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Chemical Nomenclature

There are more than three million organic compounds known. Thus, it is humanly impossible to remember the names of all these compounds. This led to a systematic classification of these organic compounds, so that they can be named with respect to their identical chemical properties.

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How to Name Chemical Compounds?

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The main function of chemical nomenclature is to make the person who listens or reads a chemical name is completely aware about the chemical compound it refers to and each nomenclature should be referring to only one substance.

It is usually understood to have least importance when we ensure that each of the substance should be carrying a single name, although the accepted list of names is very limited.
Its a expected norm that the name should convey the overall composition and contain information about its structure as well as the basic chemical nature of the compound.
  1. Chemical abstract system form an extreme example of names which do not take care of this aspect as each of these refers to a single compound but none of these contain structural information.
  2. A common name will also provide some information in identifying chemical compounds for a given set of conditions.
  3. To be the most general applicable nomenclature, the name should indicate at least the chemical formula.
  4. To be still more specific, the 3D atomic arrangement might need a specification.
  5. In a few specific conditions, such as large entities it becomes necessary that each of the compound have unique names as this requires the extra rules to be added for having a standard IUPAC system in lieu of having names which are basically longer and less trodden.
  6. Another system of nomenclature gaining popularity is the international chemical identifier.
  7. While International Chemical Identifier symbols are not that friendly as these contain complete set of information about a particular structure of compound.
  8. These makes these compounds more general than chemical abstract (CAS) numbers.
  9. The IUPAC system often has some drawbacks for the above failures when they become relevant (for example in differing reactivity of sulfur allotropes which IUPAC doesn't distinguish).
  10. While IUPAC nomenclature have advantage over CAS as it would be difficult toremember and recall the IUPAC names for bigger and specific molecules and thiese things lead to use of informal names.

Principles of Chemical Nomenclature

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In the early days of chemical discoveries, as soon as a new compound was discovered, it was given an individual name. Such a name was usually based on the source from which the chemical compound was derived. For example - Formic acid, HCOOH was named so, because it was derived from ants. (Formica - ants in Latin).

A group of sedatives was named barbiturates after the name of a woman Barbara. The structure of many of these compounds was not known at that time. An ordinary name given to a compound without reference to its structure is called a common name or trivial name. The common names are like nicknames.

IUPAC System of Nomenclature


With the rapid growth of organic chemistry, the number of compounds increased frantically, and it has now reached almost about 3 million. This rapid growth made it impossible for the chemists to maintain individual name for all these compounds.

In 1957, the International Union of Pure and Applied Chemistry evolved a scheme for giving systematic names to Organic compounds on the basis of structure. This is known as the IUPAC system. This system has set rules for naming organic as well as inorganic molecules from their structures.

The systematic name of a compound derived from its structural formula by applying IUPAC rules is referred to as its IUPAC name. One organic compound can have only one IUPAC name. It is superior to a common name as it gives an insight into the structure of the molecule. Knowing the IUPAC name of a compound, we can at once write its structural formula.

IUPAC names of a compound are different from the common names in the fact that IUPAC name are strictly based on the structure of the molecule, and not on its source or any other reasons. Currently, wherever and whenever applicable, IUPAC names are used, rather than the common names, as they are easily identifiable and can be used in whichever part of the world.

Chemical Nomenclature Rules

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Chemical nomenclature or the IUPAC nomenclature have put in rules separately for all the types of compounds, such as hydrocarbons, carbohydrates, alcohol, aldehyde etc. Thus, the IUPAC nomenclature is based on the functional group of a compound.

The basic idea of the functional group is at the heart of much of the organic chemistry. A functional group of an atom or group of atoms in a molecule that gives the molecule its characteristic chemical properties.

Functional groups serve as a basis for nomenclature of organic compounds. They serve to classify organic compounds into classes /families. All compounds with the same functional group belong to the same class.
One example of chemical nomenclature rule is of alkanes

IUPAC rules for naming Alkanes


The IUPAC system is much the same for all classes of organic compounds. IUPAC rules for naming alkanes are

Rule: 1

Select the longest continuous carbon chain. Remember that this chain does not have to be that portion of the molecule that is written horizontally.

Rule: 2

Name the longest chain. The longest carbon chain is chosen as the basis for the name.

Example:

Alkanes Naming

Rule: 3

Number the longest chain. The carbon atoms in the longest chain are numbered. The numbering is started from that end which will give numbers having the lowest value to carbons carrying substituents.

Example:

Rules for Naming Alkanes


Rule: 4

Identify the substituent. Name the substituent. Indicate its position by the number of the carbon atoms to which it is attached.

In the example II above, we see that the methyl group is attached to second carbon.

Rule: 5

Prefix the position number and name of the substituent onto the parent name. The whole name is written as one word.

Thus, Example II is named as:
2- (position of attachment) - methyl (name of the attachment - Butane (Longest chain with 4 carbons so butane)
The name would read - 2-methylbutane.

Rule: 6

When two or more different substituents are present, their names are arranged in alphabetic order and added to the name of the parent alkane, again as one word.

Naming Alkanes

Rule: 7

When naming the substituents, we follow the alphabetical order. Ethyl will come before methyl, chlorine will come after bromine, etc.

Rule: 8


In the presence of functional groups like COOH, -OH, -NH2, etc, the numbering is done from the side where the functional group is attached.

Chemical Nomenclature Chart

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Structure of the compound (Attachments in brackets)
IUPAC name
CH3 - CH(CH3 ) - CH2- CH3 2- methyl Butane
CH3 - CH(CH3 ) - CH2- CH2 - CH(Cl) -CH3 2- chloro 5- methyl Hexane
CH3 - CH (CH3) - CH2 - COOH
4 3 2 1
3- methyl butanoic acid
CH3 - C (=O) - CH3 Di-methyl Ketone (Acetone)
CH3 - CH(CH3) - CH2 - CHO 2- methyl butanal (aldehyde)
CH3 - CH(CH3) - CH2 - CH2 - OH
4 3 2 1
3- Methyl - 1- butanol (1 - here refers that OH is attached to the first carbon, as for primary alcohols)

Types of Organic Compounds

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Aims of chemical nomenclature is to provide some classification of organic compounds, so that they can be named easily and with the similar name, all throughout the world.

There are five main types of organic compounds. They are

1. Aliphatic compounds


These are organic compounds, in which the carbon atoms are present in the form of an open chain. The chain may range from 2 to 100 or more. There is no limitation to the number of members in this chain.

Examples of such compounds are:

Ethyl alcohol - CH3CH2OH
Propane - CH3CH2CH3

2. Saturated and unsaturated compounds


Compounds which contain only hydrogen and carbon are known as Hydrocarbons. A hydrocarbon is said to be saturated if it contains only C-C single bonds. A hydrocarbon is said to be unsaturated if it contains C=C or C=C multiple bonds.

Example :

Ethane - CH3-CH3 Ethane
Ethylene - CH2=CH2 Unsaturated
Acetylene - CH = CH Unsaturated.

The term UN-saturation is also sometimes used to describe a compound containing multiple bonds between other pairs of atoms, like C=O, C= N, etc.

3. Aromatic compounds


Benzene and all compounds that have structures resembling the ring of benzene are called as aromatic compounds.

Example :

Benzene, aniline, naphthalene, etc.

4. Alicyclic compounds


Cyclic compounds which consist of only carbon atoms are called as alicyclic or carbocyclic compounds.

Example :

Cyclopropane, cyclobutane, etc.

5. Heterocyclic compounds


Cyclic compounds in which the ring atoms are of carbon and some other element, say, Nitrogen, sulfur or oxygen, are called as heterocyclic compounds.

Example :

Ethylene dioxide or oxirane, pyridine, Quinoline, etc.
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