are CH₃−CO−CH₂−COOH, NH₂-(CH2)₃-NH₂, CCl₄. Whenever we do the IUPAC naming of polyfunctional group, we categorise the functional group as primary and secondary functional group according to their priority and some of the functional groups like ester and amines are known as complicated polyfunctional groups because of their complex IUPAC naming.

Classification of Polyfunctional Compounds on the Basis of Number of Functional Groups

Polyfunctional compounds are classified on the basis of two or three functional groups as:

• Bifunctional compounds

• Trifunctional compounds

Bifunctional Compounds

These are the compounds in which two functional groups are attached with the parent chain. These functional groups can be similar or different. Some of the examples of bifunctional compounds are $HO-\left({CH}_2\right)_2-COOH$ (3hydroxylpropanoic acid),$OHC-\left({CH}_2\right)_4-COOH$ (5formylpentanoic acid).

Trifunctional Compounds

These are the organic compounds in which three functional groups are attached with the parent chain. 

Example of trifunctional compounds is

$OHC-{CH}_2-\left(OH\right)\left(CH\right)-{CH}_2-COOH$ (4formyl-3hydroxylbutanoic acid).

Polyfunctional Compounds: Principal Functional Group

Whenever we write the naming of any polyfunctional groups, we select one functional group as principal (main) functional and the rest of the functional groups are considered as substituents.

Order of Priority of Functional Groups in Polyfunctional Compound

The order of decreasing priority for some functional groups is given below:

-COOH, –SO₃H, -COOR (R=alkyl group), COCl, -CONH₂, -CN, -HC = O, > C = O, -OH, -NH₂, > C = C <, -C ≡ C-.

Whenever we select the principal group, we select it according to the priority. In a polyfunctional compound, the functional group which has the highest priority is selected as the principal functional group.

Substituents in Polyfunctional Compounds

The functional groups other than the principal functional groups in a polyfunctional compound are called substituent or secondary functional groups. Example $OH-CH_2-CH_2-COOH$  in this polyfunctional compound OH is substituent and COOH is the principal functional group.

Polyfunctional Compounds Nomenclature 

During the naming of polyfunctional compounds, we always select the longest carbon chain as the principal chain in which the principal functional group is attached. When we do naming, we follow the below steps for nomenclature of polyfunctional compounds:

• Select the principal chain 

• numbering the principal chain

• Write the prefix according to alphabetical order

• Write the root word with primary suffix

• Secondary suffix (functional group)

How do We Select the Principal Chain in Nomenclature of Organic Compounds

We select that chain as the main or parent chain which contains the greatest number of carbon atoms. We give preference to the greater number of multiple bonds. Another thing we use to select the parent chain is the greater number of prefixes and lowest position for the suffixes and the multiple bonds.

Numbering in the Principal Chain

Whenever we do the numbering in polyfunctional compounds, we give the lowest possible number to the parent functional group and after that, we give priority to the double bond and after the double bond, we give priority to the triple bond and then we give priority to substituents.

Write the Prefix in the Alphabetical Order

When we write the substituents in naming, we always write them in the beginning of the naming as prefixes. If there are more than one prefix in naming, we write them in alphabetical order. Some of the prefixes of substituents are as follows:

Write the Root Word with Primary Suffix

After writing a prefix, we write the root word along with the primary suffix.

Example: In $CH_3-\left({CH}_3\right)C\left(OH\right)-CO-CH_3$, 3-hydroxy-3 methyl-2-butanone “but” is the root word and “an” is the primary suffix.

Secondary Suffix (Functional Group) in Polyfunctional Compounds

We write the suffix or principal functional group in the last in the nomenclature of polyfunctional compounds. Here is a table which shows the suffix of some functional compounds.

Example: In ${CH}_3-CH_2-CH\left(C_2H_5\right)-CO-OC_2H_5\ $ , Ethyl-2-ethylbutanoate “oate” is a secondary suffix.

Complicated Organic Compounds

Some functional compounds show complicated naming like ester and amines. Examples: In Methyl methanoate $H{COOCH}_3$, alkyl group comes at the starting of the name.

In N-Methylethanamine  ${CH}_3{CH}_2{NHCH}_3$, to donate the alkyl group, we use N-alkyl for secondary and tertiary amines.

Conclusion

Polyfunctional compounds contain more than two functional groups. Whenever we do nomenclature of polyfunctional groups, we divide the functional groups as secondary and principal functional groups according to their priority. Secondary functional groups come in the beginning of naming and principal functional groups in the last nomenclature of that compound.