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IUPAC Nomenclature of Alkanes

 

There is a general system for naming organic compounds instituted by the International Union of Pure and Applied Chemistry, I.U.P.A.C.

Here are the rules to follow to name hydrocarbons, using alkanes as examples:


1. Select the longest straight chain of carbon atoms in the molecular structure - this gives the parent name of the compound.

2. Recognize any substituent on the longest chain. There are usually the following alkyl substituents:

CH3 - methyl; CH3CH2 - ethyl; and CH3CH3CH2 - propyl.

3. Number the carbon atoms on the straight chain, such that those bearing the substituent would have the lowest number.

4. If there are two or more same substituents, use the prefixes: di- for two; tri-for three and tetra- for four, before the name of the substituent.

5. If there are two substituents on the same carbon, then the position of that carbon is indicated twice before the names of the substituents.

6. Write the names of the compound as one word, separate numbers from substituents groups and prefixes with a hyphen(-) and numbers from each other with a comma (,).

The position(s) of carbon (s) bearing the substituents is(are) written first, followed by prefixes, then the name(s) of substituents in alphabetical order, then finally the parent name.

Example: name the following:

1.

Solution: The longest straight chain is the one with 6 carbon atoms. Hence, the parent name is hexane.

The substituents are two methyl groups (- CH3). The correct numbering of the longest straight chain is

rather than




There are two same substituent groups, - the preffix- di is used. Therefore, the name of the
compound is 2,3-dimethyl hexane.

2.

Solution: the longest straight chain is the one with seven carbon atoms- the parent name is heptane. The stustituents are: one methyl and one ethyl groups. The right numbering is:


                            Positions 2 and 4

rather than
.

                        Positions 4 and 6

Therefore, the name of the compound is 4-ethyl-2- methyl heptane.

Note: ethyl is expressed first before methyl - expression of the substituent groups is based on alphabetical order of their names.

3.

Solution: The parent name is pentane. The substituents are two methyl groups attached to the same carbon (this carbon position will be indicated twice). The correct numbering is from left to right, giving the carbon bearing the substituents number 2. Therefore, the name of the compund is 2,2 - dimethyl pentane.


4.

Name: 2-chloro -2 - methyl pentane


5.

Name: 3-chloro-2,2-dimethyl pentane

Note: the prefix -di does not determine the order of expressing the substituents, rather, it is the alphabetical order of their names.

Related:

Hydrocarbon and Alkanes
Methane
Isomerism in Alkanes
Petroleum and Petrol

 

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