IUPAC Nomenclature of organic compounds [11th class]

IUPAC Nomenclature of organic compounds

The IUPAC system is the most rational and widely used system of nomenclature in organic chemistry. The most important feature of this system is that any given molecular structure has only one IUPAC name and any  given name denotes only one molecular structure.

The IUPAC name of any organic compound essentially consists of three parts,i.e.,

  1. Word root 2. Suffix and 3. Prefix.

 1. Word root. It denotes the number of carbon atoms present in the parent chain. For chains from one to four carbon atoms, special word roots are mused but for chains of five or more carbon atoms Greek number roots are given below :

No. Of carbon             atoms	Word                    Root
C1            C2            C3            C4            C5            C6            C7            C8            C9            C10            C11           C12	Meth             Eth             Prop             But              Pent              Hex             Hept             Oct              Non             Dec             Undec   .          Dodec

2. Suffix. There are two types of suffixes :   

(i) Primary suffix.  A primary suffix is always added to the word root to indicate whether the carbon chain is unsaturated. The three basic primary suffixes are given below :

Type of carbon chain	Primary suffix	General name
* Saturated(single bond) * Unsaturated (double bond) * Unsaturated (triple bond)	—ane      —ene      —yne	Alkane        Alkene        Alkyne

  

If the parent chain contains two, three, four or more double or triple bonds, numerically prefixes such as di (for two), trie (for three), tetra (for four),penta (for five) etc. are added to the primary suffix.  For example,

1. CH3—CH=CH=CH2

IUPAC Name – But 1,2 di ene

2. CH2=C=C=CH2

IUPAC name – But 1,2,3 tri ene

(ii) Secondary Suffix. A secondary suffix is then added to primary suffix to indicate nature of the functional group present in the organic compound. Secondary suffixes of some important functional groups are given below :

Class of organic compounds	Functional      Group	Secondary            suffix
Alcohols       Aldehydes       Ketones      Carboxylic acids      Acid amides      Acid chlorides       Esters      Nitriles      Thiols       Amines	–OH        –CHO       >C=O       –COOH       –CONH2       –COCl       –COOR       –CN       –SH       –NH2	–ol           –al           –one           –oic acid           –amide           –oyl chloride           alkyle...oate           nitrile           thiol           amine

Note – it may be noted that while adding secondary suffix to the primary suffix, the terminal ‘e’ of the primary suffix (I. e.ane, ene, yne) is removed if the secondary suffix begins with vowel but is it retained if the secondary suffix begins with a consonant.

For example :

1. CH3CH2OH

Name – eth an (e) ol — ethanol 

(e) is removed because secondary suffix begins with a vowel.

2.CH3CH2CN

Name – prop ane nitril — propanenitrile

(e) is not removed.

3. Prefix.There are two types of prefixes :

(i) Primary prefix. A primary prefix is used simply to distinguish cyclic from acyclic compounds. In case of carbocyclic compounds, cyclo is used immediately before word root. For example :

     CH2 

     /     \

  CH2  CH2 Cyclo  + pent  + ane =cyclobutane

     \       / p. Pref.    w.Root. p.suff. IUPAC nam. 

       CH2 

(ii) Secondary prefix. In IUPAC system of nomenclature, certain groups are not considered as functional groups but instead are treated as substituents. These are called secondary prefixes and are added immediately before the word root in alphabetical order to denote the side chain or substituent groups.

The secondary prefixes for some groups are given below:

Substituent                         group	Secondary                        prefix
—F              —Cl              —Br              —I              —NO2               —NO              —OR              OCH3(OMe)               —OC2H5              —CH3              —C2H5              —CH2CH2CH3              —CH(CH3)2              —C(CH3)3	Floro              Chloro             Bromo              Iodo             Nitro              Nitroso             Alkoxy              Methoxy              Ethoxy              Methyl              Ethyl              Propyl              Isopropyl              tert-Butyl

Thus, the complete IUPAC name of a organic compound consists of the following parts :

Secondary prefix + Primary prefix + Word root + Primary suffix + Secondary suffix 

*Numbering of carbon atom

Numbering of carbon atoms  is the first step for writing the IUPAC name of an organic compound.

Some rules of Numbering are given below :

1. the numbering of carbon chain is always done from that end which is nearer to the bonds and Substituents groups. 

2. When both double or triple bonds are present in a molecule, the numbering of carbon chain is always done from that end which is nearer to the double bonds.

3. While the numbering of carbon atoms of an organic compound the lowest set of locants for Substituents is always preferred.

Ex – (i) the set of locants (1,4,5) is preferred over the set (2,3,5).

Some examples :

      1         2 3       4 5             

(a) CH3—CH—CH2—CH2—CH3

                 /

               CH3

Step 1. Numbering – Numbering is always done from that end which is nearer to Substituent.

Step 2. Secondary prefix – Methyl.

Step 3. Primary prefix – No primary prefix.

Step 4. Word root – pent.

Step 5. Primary suffix – ane (because no double or triple bond).

Step 6. Secondary suffix – no secondary suffix.

Finally the IUPAC name – 2-methyl pentane

I hope this information is helpful for you.
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