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Definition of organic chemistry (Historical Background) :-

Old concept :-

 The word 'organic 'signifies Life. Therefore all substances which were obtained directly or indirectly from living organisms, plants and animals, were called organic compound and the branch of chemistry which deals with these compound was called organic chemistry. 

Re-classification of natural substances into two categories. 

👉all those which could be obtained from vegetables or animals i.e., the substances which were produced by living organisms. These substances were classified as organic compounds.

👉all those which were not obtained from the living organisms. These substances were classified as inorganic compounds.

Modern definition of organic chemistry :-

Since carbon is an essential constituent of all organic compounds, organic chemistry is defined as the chemistry of carbon compounds. In fact, out of millions of organic compounds now known, there is not a single compound which does not contain carbon. However, there are compound of carbon such as carbon monoxide, carbon dioxide, metal carbonates and bicarbonates,metal cyanides,metal carbides etc which are studied in inorganic chemistry because of their properties. 

👉Hydrocarbons :-

Simplest organic compounds are those which are composed of carbon and hydrogen only and are called hydrocarbon.organic chemistry may also defined as the chemistry of hydrocarbon and their derivatives. 

👉Reasons For Large Number OF organic compound :-

1.Catenation :-

The tendency of an element to form chains of identical atoms is called Catenation. This tendency is observed in the case of non -metals showing covalency of two or more. 

2.Tendency to combine with other non -metals :-

Carbon has the ability to form strong covalent bonds with other atoms such as H  ,O, S, N, Cleaning, Br,I. etc The C -C bond energy is approximately equal to the magnitude of bond energies between carbon and various other atoms. 

C-N. = 73.0 (bond energy  )

3.Isomerism:-

The phenomenon of isomerism shown by organic compounds is also responsible for large number of organic compounds. The compound are said to be isomers if they have same composition that is same chemical formula but different properties. 

👉General characteristics of organic compounds :-

1.Composition :-

Organic compounds are mainly composed of ten elements.There are C, H, O, N, S, P, F, Cl, Br and I. Compound obtained from plants such as starch, sugar, cellulose, oils, fats etc, contain mainly carbon, hydrogen and oxygen while those obtained from animals such as urea, uric acid, protein etc 

2.Quadri -covalency of carbon :-

The carbon present in organic compounds is always quadri -covalent in nature. It combines with other atoms by covalent bonds. 

Organic compounds are essentially covalent compounds.They show the following characteristics of covalent compounds :-

(a) They are volatile in nature. 

(b) They have low melting and boiling point. 

(c) They are generally insoluble  in water but soluble in organic solvents. 

(d) They are bad conductors of electricity. 

(e) They do not furnish ions. They show molecular reactions.

3.Colour and odour :-

Organic compounds posses distinct colour and odour. 

4.Action of Heat :-

Organic compounds are not stable towards heat. They decompose on heating at high temperatures. Some of them decompose on heating leaving a black residue. 

5.Combustion:-

Organic compounds readily burn in air. They burn with smoky or non-smoky flame. One of the products of combustion is always carbon dioxide. 

6.Homologous series :-

Organic compounds have been classified into many classes are known as homologous series. The compounds belonging to the same class show similar chemical properties. 

7.Polymerism:-

Quite a number of organic compounds exhibit polymerism (when a molecular formula of an organic compounds is a simple multiple of the other). 

8.Isomerism :-

Organic compounds show the phenomenon of isomerism.