Classification of Organic Compounds

a) Acyclic or open chain compounds:  These contain alkanes, alkenes, alkynes and their derivatives. These are also called aliphatic compounds.

b) Cyclic or closed chain compounds:

  • Cyclic compounds whose rings are made up of only one kind of atoms, i.e. carbon atoms are called homocyclic or cabocyclic compounds. Aliphatic cyclic compounds are called alicyclic compounds eg cyclopropane, cyclobutane etc

  • Organic compounds containing one or more fused or isolated benzene rings and their functinalized derivatives are called benzenoids or aromatic compounds, eg benzene, toluene, naphthalene, anthracene etc.

  • Cyclic compounds containing one or more heteroatoms (usually O,N, S etc) are called hetrocyclic compounds eg ethylene oxide, tetrahydrofuran (THF), furan, pyrole etc

IUPAC Nomenclature of Organic Compounds:

a) Nomenclature of saturated hydrocarbons:

  • Select the longest continuous chain of carbon atoms in the molecule. The compound is named as a derivative of this alkane

  • Number the carbon atoms in the parent chain starting from the end which gives lowest possible sum for the numbers of the carbon atoms carrying the substituents

  • That set of locants is preferred, which when compared term by term with other set of locants, each in order of increasing magnitude, has the lowest term at the first point of difference. For example the set of locants (2,7,8) is preferred over the set of locants (3,4,9) since 2 comes before 3 even though the sum of locants in the former case is 17 while in the latter case, it is 16

  • The correct name is: 2,7,8 - Trimethyldecane and not 3,4,9 - Trimethyldecane

b) Nomenclature of compounds containing functional group or multiple bonds:

  • Select the longest continuous chain containing the carbon atoms having the functional group or those involved in the multiple bonds

  • The numbering of atoms in the parent chain is done in such a way that the carbon atom bearing the functional group or those carrying the multiple bond gets the lowest possible number

  • While writing the name of alkene (double bond) or alkyne (triple bond), the primary suffix 'ane' of the corresponding alkane is replaced by 'ene' and 'yne' respectively. However, if the multiple bond occurs twice or thrice in the parent chain, the prefix di- or tri- is attached to the primary suffix ene or yne

  • In naming the organic compounds containing one functional group a suffix known as secondary suffix is added to the primary suffix (giving number of carbon atoms in the chain) to indicate the nature of the functional group. A few important secondary suffixes are:

Functional group Secondary suffix Functional group Secondary suffix
Alcohols (-OH) -ol Aldehydes (-CHO) -al
Ketones (>C=O) -one Carboxylic acids (-COOH) -oic acid
Amines (-NH2 ) -amine Acid amides (-CONH2 ) -amide
Acid chlorides (-COCL) -oyl chloride Esters (-COOR) -oate
Nitrites (-C=N) -nitrite Thioalcohols (-SH) -thiol

Nomenclature of compounds having polyfunctional groups:

When an organic compound contains two or more functional groups, one group is called the principal functional group while the others are called the secondary functional groups and are treated as substituents: The order of preference for principal group is: Carboxylic acid > acid anhydrides > esters > acid halides > amides > nitrites > aldehydes > ketone > alcohols > amines > double bond > triple bond

When the functional groups act as substituents, they ar named as:

Functional group Prefix Functional group Prefix
- COOH Carboxy -CHO Formyl
-COOR Alkoxy cabonyl or Carbalkoxy >CO Oxo or Kelo
-COCL Chloroformyl -OH Hydroxy
-CONH2 Carbamoyl -SH Mecaplo
-CN Cyano -NH2 Amino
-OR Akoxy =NH Imino
-X Halo -NO2 Nitro

Nomenclature of simple aromatic compounds:

a) Nuclear substituted: In these the functional group is directly attached to the benzene ring. Most of these compounds are better known by their common and historical names. In the IUPAC system, they are named as derivatives of benzene.

b) Side chain substituted: In these the functional group is present in the side chain of the benzene ring. Both in the common and IUPAC systems, these are usually named as phenyl derivatives of the corresponding aliphatic compounds.