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Nomenclature Chemistry

The purpose of nomenclature is to give a unique name for a compound. This means each formula should have only one name and each name should represent only one compound.

In ancient times organic compounds are named in according to their source, parent compound name etc. But by this method only limited number of compounds can be named. To avoid this, now we are following IUPAC norms to name an organic compound.

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Trivial Nomenclature

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Trivial means 'local' or 'vernacular' and in trivial nomenclature the organic compounds are named in different ways.

1. By the parent compound name


For example the aldehyde prepared by the reduction of acetic acid is called as acetaldehyde.

Acetaldehyde


2. By the source of the compound


For example the acid prepared by the distillation of ants is called as Formic acid. The acid present in the milk is called as lactic acid. By prefixing the words like 'iso', 'neo', 'sec' to the parent structure. For example the following structure is called as n-butane.

CH3-CH2-CH2-CH3

And hence the following structure with same four number of carbon atoms is called as iso-butane.

CH3-CH(CH3)-CH3

Advantages of trivial name


It is simple and easy to understand. For example the IUPAC name of tartaric acid is 2,3 dihydroxy 1,4 butane dioic acid which is more complex then the trivial name.
It was used in classical times and many references are given in trivial name only.

Disadvantages of trivial name

  1. It is not scientific and confusion may arise for the use of iso, neo in a compound name.
  2. One compound may contain many trivial name in different regions. For example phenol is having around seven trivial names like phenol, carbolic acid, hydroxy benzene etc.
  3. It is restricted to only few compounds. Many compounds like one given below cannot be named in trivial name.
Trivial Name

Nomenclature of Alkanes

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The nomenclature of alkanes is done in according to IUPAC norms now. The nomenclature of alkanes can be studied under the following headings.
  • Naming straight chain alkanes.
  • Naming branched alkanes.
  • Naming cycloalkanes.
  • Naming bridged alkanes.

Naming Straight Chain Alkanes

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The following table gives the name of first 12 straight chain alkanes. They have general formula CnH2n+2.

S.No.
Number of
carbon atoms
Formula
Name
1
1
CH4
Methane
2
2
CH3-CH3
Ethane
3
3
CH3-CH2-CH3
Propane
4
4
CH3-CH2-CH2-CH3
Butane
5
5
CH3-CH2-CH2-CH2-CH3
Pentane
6
6
CH3-CH2-CH2-CH2-CH2-CH3
Hexane
7
7
CH3-CH2-CH2-CH2-CH2-CH2-CH3
Heptane
8
8
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Octane
9
9
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Nonane
10
10
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Decane
11
11
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Undecane
12
12
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Dodecane

Naming Branched Alkanes

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In naming the branched alkanes, in according to IUPAC norms, the longest chain containing maximum number of carbon atoms is selected as a root alkane. Other groups are treated as substituents.

For example the following alkane contains 6 carbon atoms in the longest chain hence the parent chain name is hexane.

Branched Alkanes

  • The chain is numbered in such a way that the substituent gets the least number. For example in the following compound the chain is numbered from left to right so that the methyl group gets the number 2.
Methyl Group


  • If more than one substituent is present in the chain then the chain is numbered in such a way that the first substituent gets the lowest number. Then the alkane is named as

LOCANT + SUBSTITUENT + PARENT ALKANE NAME

2,4 Dimethyl Hexane
For example the longest chain contains 6 carbon atoms and hence the parent alkane name is hexane. The chain is numbered from right to left so that the first methyl group least number. Hence there is two methyl groups at 2,4 positions. So the name of the compound is 2,4 dimethyl hexane.

If the attached group itself is a complex chain, it is numbered from the point of attachment in the chain. For example the name of the given compound is 3,6,6 trimethyl 5,5 diethyl 4 (1-methyl ethyl) nonane.

3,6,6 Trimethyl

Naming Cycloalkanes

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In naming the cycloalkanes the prefix cyclo is added before the root alkane name. For example the following compound is named as cyclobutane.

Cyclobutane

The numbering of carbon chain is done in line with linear alkanes. For example the following compound is named as 1-methyl cyclopropane.
1-Methyl Cyclopropane

Naming Bicyclic Alkanes

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This are alkanes with two ring within a molecule. The two ring structure can be obtained in two ways.

Bicyclic alkanes having one carbon atom as common. A common example is given below.

Bicyclic Alkanes

In this there are a total of nine carbon atoms. Hence the root name is nonane. Then the word spiro is prefixed to indicate the bicyclic system followed by the number of non-common carbon atoms in both sides. Here there are 4 carbon atoms in both sides. Hence the compound name is spiro[4,4] nonane.

Bicyclic alkanes having a bridge carbon bond across two carbon atoms (bridge heads). A common example is given below.

Bicyclic Alkanes Having Bridge Head

It is named as follows.
  1. The total number of carbon atoms is counted and taken as root carbon atom. In this molecule there are seven atoms. Hence it is heptane.
  2. The number of carbon atoms to the left and right of bridge head is counted. In this example there are 2 and 2 carbon atoms.
  3. The number of carbon atoms in the bridge is counted. In our example there is one carbon atom in the bridge.
  4. Now the compound is named as bicyclo (2,2,1) heptane.
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