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IUPAC Nomenclature

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The nomenclature developed and recommended by IUPAC has emphasized the generation of unambiguous names that accord with the historical development of chemistry. In the development of nomenclature several systems have emerged for the construction of chemical names; each system has its own inherent logic and set of rules.

Some systems are broadly applicable whereas practice has led to the use of specialized systems in particular areas of chemistry. The systematic IUPAC nomenclature of compounds tries to characterize compounds by a unique name. The basic aim of the IUPAC nomenclature is to describe particular parts of the structure (fragments) in a systematic manner, with special expressions from a vocabulary of terms.

IUPAC Naming

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The formal system of nomenclature used today is one proposed by the International Union of Pure and Applied Chemistry (IUPAC). This system was first developed in 1892 and has been revised at irregular intervals to keep it up to date.

IUPAC name generator formulated a set of rules for naming the organic compounds. Underlying the IUPAC system of nomenclature for organic compounds is a fundamental principle: Each different compound should have an unambiguous name. Thus through a systematic set of rules the IUPAC system provides different names for the more than 16 million known organic compounds, and names can be devised for any one of millions of other compounds yet to be synthesized.
In addition the IUPAC system is simple enough to allow any chemist familiar with the rules to write the name for any compound that might be encountered. In the same way, one is also able to derive the structure of a given compound from its IUPAC name.

A single compound can have a several acceptable names but no two compounds can have the same name.

IUPAC Nomenclature Rules

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Following rules should be used as a basic guideline for the IUPAC nomenclature of organic compounds. IUPAC naming rules are listed below.

1. Selection of parent chain

  • Locate the longest continuous chain of carbon atom as parent chain.
  • The parent chain must include functional group/multiple bond, although it may not be longest.
  • When two chains of equal length complete for selection as the parent chain, choose the chain with the greater number of substituents.


2. Numbering of carbon atoms of parent chain

  • The carbon atoms of the parent chain are numbered on the basis of following rules.
  • The locant (carbon atom carrying functional group/multiple bond/substituent) must get lowest number)
  • If two or more substituents are present one of them is regarded as senior group and rest junior groups. The locant containing senior group must get lowest number. The order of seniority is as follows.
Acids > Acid derivatives excluding nitriles > Aldehydes > nitriles > Ketones > Alcohols > Amines > Ethers > => substituents
  • Lowest sum rule: The sum of the numbers used to indicate the position of substituents should be minimum.
  • The carbon in chain terminating functional groups like -COOH, -CONH2, must always be given number 1.


3. Other Rules

  • If there are more than one substituents in side chains, they should be prefixed in an alphabetical order.
  • When 2 or more identical substituents/side chains are present numerical di, tri etc, are used before the names of such substituents/side chain. However these numerical prefixes are not considered while arranging the substituents/side chains alphabetically.


The IUPAC name for the first twelve continuous chain alkanes.


Molecular formula
IUPAC prefix
IUPAC names
Condensed structural formula
CH4 meth- Methane CH4
C2H4 eth-
Ethane CH3-CH3
C3H6
prop-
Propane
CH3-CH2-CH3
C4H10
but-
Butane
CH3-CH2-CH2-CH3
C5H12
pent-
Pentane
CH3-CH2-CH2-CH2-CH3
C6H14
hex-
Hexane
CH3-CH2-CH2-CH2-CH2-CH3
C7H16
hept-
Heptane
CH3-CH2-CH2-CH2-CH2-CH2-CH3
C8H18
oct-
Octane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
C9H20
non-
Nonane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
C10H22
dec-
Decane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
C11H24undec- Undecane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
C12H26
dodec-
Dodecane
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3


Names for the first eight continuous chain alkyl groups.

Number of carbon atoms
Structural formula
Stem of alkane name
Suffix Alkyl group name
1 -CH3 meth- -yl Methyl
2
-CH2-CH3
eth-
-yl
Ethyl
3
-CH2-CH2-CH3
prop-
-yl
Propyl
4
-CH2-CH2-CH2-CH3
but-
-yl
Butyl
5
-CH2-CH2-CH2-CH2-CH3
pent-
-yl
Pentyl
6
-CH2-CH2-CH2-CH2-CH2-CH3
hex-
-yl
Hexyl
7-CH2-CH2-CH2-CH2-CH2-CH2-CH3 hep- yl- Heptyl
8
-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3 oct-
yl-
Octyl

IUPAC Nomenclature Examples

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The most important of the IUPAC rules for alkane nomenclature. IUPAC nomenclature can be explained by taking C6H14 isomers as example.

The un-branched C6H14 isomer is hexane.

CH3-CH2-CH2-CH2-CH2-CH3

The IUPAC nomenclature for branched substituted C6H14 isomer represented by the structure.

CH3-CH(CH3)-CH2-CH2-CH3

Step-1

Pick out the longest carbon chain. In this case it has five carbon chains, the compound is named as a derivative of pentane. All that matters is the number of carbons linked together in an uninterrupted sequence.

Step-2

Identify the substituent attached to the parent chain. Here the parent pentane chain bears a methyl group as a substituent.

Step-3

Number the longest continuous chain in the direction that gives the lowest number to the substituent group at the first point of branching. In this case two numbering scheme is possible but both have five carbon atoms in their longest continuous chain and bear a methyl group as a substituent at the second carbon.

Step-4

Write the name of the compound. The parent alkane is the last part of the name and is preceded by the names of the substituent groups and their numerical locations (locants). Hyphens separate the locants from the words.

CH3-CH(CH3)-CH2-CH2-CH3
IUPAC name - 2-methyl pentane

Since we considered the isomers, three more structure is possible as represented below.

Possible structures
IUPAC names
CH3-CH2-CH(CH3)-CH2-CH3 3-methyl pentane
CH3-C(CH3)2-CH2-CH3 2,2-dimethyl butane
CH3-CH(CH3)-CH(CH3)-CH3 2,3-dimethyl butane


IUPAC Organic Nomenclature

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Formal systematic rules exist for generating IUPAC nomenclature of organic chemistry. The IUPAC name of any organic compound essentially consists of three parts which can be described as follows.

1. Stem name


It is the basic unit of the systematic name which describes the carbon skeleton. It is also referred as the "word root". The stem name are derived from Greek numerals.

2. Prefix


The stem name may have certain groups or atoms attached to it which are termed substituents. The names for these substituents are prefixed to the stem name. If the molecule contains more than one functional group than as per the priority order, lower priority group is treated as substituent ans written as a prefix.

3. Suffix


A suffix denotes the functional group present in a compound and is always added after the stem name (word root).

IUPAC Nomenclature Practice

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The IUPAC nomenclature practice problems are given below.

Solved Examples

Question 1: Find the IUPAC name for the following compound.

Hexadecane
Solution:
 
  1. The longest chain is ten, so it is a decane.
  2. There are two isopropyl or (1-methylethyl) groups.
  3. Numbering from the left gives us the lowest number for the first group.
  4. Either 4,5-diisopropyldecane or 4,5-bis(1-methylethyl)decane are acceptable.

 

Question 2: Name the following compound using IUPAC rule.

Substituted Nonane
Solution:
 
Nonane

  1. The longest chain is seven, so it is a heptane.
  1. There are two methyl groups, so it is a dimethylheptane.
  2. We number from the right end since we run into a methyl group attached to carbon 2. If we numbered from the left end we cant hit a methyl group until carbon 3.
  3. Numbering from the right the methyl groups are on carbon2 and 5 of the heptane, so the name is 2,5-dimethylheptane.