IUPAC NOMENCLATURE
This is a systematic method of naming organic compounds using IUPAC system. That is, the International Union of Pure and Applied Chemistry.
The IUPAC system of naming organic compounds, the following rules are to be followed.
Every organic compound has 2 or three parts to its name,
**the root and the suffix or
**the prefix, the root and thwe suffix
Organic Compounds without branches or that does not have any subtituents apart from the main the main functional group will have only two parts to their name.
While those with branches will have three parts to their names.
***The ROOT: - This is got from the longest continuous carbon chain in the molecule. We use the Greek system of numbering to indicate the root as follows
|
Number of C-atom |
Name |
|
one C-atom |
Meth- |
|
two C-atoms |
Eth- |
|
Three C-atoms |
Prop |
|
Four C-atoms |
But- |
|
Five C-atom |
Pent - |
|
Six C-atoms |
Hex- |
|
Seven C-atoms |
Hept- |
|
Eight C-atoms |
Oct- |
|
Nine C-atoms |
Non- |
|
Ten C-atoms |
Dec- |
*** The SUFIX: - This is usually added after the root hydrocarbon, it indicates the family or homologous series that the organic compound belongs to.
***PREFIX / PREFIXES: - These are usually hanging atoms or groups that are not the main functional group of the compound. They are usually named before the root hydrocarbon. Their positions are usually mentioned (using arithmetic integers like 1,2,3,...) before their names, and where you have more than one of the same types, we use mono -, di-, tri-, tetra- to indicate the number.
when we have more than one functional group in a compound, then one is named as the prefix and the other the sufix. In this case we will not use the name as a suffix but as a prefix. (see functional groups)
RULE 1: Select the longest continuous carbon chain in the molecule and use it as the root or the parent hydrocarbon name of this chain.
RULE 2: Every branch off the main chain should be considered as a substituent derived from a corresponding hydrocarbon or any other hanging group that is not a hydrogen
CH4 -methane CH3 – methyl,
C2H6-ethane CH3CH2- ethyl,
C3H8 - propane C3H7 - propyl
All of these functions as prefix
RULE 3: Number the carbon atoms of the continuous chain from a direction that gives the Carbon atom carrying the substituent ( i.e the Carbon atom carrying anything other than a H-atom will have a lower number) a small number
RULE 4: Give each substituent a name and number
RULE 5: For identical substituents, use di, tri, tetra, penta, hexa, and so on to indicate the number of identical substituents.
RULE 6: Where you have more than one substituent that are different, name them alphabetically
RULE 7: Give the lowest possible number to the functional group.
NOTE: Halogen when they occur in organic compounds are named thus, chlorine = chloro, fluorine = fluoro, bromine = bromo, iodine = iodo.
Always show your bonds when you write or draw structures of organic compounds
E.g .1 write the structure of 2-methyl butane. This should be written as shown below:
|
H H H H |
Following strictly the above rules, one can then name perfectly any organic compound with ease. For example, in naming the compound below, following the rules
|
C1H3C2H2C3H2C4H2C5H2C6H3
|
CH3
In the above structure, there are six carbon atoms in the longest continuous chain. Therefore, the parent or the ROOT hydrocarbon has a name Hex (RULE 1). There are 2 methyl substituents derived from another hydrocarbons (RULE 2)
Counting the position of the substituents on the continuous root chain, they both stand on the 4th carbon atom when counting from left to right, but when counting from right to left, they appear on the 3rd carbon atom, hence the lowest position of 3rd carbon atom is considered (RULE 3)
Since the two substituents are similar, they are named dimethyl (RULE 4), hence the two substituents have a number 3 and name 3 and so can be named as 3,3-dimethyl (RULE 5). When this is combined with the parent hydrocarbon (RULE 1), the name of the compound becomes 3,3-dimethylhexane.
Other examples with their names are shown below following the stated rules.
CH3 CH3
(i) | |
C1H3-C2-C3H2-C4H-C5H3 (ii) C1H3C2H2C3H-C4HC5H2C6H3
| | |CH3 CH3 CH3
2,2,4-trimethylpentane 3,4-dimethylhexane
| |
(iii) C1H3C2H2-C3-C4H2C5H3 (iv) C1H3-C2-C3H3
| |
CH2CH3 CH3
3,3-diethylpentane 2,2-dimethylpropane
| |
(v) CH3CH2CHCH3 (vi) CH3—C−CH2-CHCH3| |
CH3 CH3
2-methylbutane 2,2,4-trimethylpentane
(vii) CH3CH2CH—CHCH3 (viii) CH3CH2CHCH3
׀ ׀ ׀
CH3 CH3 CH3
2,3-dimethylpentane 2-methylbutane
CH3 CH3
(ix) CH3 − C −CH− CH2−CH2 CH2CH3 (x) CH3CH2CHCH2CHCH2CH2CH3
CH3 CH2CH3 CH2CH3
2,2-dimethyl, 3-ethylheptane 5-ethyl, 3-methyloctane
Cl
(xi) CH3CH2CHCH3 (xii) CH3-C-CH2CH3
Cl Cl
2-chlorobutane 2,2-dichlorobutane
(xiii) CH3CH2CHCH2CHCH3 (xiv) CH3CHCH2CHCH3
Cl Br Cl Cl
2-bromo, 4-chlorohexane 2,4-dichloropentane
Cl Br Cl
(xv) C1H3CH2-C-CH2-C-CH3 (xvi) CH3CHCH2-C-CH3
Cl Br Cl Cl
2,2-dibromo, 4,4-dichlorohexane 2,2,4-trichloropentane
CH3 H H CH3 CH3 H H CH3
(xvii) CH3 – C – C – C – C – CH3 (xviii) CH3 – C – C – C – C – CH3
CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2
CH3 CH3 CH3 CH3 CH2 CH3 CH3 CH2
4,5-diethyl,3,3,6,6-tetramethyloctane CH3 CH3
5,6-diethyl,4,4,7,7-tetramethyldecane
(xix) CH3 –
CH2 – CH – CH3
CH2Cl
1-chloro – 2-methylbutane
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