Nomenclature of Organic Compounds having Functional Groups | Chemistry Class 11 - NEET PDF Download

A functional group is an atom or a group of atoms bonded together in a characteristic way that is usually the site of chemical reactivity in an organic molecule.

• Compounds having the same functional group undergo similar chemical reactions. For example, CH₃OH, CH₃CH₂OH and (CH₃)₂CHOH - all containing the -OH functional group - liberate hydrogen on reaction with sodium metal.
• The presence of functional groups allows organic compounds to be systematised into different classes (alcohols, aldehydes, ketones, carboxylic acids, etc.).
• To name an organic compound, first identify the principal functional group, because it determines the suffix of the name.
• Choose the longest carbon chain that contains the principal functional group and number the chain so that the principal functional group receives the lowest possible locant (lowest possible number).
• When more than one functional group is present, one is chosen as the principal functional group and the compound is named on that basis; remaining functional groups are treated as substituents and are indicated by appropriate prefixes.
• The choice of the principal functional group is made using the accepted order of preference (IUPAC priority rules); groups such as -R, C₆H₅-, halogens (F, Cl, Br, I), -NO₂, -OR (alkoxy) are always treated as prefix substituents.
• If more than one functional group of the same type is present, their number is indicated by multiplicative prefixes di, tri, etc., placed before the class suffix. In such cases the full name of the parent alkane is written before the class suffix.
• The suffix of the parent alkane (-ane) is dropped when indicating more than one double or triple bond; for example, CH₂=CH-CH=CH₂ is named buta-1,3-diene.

Examples that illustrate choice of principal group and prefix/suffix assignment are:

• A compound containing both an alcohol and a keto group is named as hydroxyalkanone when the keto group is the principal group (because the keto group has higher priority than hydroxyl for suffix assignment). Thus, HOCH₂(CH₂)₃CH₂COCH₃ is named 7-hydroxyheptan-2-one and not 2-oxoheptan-7-ol.
• When both a halogen and a C=C bond are present, the halogen is a prefix substituent and the double bond is part of the suffix. Thus BrCH₂CH=CH₂ is named 3-bromoprop-1-ene and not 1-bromoprop-2-ene.
• CH₂(OH)CH₂(OH) is named ethane-1,2-diol.

Some Functional Groups and Classes of Organic Compounds

Q.1. Write the IUPAC names of the compounds i-iv from their given structures.

(i)

Sol.

The functional group present is an alcohol (-OH); hence the suffix is -ol.

The longest chain that contains the -OH group has eight carbon atoms; the corresponding parent hydrocarbon is octane.

The -OH group is on carbon atom 3 and a methyl substituent is attached at carbon 6.

Therefore, the systematic name of this compound is 6-methyloctan-3-ol.

(ii)

Sol.

The functional group present is a ketone (>C=O); hence the suffix is -one.

There are two keto groups; this multiplicity is indicated by di, giving the combined suffix -dione.

The two keto groups are located at carbon atoms 2 and 4 of the longest chain, which has six carbon atoms; the parent hydrocarbon is hexane.

Thus, the systematic name is hexane-2,4-dione.

(iii)

Sol.

Two functional groups, a ketone and a carboxylic acid (-COOH), are present.

The carboxylic acid is the principal functional group and therefore the parent chain is named as a -oic acid; numbering of the parent chain starts from the carbon of the -COOH group.

The ketone (keto) group is present at carbon 5 and is indicated by the prefix oxo.

The longest chain containing the carboxylic acid has six carbon atoms; hence the parent is hexanoic acid.

The compound is therefore named 5-oxohexanoic acid.

(iv)

Sol.

The molecule contains two C=C double bonds and one C≡C triple bond.

Double bonds are indicated by the suffix -diene and the triple bond by -yne; positions are given by locants.

The longest chain containing all unsaturations has six carbon atoms; hence the parent hydrocarbon is hexa-.

The two double bonds are at positions 1 and 3 and the triple bond is at position 5.

The name is therefore hexa-1,3-dien-5-yne.

Q.2. Derive the structure of (i) 2-Chlorohexane, (ii) Pent-4-en-2-ol, (iii) 3-Nitrocyclohexene, (iv) Cyclohex-2-en-1-ol, (v) 6-Hydroxyheptanal.

Sol.

(i)

'Hexane' indicates a straight chain of six carbon atoms; the substituent chloro is at carbon 2.

The structural formula is CH₃CH(Cl)CH₂CH₂CH₂CH₃ (2-chlorohexane).

(ii)

'Pent' indicates five carbon atoms in the parent chain. 'en' indicates a C=C double bond and 'ol' indicates an -OH group.

Numbering places the double bond at carbon 4 and the hydroxyl at carbon 2.

The structural formula is CH₂=CH-CH₂-CH(OH)-CH₃ (pent-4-en-2-ol).

(iii)

'Cyclohexene' indicates a six-membered ring with one C=C double bond; the ring is numbered so that the double bond carbons receive the lowest possible numbers.

A nitro group at position 3 gives 3-nitrocyclohexene; the nitro group is a prefix and the double bond is part of the suffix.

(iv)

'Cyclohex-2-en-1-ol' indicates a six-membered ring in which an -OH group is at carbon 1 and a C=C double bond is between carbons 2 and 3.

Numbering starts at the ring carbon bearing the -OH group because -OH is the principal functional group in this name.

(v)

'Heptanal' indicates an aldehyde (-CHO) with seven carbon atoms in the parent chain; the -CHO carbon is included in the numbering as C-1.

'6-hydroxy' indicates an -OH substituent at carbon 6.

The structural formula is CH₃CH(OH)CH₂CH₂CH₂CH₂CHO (6-hydroxyheptanal).

Nomenclature of Substituted Benzene Compounds

For IUPAC nomenclature of substituted benzene derivatives, the substituent is named as a prefix to the word benzene. Many such compounds also have widely used common (trivial) names which are often given in brackets after the systematic name.

When a benzene ring is disubstituted, the positions of substituents are indicated by numbering the ring carbon atoms so that substituents receive the lowest possible set of locants. For example, the compound shown as (b) is named 1,3-dibromobenzene (not 1,5-dibromobenzene).

In the traditional/trivial system, the prefixes ortho (o-), meta (m-) and para (p-) are used to denote relative positions 1,2- ; 1,3- and 1,4- respectively on the benzene ring.

Thus, 1,3-dibromobenzene is commonly named m-dibromobenzene, and the 1,2- and 1,4- isomers are named o- and p-dibromobenzene respectively.

For tri- or higher substituted benzene derivatives the ortho/meta/para prefixes are not used; instead substituent positions are specified by numbers following the lowest locant rule. The substituents are listed in the name in alphabetical order (ignoring multiplicative prefixes such as di, tri).

1-Chloro-2,4-dinitrobenzene (not 4-chloro,1,3-dinitrobenzene)

2-Chloro-1-methyl-4-nitrobenzene (not 4-methyl-5-chloro-nitrobenzene)

2-Chloro-4-methylanisole 4-Ethyl-2-methylaniline

3,4-Dimethylphenol

When a benzene ring is attached to an alkane that contains a functional group, the benzene ring may be treated as a substituent rather than the parent. The name for a benzene substituent is phenyl (C₆H₅-), often abbreviated as Ph-.