Naming a Complex Side Chain

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If a substituent or side chain on the parent chain is complex i.e., it has a branched chain then, it is named as a substituted alkyl group. The substituted alkyl group is then named as a separate carbon chain. The carbon atom of this substituted alkyl group attached to the parent chain is numbered as 1. The name of such a substituent is enclosed in parenthesis to avoid confusion with the numbering in the parent chain. For example, the sidechain in the following molecule is named as -(1,2-dimethylpropyl).

The side chain is named as -(1,2-dimethylpropyl)

While naming the compound, the position of this side chain in the parent chain is indicated by a numeral placed outside the parenthesis. For example, in the above molecule the side chain is linked to carbon 5 of the parent chain. So, the side chain can be described by, 5-(1,2-dimethyl propyl).

Naming the Branched Alkane

After the word root (from the longest chain), side chains, positions of the side chains are identified; the name of the branched alkane can be written as:

Prefix + Word root + Primary suffix

For alkanes, the primary suffix is -ane. So,

Prefix + Word root + ane

Naming alkenes and alkynes

Alkenes are named after the parent alkane by replacing '-ane' of the alkane by '-ene'. Alkynes are named after the parent alkane by replacing the suffix '-ane' of the alkane by '-yne'.

Thus,

Suffix for alkenes: -ene

Suffix for alkynes: -yne

Longest chain rule

The longest continuous carbon chain containing the multiple (double or triple) bonds is selected. It is to be remembered that the parent chain must contain double or triple bond even if it is not the longest chain in the formula. For example,

           

On the basis of the number of carbon atoms in the parent chain, the parent alkane is determined. Then,

• For alkene (double-bonded compound) the suffix -ane is replaced in the name of the parent alkane by -ene.

• For alkyne (triple-bonded compound) the suffix -ane is replaced in the name of the parent alkane by -yne.

Lowest number rule

The carbon atoms of the parent chain are numbered as 1, 2, 3..., so as to assign the lowest number to the carbon atom where the multiple bond (double or triple) begins. The position of the multiple bond is indicated by adding the serial number of the first carbon of the multiple bond before the name of the alkene. Sometimes this number is mentioned after the name of the alkene, or before the suffix -ene also. For example,

                     

Naming the side chains (or substituent)

If there is any other substituent on the chain, the compound is named as the derivative of the corresponding alkene. The side chains (or substituents) are named as per rules for alkanes.

Primary suffix for compounds containing more than one multiple bonds

If there are two or three multiple bonds in the parent chain, then the suffix -diene (for two) or triene (for three) are used respectively. The positions of the multiple bonds are indicated by the numerals indicating the carbon atoms at which multiple bonds start. The numbering of the chain is done so that the sum of the numerals assigned to the carbon atoms linked by the multiple bonds is the lowest.

If both the double and triple bonds are present in the same molecule, their locants are written either before their respective suffixes, or the locant of the double bond is always written before the word root and that of the triple bond before its suffix. In either case, the terminal 'e' from the suffix 'ene' is dropped while writing the complete name of the organic compound.

The rules of naming the alkenes and alkynes are illustrated through the following examples.

                

     

          

Problem

4. Give IUPAC names of the following alkanes:

(a) (C₂H₅)₂ CH CH₃

(b) CH₃ C(CH₃)₂ CH (CH₃) CH₃

(c) (CH₃)₂ CHCH₂CH₂CH(CH₃)₂

(d) C₂H₅ CH(CH₃) CH₂ CH(CH₃)₂

Solution

(a) The formula of the given compound is (C₂H₅)₂ CH CH₃

This can be written as

           

The longest chain contains five carbon atoms and the main chain has a side chain. According to the rules, the numbering is done so as to give the carbon having side chain the lowest number.

Both I and II are right.

The parent alkane for this compound is pentane. Therefore, the name of the compound is, 3-methylpentane.

(b) The compound, CH₃ C(CH₃)₂ CH (CH₃) CH₃ may be described as

The longest continuous chain consists of four carbons. The two different side-chains are at equal distance from the two ends of the chain. Therefore, the numbering is done from the end closer to the carbon having more branching viz.,

The compound thus is named as 2,2,3-trimethylbutane

(c) The compound (CH₃)₂-CH-CH₂-CH₂-CH(CH₃)₂ may be written as

The main chain contains six carbon atoms and has two side-chains of one carbon each, at equal distance from the ends. Thus, the chain is numbered and named as,

(d) The compound, C₂H₅-CH(CH₃)-CH₂-CH(CH)₃ may be written as,

The main carbon chain consists of six carbon atoms and hence the parent alkane is hexane. The numbering is done from the right end of the chain (as shown above). The two methyl groups are attached to the main chain at carbon number 2 and 4. Thus, the name of the compound is, 2,4'dimethylhexane.