Nomenclature of Organic Compounds having Functional Group(s)

A functional group is an atom or a specific arrangement of atoms that gives an organic molecule its characteristic chemical behaviour. The same functional group, regardless of the carbon skeleton it is attached to, undergoes similar reactions. For instance, methanol ($\mathrm{CH_3OH}$), ethanol ($\mathrm{CH_3CH_2OH}$), and propan-2-ol ($\mathrm{(CH_3)_2CHOH}$) all contain the hydroxyl ($-\mathrm{OH}$) group, and each liberates hydrogen gas when treated with sodium metal. This uniformity allows us to classify the vast number of organic compounds into a manageable set of families, each defined by its functional group.

The systematic naming of such compounds follows a clear sequence: identify the functional group, select the appropriate suffix from the standard table, find the longest carbon chain that includes the functional group, number the chain so that the functional group gets the lowest possible locant, and then assemble the name.

Identifying the Functional Group and Choosing the Suffix

The first step is to locate the functional group in the molecule. Table 8.4 lists the common functional groups along with their structural formulas, the prefix used when the group is treated as a substituent, and the suffix used when the group is the principal functional group. For example, the hydroxyl group ($-\mathrm{OH}$) has the prefix hydroxy- and the suffix -ol; the carbonyl group ($>\mathrm{C=O}$) has the prefix oxo- and the suffix -one; the aldehyde group ($-\mathrm{CHO}$) has the prefix formyl or oxo- and the suffix -al.

Once the functional group is identified, the longest continuous chain of carbon atoms that contains that functional group is selected as the parent chain. This chain is numbered from the end that gives the functional group the smallest possible locant number. The suffix is then attached to the name of the parent alkane, with the locant placed immediately before the suffix.

Polyfunctional Compounds: The Order of Preference

Many organic molecules contain more than one functional group. In such cases, one functional group is chosen as the principal functional group, and the compound is named on that basis. The remaining functional groups are treated as substituents and are indicated by their appropriate prefixes. The choice of which group becomes the principal one is governed by a fixed order of decreasing priority.

The group highest in this list becomes the principal functional group and determines the suffix. All groups lower in the list, as well as groups that are never principal (like halogens, nitro, alkoxy, alkyl, and aryl), are named as prefixes.

Worked Example: Alcohol and Ketone

Consider the compound $\mathrm{HOCH_2(CH_2)_3CH_2COCH_3}$. It contains both a hydroxyl group ($-\mathrm{OH}$) and a keto group ($>\mathrm{C=O}$). According to the priority order, the keto group ($>\mathrm{C=O}$) has higher priority than the hydroxyl group ($-\mathrm{OH}$). Therefore, the keto group is the principal functional group, and the suffix will be -one. The hydroxyl group becomes a substituent, indicated by the prefix hydroxy-.

The longest chain containing the keto group has seven carbon atoms. Numbering from the end nearer the keto group gives the carbonyl carbon the locant 2. The hydroxyl group is on carbon 7. The correct IUPAC name is 7-hydroxyheptan-2-one.

Worked Example: Alkene and Halogen

Consider $\mathrm{BrCH_2CH=CH_2}$. This molecule contains a bromine atom (a halogen, always a prefix) and a carbon-carbon double bond. The double bond is higher in priority than the halogen. The parent chain is three carbons long (propene). Numbering is done to give the double bond the lowest locant, which is 1. The bromine is on carbon 3. The correct name is 3-bromoprop-1-ene.

Multiple Identical Functional Groups …