Nomenclature of saturated hydrocarbons

12 Feb

The alkanes are named according to the following rules

1. Longest chain rule:

The longest continuous chain of carbon atoms in the molecule, which may or may not be straight is selected as the parent chain and it gives the name of the parent hydrocarbon. The other parts which are not included in the parent chain are identified as substituents or branched chains.


If two different chains of equal lengths are possible, the chain with maximum number of side chains or alkyl groups is selected.

2.Lowest set of locants:

Number the carbon atoms in the parent chain as 1,2,3…etc., starting from the end which gives smaller numberto the carbon atoms carrying the substituent.

The numbering of the carbon chain as given in the second structure is wrong as it gives higher number to the carbon atom carrying the substituent.

The number that indicates the position of the substituent or side chain is called locant. The position of the locant in the above structure is 3.

When there are two or more different substituents attached to the parent chain, then the end of the parent chain which gives lowest set of locants is preferred for numbering. This rule is called lowest set of locants. This means that when two or more different sets of locants are possible, the lowest set of locants is that set of locants which when compared term by term with other sets, each in order of increasing magnitude, has the lowest term at the first point of difference. This rule is used irrespective of the nature of the substituents.

The correct set of locants is 3,4,6 and not 3,5,6. The first set is lower than the second set because at the first point of difference 4 is less than 5 (note that the first locant is same in both sets 3,3 and the first difference is with the second locant. We can compare term by term 3 – 3 ; 4 – 5 (first difference); 6 – 6. Only first point of difference is considered for preference).

3. Presence of same substituent more than once:

If the same substituent occurs more than once on the parent chain, it is indicated by the prefixes such as di, tri, tetra,… to indicate 2, 3, 4,… same substituents.

The locants of each substituent whether same or different are indicated separately and a comma is used between locant and locant.

4. Naming different substituents:

If two or more substituents are present on the parent chain, they are named in the alphabetical order along with their appropriate positions.

It may be noted that di, tri, etc. are not considered while comparing the substituents.

5. Naming different substituents at equivalent positions:

If two different substituents are present in equivalent positions from the two ends of the chain ( two sets of locants are equally possible), then the numbering of the chain is done in such a way that the substituent which comes first in the alphabetical order ( written first in the name) gets lower number.

6. Naming the complex substituents ( or substituted substituents):

If the substituent on the parent chain is complex (i.e., it is branched), it is named as a substituted alkyl group by numbering the carbon atom of this group attached to the parent chain as 1. The name of such substituent is written in brackets in order to avoid any confusion with the numbering of the parent chain.

Important Note:

1. The name of the complex substituent is always written in brackets.

2. While deciding the alphabetical order of the various substituents, the name of the complex substituent is considered to begin with the first letter of the complete name. It may be remembered that in case of simple substituents, however, the multiplying prefixes are not considered. The names of the simple substituents are first alphabetized and then multiplying prefixes are inserted.

“dimethylbutyl” (as a complete substituent) is alphabetized under ‘d’ and not under ‘m’. Therefore, it is cited before “ethyl(e)”.

3. When the names of two or more complex substituents are composed of identical words, priority for citation is given to the substituent which has lowest locant at the first cited point of difference within the complex substituent.

The substituent (1–methylbutyl) is cited first because it has lower locant than the substituent (3–methylbutyl).

4. When the same complex substituents ( substituted in the same way) occur more than once, it is indicated by the multiplying prefix bis–(for two), tris–(for three), tetrakis–(for four) ..etc.

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