Introduction to Organic Compounds

The branch of chemistry that deals with the study of the study of carbon and its derivative is called organic chemistry.
In chemistry there are two fundamental classes of chemical compounds on the basis of origin. They are organic and inorganic compounds.
Inorganic compounds: Those organic compounds which are obtained from non-living sources are called inorganic compounds. Example: Rock, minerals etc.
Organic compounds: Those organic compounds which are obtained from living sources such as plants and animals etc are called organic compounds.
Origin of organic compounds:
In the earlier days, it was believed that organic compounds could be produced only by living organism. Berzelius proposed that there is a natural force inside the body of living plants and organism which can generate organic compounds.  This theory was called as vital force theory. As this concept was prevailing in chemical world, no efforts were made to produce organic compounds in the laboratory.
But, with the growing research in organic chemistry, German chemist Friedrich Wholer synthesized an organic compound in laboratory for the first time in the world. The compound was urea. He prepared urea by reacting ammonium sulphate  and potassium cyanide as given below.

(NH4)2SO4Ammonium sulphate+2KCNOPotassium cyanide−→Δ2NH4CNOAmmonium cyanide+K2SO4

NH4CNO−→ΔNH2−CO−NH2Urea

After then, more and more organic compounds were synthesized in the laboratory by artificial medium. There are more than five millions of organic compounds synthesized by artificial and natural medium till today. Due to the extensive application of organic compounds, more and more research is going on in organic chemistry.

Applications and Sources of Organic Compounds

Application of organic compounds:
The application of organic chemistry in today’s world is considered as very vital. There are more than five millions of organic compounds till now.  Many compounds are yet to be discovered. Some of the major applications of organic compounds are:

• Poly-ethene is is used to make plastic buckets, bowls, plastic bags etc.
• Poly-phenylethtane is used to make yoghurt cartoons and packaging.
• Poly tetra fluroethane is used to make non stick coating in frying pans.
• Poly propane is used to make strong ropes.
• Different hydrocarbons are used to manufacture medicines.
• Ethene is used as a fuel for vehicles, airplanes etc.
• Likewise, different organic compounds are used in cosmetics, agriculture, clothing etc.

Sources of organic compounds:
The main sources of organic compounds are living organisms i.e., plants and animals. The various organic compounds have been prepared artificially in laboratories. Some of the important sources of organic compounds are as follows:
1. Plants: Most of the organic compounds are obtained from plants and animals by suitable methods of isolation. Example: Carbohydrates, protein, perfumes, colors are obtained from plants.
2. Coal: Coal is another main source of organic compounds. From coal, hundreds of useful organic substances like solvents, synthetic rubber, explosives, plastics etc are produced by carrying suitable synthetic reactions.
3. Fermentation: One of the useful method for obtaining some organic substances like alcohols, acids, vitamins, antibiotics is fermentation. Fermentation is the process carried in presence of micro-organisms. In this process, the larger micro organisms are converted into simple organic compounds by micro-organisms.
4. Artificial synthesis: Nowadays, a large number of organic compounds are artificially synthesized in the laboratory. Examples: plastics, fibres, drugs, vitamins, dyes etc. However, the starting substances required for the artificial synthesis are obtained mainly from petroleum and coal.

Homologous Series

The series of organic compounds in which the consecutive member differs by CH₂ group is called homologous series.  Examples:
CH₄: Methane
C₂H₆: Ethane
C₃H₈: Propane
C₄ H₁₀: Butane
Also,
CH₄OH: Methanol
CH₃CH₂OH: Ethanol
CH₃CH₂CH₂OH: Propanol
CH₃CH₂CH₂CH₂OH: Butanol
Properties of homologous series:

• Consecutive members differ by CH₂ units.
• All the members of homologous series have similar properties.
• Physical properties of homologous series gradually changes with the change in molecular weight.
• All the members of homologous series can be represented by a common general formula. Example: Alkanes can be represented by the general formula. C_nH_2n+2, where n is the number of carbon atom.
• Two consecutive homologues differs by 14 units in molecular weight.

Catenation Property of Carbon

It is the property of carbon or any other element by which the large number of atoms can be linked to form a long chain. Carbon has the maximum catenation property. Carbon itself combines with other carbons to form a long chain. This combination may give cyclic or acyclic structure.

Catenation

The carbon-carbon bond enthalpy is about 347.0 KJ^– which is very high. High bond enthalpy is the main cause for very effective catenation property in carbon.   Strong covalent bond between the carbon atoms favors the catenation property in carbon.
Carbide: Compounds of carbon with metals is called carbides.

Classification of Organic Compounds

For the detailed study, organic compounds are classified into different groups and sub groups on the basis of their physical structure and chemical properties. The main classification of organic compounds is given below.
Organic compounds are broadly classified into the following classes:

• Open chain or cyclic compounds: The compounds which contain an open chain system of carbon atoms is called open chain compound. The chains may be straight chains (non-branched) or branched. The open chain compounds are also called aliphatic compounds. The name aliphatic is derived from the Greek word aleiphar meaning fats, as the earlier known compounds of this class were either obtained from animal or vegetable fats, or had fat like character. Example: 1 Butanol, Propane, 2-methyl propane are the examples of open chain compounds. Cyclic compounds are either branch chained or straight chained.

• Closed chain or acyclic (or ring) compounds: The homocyclic compound containing a ring of three or more carbon atoms and resembling aliphatic compounds are called alicyclic compounds. Benzene, anthracene are the examples of closed chain compounds. Closed chain compounds are either homocyclic  (compounds containing carbon ring only like benzene)or heterocyclic (compounds containing carbon and other elements in the ring like pyridine, furan etc).

Functional Group

Atom or group of atoms which determines the chemical properties of an organic compound is called functional group.  Example: –COOR is the functional group of ester. An organic compound may contain one or more functional group. In this case, the naming of organic compound is made according to priority order.
The  priority order of the functional group is given below:
Sulphonic acid > Carboxylic acid >Acid Anhydride > Ester > Acid chloride > Amide > Aldehyde > Nitrile > Ketone > Alcohol > Amines > Ethers > Alkenes > Alkynes
Some of the important functional groups and their examples are listed below:

Significance of functional group:

• Functional group denotes the naming of organic compounds.
• Functional group specifies the classes of organic compounds.
• Functional group denotes the reactivity of organic compounds.

Detection of Foreign Elements in an Organic Compound

The presence of foreign elements in organic compounds can be detected by Lassaigne’s test. This test is also known as sodium extract test or sodium fusion test. It is the most reliable method for the detection of foreign compounds in organic compounds.
Preparation:
In Lassagines’ test, the given sample of organic compounds is heated with sodium metal in an ignition tube. Thus, heated ignition tube is plunged in an porcelain basin containing distilled water. The content in the basin is crushed, boiled and then filtered. The filtrate is called Lassaigne’s solution which is used for further test for the identification of element.
The Lassigne’s solution may contain the following compounds:
If nitrogen is present:
$$\ce{Na + C + N-> NaCN <=>Na^++ + CN^-}$$
If sulphur is present,
$$\ce{Na + S->Na2S->2Na^+ + S^– }$$
If both nitrogen and sulphur are present,
$$\ce{Na + CN + S->NaCNS<=>Na^+ + CNS^- }$$
If halogen is present,
$$\ce{Na + X->NaX <=>Na^+ + X^-}$$
Detection of nitrogen:
At first the sodium extract is made alkaline by adding NaOH. Then to the alkaline solution of sodium extract, freshly prepared FeSO₄ solution is added. After then there is a formation of dirty green ppt. which is boiled from some time and then dissolved by adding dilute HCl. Finally, ferric chloride is added to the solution and the prussian blue color is formed which confirms the presence of nitrogen. The reactions involved are as follows:
Preparation of sodium extract:
$$\ce{Na + C + N-> $\underset{\text{Sodium cyanide (Sodium extract)}}{\ce{NaCN}}$}$$
Addition of ferrous sulphate and dilute hydrochloric acid:
$$\ce{FeSO4 + 2NaOH->Fe(OH)2 + Na2SO4}$$
$$\ce{Fe(OH)2 + 2NaCN->Fe(CN)2 + 2NaOH}$$
$$\ce{Fe(CN)2 + 4NaCN->$\underset{\text{Sodium ferrocyanide}}{\ce{Na4Fe(CN)6}}$}$$
$$\ce{2Cu + $\underset{\text{Steam}}{\ce{H2O}}$ ->$\underset{\text{Copper Oxide}}{\ce{CuO}}$ + H2}$$
Confirmation for presence of nitrogen:
$$\ce{3Na4Fe(CN)6 + 4FeCl3->$\underset{\text{Ferric ferrocyanide (Prussian blue)}}{\ce{ Fe4[Fe(CN)6]3}}$ + 12NaCl}$$
Detection of both nitrogen and sulphur:
 If nitrogen and sulphur both are present, blood red coloration is formed.
$$\ce{Na + C + N +S->NaCNS}$$
$$\ce{NaCNS + FeCl3->$\underset{\text{Blood red}}{\ce{Fe(CNS)3}}$ + NaCl}$$
Detection of Sulphur
Sulphur is detected in either by lead acetate test or sodium nitroprusside test:

• Lead acetate test: The Lassaigne’s test is first acidified with acetic acid and then lead acetate solution is added. Formation of black precipitate confirms the presence of sulphur.

$$\ce{Na + S->Na2S}$$
$$\ce{Na2S +(CH3COO)2Pb->$\underset{\text{Black}}{\ce{PbS ^ }}$ + CH3COONa}$$

• Sodium Nitroprusside test: To a small portion of sodium extract, sodium nitroprusside solution is added. Formation of violet coloration shows the presence of sodium sulphur in organic compound.

$$\ce{Na2S + $\underset{\text{sodium nitroprusside}}{\ce{Fe(CN)5NO}}$ ->$\underset{\text{Sodium sulphonitroprusside}}{\ce{Na4[Fe(CN)5NOS]}}$}$$
Detection of halogens:
Few drops of concentrated HNO₃ is added to the sodium extract. The solution is then boiled to remove any gas is produced. A few drops of AgNO₃ solution is added to the Lassaigne’s test.
Formation of curdy white ppt soluble in ammonia solution confirms the presence of chlorine.
$$\ce{Na + Cl->NaCl}$$
$$\ce{NaCl + AgNO3-> $\underset{\text{Curdy white}}{\ce{AgCl v }}$}$$
Formation of pale yellow ppt. partially soluble in ammonia solution confirms the presence of bromine.
$$\ce{Na + Br->NaBr}$$
$$\ce{NaBr + AgNO3->$\underset{\text{Pale yellow}}{\ce{AgBr v }}$ + NaNO3}$$
Formation of pale yellow precipitate shows the presence of iodine.
$$\ce{I + Na->NaI}$$
$$\ce{NaI + AgNO3->$\underset{\text{Silver iodide}}{\ce{AgI v }}$ + NaNO3}$$