Nomenclature of Organic Compounds

NOMENCLATURE OF ORGANIC COMPOUNDS

1. Why do we need nomenclature?

In ancient days, the names of organic compounds were related to the natural things from which they were obtained. For example, the formic acid was initially obtained by distillation of ‘red ants’. Latin name of the red ant is ‘Formica’. So, the name of the formic acid was derived from the Latin name of its source Later, the organic compounds were synthesized from sources other than the natural sources. So scientists framed a systematic method for naming the organic compounds based on their structures. Hence, a set of rules was formulated by IUPAC (International Union of Pure and Applied Chemistry) for the nomenclature of chemical compounds.

2. Components of an IUPAC name

The IUPAC name of the any organic compound consists of three parts:

i. Root word

ii. Prefix

iii. Suffix

These parts are combined as per the following sequence to get the IUPAC name of the compound:

(i) Root word: It is the basic unit, which describes the carbon skeleton. It gives the number of carbon atoms present in the parent chain of the compound and the pattern of their arrangement. Based on the number of carbon atoms present in the carbon skeleton, most of the names are derived from Greek numerals (except the first four). Table 11.3 shows the root words for the parent chain of hydrocarbons containing 1to10 carbon atoms:

(ii) Prefix: The prefix represents the substituents or branch present in the parent chain. Atoms or group of atoms, other than hydrogen, attached to carbon of the parent chain are called substituents. Table 11.4 presents the major substituents of organic compounds and respective prefix used for them:

(iii) Suffix

The suffix forms the end of the name. It is divided into two parts such as (a) Primary suffix and (b) Secondary suffix. The primary suffix comes after the root word. It represents the nature in carbon to carbon bonding of the parent chain. If all the bonds between the carbon atoms of the parent chain are single, then suffix ‘ane’ has to be used. Suffix ‘ene’ and ‘yne’ are used for the compounds containing double and triple bonds respectively. The secondary suffix describes the functional group of the compound.

3. IUPAC rules for naming organic compounds:

� Rule1: Identify the longest chain of carbon atoms to get the parent name (root word).

� Rule 2: Number the carbon atoms of the parent chain, beginning at the closest end of the substituent or functional group. These are called locant numbers. If both functional group and substituent are present, then the priority will be given to the functional group.

� Rule 3: In case of alkenes and alkynes, locate the double bond or triple bond and use its locant number followed by a dash and a primary suffix. The carbon chain is numbered in such a way that the multiple bonds have the lowest possible locant number.

� Rule 4: If the compound contains functional group, locate it and use its locant number followed by a dash and a secondary suffix.

� Rule 5: When the primary and secondary suffixes are joined, the terminal ‘e’ of the primary suffix is removed.

� Rule 6: Identify the substituent and use a number followed by a dash and a prefix to specify its location and identity.

4. IUPAC Nomenclature of hydrocarbons – Solved examples

Let us try to name, systematically, some of the linear and substituted hydrocarbons by following IUPAC rules:

Example 1: CH₃-CH₂-CH₂-CH₂-CH₃

Step 1: It is a five- carbon chain and hence the root word is ‘Pent’. (Rule 1)

Step 2: All the bonds between carbon atoms are single bonds, and thus the suffix is ‘ane’.

So, its name is Pent + ane = Pentane

Example 2:

Step 1: The longest chain contains five carbon atoms and hence the root word is ‘Pent’.

Step 2: There is a substituent. So, the carbon chain is numbered from the left end, which is closest to the substituent. (Rule 2)

Step 2: All are single bonds between the carbon atoms and thus the suffix is ‘ane’.

Step 3: The substituent is a methyl group and it is located at second carbon atom. So, its locant number is 2. Thus the prefix is ‘2-Methyl’. (Rule 6).

The name of the compound is

2-Methyl + pent +ane = 2-Methylpentane

Example 3:

Step 1: The longest chain contains seven carbon atoms and hence the root word is ‘Hept’.

Step 2: There is a substituent. So, the carbon chain is numbered from the end, which is closest to substituent. (Rule 2)

Step 2: All are single bonds between the carbon atoms and thus the suffix is ‘ane’.

Step 3: The substituent is a methyl group and it is located at third carbon. So, its locant number is 3. Thus the prefix is ‘3-Methyl’. (Rule 6)

Hence the name of the compound is 3-Methyl + hept + ane = 3 –Methylheptane

Example 4: CH₃-CH₂-CH₂-CH=CH₂

Step 1: It is a ‘five- carbon atoms chain’ and hence the root word is ‘Pent’. (Rule 1)

Step 2: There is a carbon to carbon double bond. The suffix is ‘ene’.

Step 3: The carbon chain is numbered from the end such that double bond has the lowest locant number as shown below: (Rule 3):

Step 4: The locant number of the double bond is 1 and thus the suffix is ‘-1-ene’.

So, the name of the compound is Pent + (-1-ene) = Pent-1-ene

5. IUPAC Nomenclature of other classes – Solved examples

Example 1: CH₃-CH₂-CH₂-OH

Step1: The parent chain consists of 3 carbon atoms. The root word is ‘Prop’.

Step 2: There are single bonds between the carbon atoms of the chain. So, the primary suffix is ‘ane’.

Step 3: Since, the compound contains – OH group, it is an alcohol. The carbon chain is numbered from the end which is closest to –OH group. (Rule 3)

Step 4: The locant number of –OH group is 1 and thus the secondary suffix is ‘1-ol’.

The name of the compound is Prop + ane + (1-ol) = Propan-1-ol

Note: Terminal ‘e’ of ‘ane’ is removed as per Rule 5

Example 2: CH₃COOH

Step1: The parent chain consists of 2 carbon atoms. The root word is ‘Eth’.

Step 2: All are single bonds between the carbon atoms of the chain. So the primary suffix is ‘ane’.

Step 3: Since the compound contains the–COOH group, it is a carboxylic acid. The secondary suffix is ‘oic acid’

The name of the compound is Eth + ane + oic acid) = Ethanoic acid

Table 11.6 lists IUPAC names homologs of various classes of organic compounds

Test yourself:

Obtain the IUPAC name of the following compounds systematically:

(a) CH₃CHO

(b) CH₃CH₃COCH₃

(c) ClCH₂-CH₂-CH­₂-CH₃