Classification of Organic Compounds: open chain, cyclic compounds

Classification of Organic Compounds : open chain and cyclic compounds

The organic compounds are classified into two main types, namely.

(1) Open - chain or acyclic compounds or aliphatic compounds

The open chain or acyclic compounds are those in which carbon atoms are linked to each other either in linear or branched fashion such that the molecule is having open chain structure. These compounds are also called aliphatic compounds (Greek word : aliphar meaning fat).

Examples

CH3 - CH3 - CH3 propane

(2) Closed chain or cyclic compounds

Organic compounds with closed chain of atoms are called closed chain compounds or ring compounds.

These compounds are further classified into

(a) Homocyclic or carbocyclic compounds

(b)Heterocyclic compounds

(a) Homocyclic compounds : In these compounds the ring structure is made up of only carbon atoms.

These compounds are further classified into

               i.Aromatic compounds and

             ii.  Alicyclic compounds

(i) Aromatic compounds (Benzenoid) : Compounds containing one or more benzene rings in their structure are classified as aromatic benzenoid compounds. Most of these compounds have pleasant odour (Greek : Aroma - sweet smell).

(b) Heterocyclic compounds (Non - benzenoid aromatic) : Cyclic compounds in which the ring atoms are made up of hetero atoms like nitrogen, oxygen and sulphur in addition to carbon atoms are called heterocyclic compounds.

The above compounds are aromatic non-benzenoid compounds.

(ii) Alicyclic compounds : Cyclic compounds with ring structure containing only carbon atoms are called alicyclic or carbocyclic compounds. Though these compounds possess a ring structure, they behave more like aliphatic compounds.

Characteristics of organic compounds

All organic compounds have the following characteristic properties

1.     Many organic compounds are inflammable

2.     They are mostly covalent compounds

3.     They are generally soluble in non - polar solvents like carbon tetrachloride, benzene etc.

4.     They have generally low boiling point and melting point.

5.     They exhibit isomerism

Homologous series

A group or class of organic compounds related to each other by a general molecular formula contributes homologous series.

Characteristics of homologous series

Homologous series have the following characteristics :

(1) All members of a series contain same elements and the same functional groups.

(2) All the members of a homologous series can be represented by a general formula

Examples

Alkanes          CnH2n+2

Alkenes          CnH2n

Alkynes         CnH2n-2

(3) All the members of a homologous series can be prepared by similar methods.

(4) All members of a homologous series usually undergo similar chemical reactions.

(5) Successive members in a series differ by a -CH2 group

(6) The physical properties of the members of a homologous series vary in a regular way down the series. For example, boiling point, melting point and density of the alkane series vary in a regular way with increasing number of carbon atoms.