Introduction:

We are familiar that the numbers of known organic compounds are in millions, and continue to increase as more naturally occurring compounds are discovered and many more are made. Can you assume what it takes to consider over a million names? There are numerous organic compounds that it is a real problem to give than all with different, yet sensible, names. It is instead like trying to find out different, yet associated, surnames for everyone in the world. At the onset of organic chemistry, the names of organic compounds are mainly based on the source from which the compound was obtained or on the structure or properties of the compound. This is termed as trivial system and the names are known as trivial names or common names example:

Trivial Name      Source

Urea                   Urine

Citric acid          Citrus fruit

This naming system couldn't be sustained due to the enormous increase in the number of acknowledged organic compounds. The new system (or rule) for naming organic compounds, that is now broadly accepted, is in accordance by the rules formulated by the International Union of Pure and Applied Chemistry (IUPAC) and is termed as IUPAC System of Nomenclature.  The full set of rules to cover each and every known compound is very complicated.

Nomenclature of Organic Compounds:

The hydrocarbons:

These are the organic compounds comprising of only carbon and hydrogen atoms. There are three most significant homologous series of hydrocarbons namely alkanes, alkenes and alkanes. We are familiar with the categorization of organic compounds as alkanes are saturated hydrocarbons whereas alkenes and alkanes are the unsaturated hydrocarbons. Illustrations of such organic compounds classes are described in the figure shown below:

Fig: Illustrations of three classes of hydrocarbons

(a) Alkanes:

The general formula of alkanes is C_nH_2n+2 (where n = whole number). Alkanes are considered to be the parent compound and contain the basic formula for writing the structure and name of the organic compounds as described in the table shown below. The alkanes have an -ane ending (suffix) and the prefix for members of the series is based on the carbon atoms involved. In the table shown below, Meth-, eth-, Prop- and but - prefixes for the first four members.

The hydrocarbon group made by the removal of one hydrogen atom from an alkyne is known as an alkyl group, and is named by dropping 'me' from the name of the corresponding alkane and adding the suffix 'yl' as illustrated in the table.

Table: IUPAC nomenclature of simple alkanes

In alkanes with branched (that is, groups not part of the longest chain of carbon) chains the IUPAC name is mainly based on the number of carbon atoms in the longest chain. This longest chain is then numbered from one end and in such a way the position of the branched chains can be point out.

The longest chain of carbon atoms can be numbered from either end; the direction selected is that which allows the lowest numbers to be used. For illustration:

Fig: 2-methylhexane

Whenever two or more branched chains are present they are listed in alphabetic order.

Fig: 3-ethyl-2-methylhexane

(b) Alkenes:

The general formula for alkenes is C_nH_2n illustrating that they contain two hydrogen atoms less than the corresponding alkanes. Remember the prefixes /or the alkanes; that depend on the number of carbon atoms in the organic compound; the similar prefixes are retained for the alkenes. Though, organic compounds that contain double bonds (alkenes) have the suffixes - ene (compare this by the suffixes for alkanes -ane).

  CH₃ - CH₃                     CH₂ = CH₂

ethane, an alkane          ethene, an alkene

Table: IUPAC nomenclature of simple alkenes

The hydrocarbon residue made by the removal of one hydrogen atom from an alkene is termed as an alkenyl. Example: CH₂ = CH₂ → (- H) → - CH = CH₂ ethenyl; CH₃CH = CH2 → (- H) → -CH₂CH = CH₂

(c) Alkynes

The general formula for alkynes is C_nH_2n-2. The pattern you learnt for alkanes and alkenes is as well employed for alkynes apart from that the suffix 'me' for the alkanes is substituted by 'yne' for the alkynes series.

Example: CH₃ - CH₃               CH₂ = CH₂            CH ≡ CH

           ethane, an alkane     ethene an alkene   ethyne an alkyne

Rules for IUPAC Nomenclature:

1) Introduction:

We are familiar with the nomenclature of hydrocarbons. This is not an oversight; it is since most organic compounds are assumed as derivatives (derived from) of the alkanes formed whenever the hydrogen(s) of alkanes are substituted via different groups or substituent. Therefore, the name of any organic compound based on the correct selection of the parent hydrocarbon or the fundamental carbon skeleton.

2) The rules of IUPAC nomenclature:

a) The very first step is to choose the longest possible continuous chain of carbon atoms that is termed as the parent chain.

The parent chain is found out by the number of carbon atoms. All other carbon chains linked to the parent's chain are referred to as side chains or substituent. Therefore, if the parent chain includes six carbon atoms, the parent hydrocarbon is hexane (that is, structure 1)

Fig: rules of IUPAC nomenclature

In the second structure (above), the straight chain includes only five carbon atoms while the longest continuous chain of carbon atoms includes eight carbon atoms and therefore it is an octane chain. Straight chain is not similar as continuous chains so what are significant are the longest continuous carbon atoms and not the straightness or zigzag nature of the chain.

It is possible that at times there might be two carbon chains having similar number of carbon atoms in the molecule. In such a situation, the subsequent rule will teach you how to choose the longest continuous chain.

b) The second step is to number (Arabic numerical 1, 2, 3 ... and so on) the longest continuous chain from one end to the other in such a way that the carbon atom carrying substituent's get the lowest numbers.

The positions of the side chains or substituent's are pointed out by the numbers assigned to the carbon atoms to which they are linked. If there is more than one substituent then the numbering of carbon atoms is done in such a manner that the sum of the numbers employed to locate the subsituents is minimum. This is as well known as the lowest sum rules. The number which locates the position of a substituent is termed as Locant. (Structure shown below).

Fig: rules for IUPAC Nomenclature-Locant

Therefore, the structure above is 2, 5, 6, trimethyloctane and not 3, 4, 7, trimethyloctane

c) The name of the substituents is prefixed, preceded via the Locant, to the name of the parent chain. The hyphen separates the Locant from the name of the substituents. Example: 2-methylpentane. (Structure shown below)

Fig: 2-methylpentane.

• Longest chain includes 5 carbon atoms, therefore parent is pentane.
• a -CH3 (methyl) group positioned at 2 or 4; though 2 is smaller than 4, thus 2-methylpentane

d) Whenever there are more than one subsituents, each substituent is prefixed via its Locant and arranged in alphabetical order preceding the name of the parent's chain. The prefix di-, tri- or tetra- (that is, for two, three or four substituents), is linked to the substituents name if more than one of a type of substituents is present and the Locant are written in increasing order separated by commas amongst themselves. (See the above structure). Though the prefixes di-, tri- or tetra-and so on are not considered while deciding the alphabetical order of the substituents.

e) If a double or triple bond is present in the molecule then the longest chain of carbon atoms is selected as to comprise the multiple bonds even if it is not longest continuous chain of carbon atoms. This rule is described in the structure shown below.

Fig: 2-ethyl-l-hexene

However the longest continuous chain is of 7 atoms however since it doesn't include the double bond, it is discarded as the parent chain and the carbon chain of 6 atoms (which is shorter) comprising the double is chosen as the parent chain for naming the compound The position of double or triple bond is pointed out by prefixing the number of the carbon atom preceding the multiple bond.

Fig: 6-methyl-3-heptene

The preferred parent carbon chain is numbered in a way which gives the lowest number to the multiple bonds. For illustration: structure above is named 6- methyl-3-heptene and not 2-methyl-4-heptane.

Naming non-hydrocarbon compounds: Compounds with functional groups

Whenever an organic compound includes a functional group, the longest carbon chain should be selected as to comprise the functional group. The parent chain chosen is numbered in a manner to provide the lowest number to the functional group even if it violates the lowest sum rule.

Fig: 2-ethyl-l-butanol

(C₄ not C₅ as only the C₄ include the functional group)

The name of the substituents is prefixed to the parent hydrocarbon according to the IUPAC rules illustrated above and the 'e' of the parent hydrocarbon name is substituted by the suffix of the functional group, example:

• Butane to butanol for alkanol
• Hexane to hexaoic acid for alkanoic acid
• Butane to butanoate for alkanoate

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