Naming Aliphatic Compounds

Naming Alkanes

There are at least three systems in use for naming alkanes. However, the IUPAC system of nomenclature is used almost universally. According to this system, the first four members of the alkane series have special names related to their history. Thereafter, the alkanes are named after the number of carbon atoms it contained in the continuous open chain. The names of all alkanes end with the suffix '-ane'.

Names of some typical n-alkanes

The branched-chain alkanes are named as the derivatives of the parent alkane, which is identified by the number of carbon atoms in the continuous, straight chain of carbon atoms. The IUPAC rules for naming alkanes are:

Rules For Naming Straight and Branched Chain Alkanes

Rule 1

Long chain rule

Select the longest continuous chain of carbon atoms in the molecule of the given compound. This longest chain is regarded as the parent chain. The number of carbon atoms in the parent chain determines the word root. Carbon atoms, which are not included in the parent chain are identified as substituents or branched chains.

The longest chain may or may not be straight, but it must be continuous. The selection of the longest continuous chain in molecules is illustrated below:

Longest chain consisting of six carbon atoms

Longest chain consisting of seven carbon atoms

Rule 2

Larger number of side-chains rule

If two different chains of equal lengths (equal number of C atoms) are possible, then select the chain which has larger number of side-chains i.e., select the more-branched chain. For example, in the following molecule, there could be two continuous chains of eight carbon each. The correct choice is the chain as shown because it has more side-chains.

Rule 3

Lowest number rule

When only one side chain is present:

The carbon atoms in the selected parent chain are numbered as 1, 2, 3... from one end to the other. The numbering is done in such a way that the carbon atom carrying the side-chain is given the lowest possible number. For example,

The numbering in formula 1 is correct because the carbon at which the substituent is present gets a lower number.

The number indicating the position of the substituent or side chain on the main chain is called its position number or locant.

When more than one side chains are present:

• If more than one side-chains containing the same number of carbon atoms are present, then the numbering is done so that the sum of the numbers given to the carbon atoms (i.e., sum of the locants) is the lowest. This is called the lowest sum rule. For example, the numbering of chain as done in I is correct because the sum of the locants in this chain is lower than that in II.

Sum of the locants = 2 + 4 + 9 = 15

Sum of the locants = 2 + 7+ 9 = 18

Sum of the locants =2 + 2 + 4 = 8 Sum of the locants = 2 + 4 + 4= 10

• If the side chains containing different number of C atoms are at equal distance from the two ends of the chain, then numbering of the chain is done in such a way that the alkyl group which comes first in alphabetic order gets the lower position. For example, if ethyl and methyl groups are present at equivalent positions, then carbon bearing ethyl group should get the lower number.

Naming the Side Chains (or substituents)

• Each side chain is named on the basis of number of carbon atoms it contains. For example, the side chain containing one carbon atom (- CH₃) is called methyl, that containing two carbon atom (-C₂H₅) is called ethyl and so on.

• If the same substituent or side chain occurs more than once in the molecule, the name of the substituent or side chain is prefixed with a suitable prefix: di for two, tri for three. For example, if methyl group occurs twice on the chain then the substituent is designated as dimethyl.

• Names of the side chains are arranged alphabetically. The prefixes such as, di-tri- etc., are not considered while arranging the names of the side-chains alphabetically. For example, if a molecule contains one ethyl (-C₂H₅) and one methyl (-CH₃) group then while naming, the ethyl is written first followed by methyl i.e., as ethyl-methyl.

• The position of each substituent (or side chain) is indicated by the serial number of the carbon atom (in the chain) to which it is attached. The position number is written before the name of the side chain (or substituent) and separated from it with a hyphen. For example, if a methyl group occurs at carbon atom number 2 in the chain, then it is designated as 2-methyl.

• If in a molecule, one methyl group occurs at carbon number 2 and an ethyl group at carbon number 4, then the side chains are written as 4-ethyl-2-methyl.

• If in a molecule, two methyl groups occur at carbon number 3 and one ethyl group at carbon number 5, then the side chains are described as 5-ethyl-3,3-dimethyl.