NCERT Solutions for Class 11 Chemistry Chapter 8 Organic Chemistry Some Basic Principles and Techniques

carbon has the unique property called catenation due to which it forms covalent bonds with other carbon atoms. It also forms covalent bonds with atoms of other elements like hydrogen, oxygen, nitrogen, sulphur, phosphorus and halogens. The resulting compounds are studied under a separate branch of chemistry called organic chemistry.

fundamental concepts of molecular structure help in understanding and predicting the properties of organic compounds. tetravalence of carbon and the formation of covalent bonds by it are explained in terms of its electronic configuration and the hybridisation of s and p orbitals. It may be recalled that formation and the shapes of molecules like methane (CH₄), ethene (C₂H₄), ethyne (C₂H₂) are explained in terms of the use of sp³, sp² and sp hybrid orbitals by carbon atoms in the respective molecules.

Structures of organic compounds are represented in several ways. The Lewis structure or dot structure, dash structure, condensed structure and bond line structural formulas are some of the specific types. The Lewis structures, however, can be simplified by representing the two-electron covalent bond by a dash (–). Such a structural formula focuses on the electrons involved in bond formation. A single dash represents a single bond, double dash is used for double bond and a triple dash represents triple bond.

Organic compounds are broadly classified as follows:
1. Acyclic or open chain compounds
2. Cyclic or closed chain or ring compounds
Acyclic or Open Chain Compounds
These compounds are also called as aliphatic compounds and consist of straight or branched chain compounds, for example: Ethane (CH₃CH₃), Isobutane {CH₃CH(CH₃)₂}, Acetic acid (CH₃COOH).

Cyclic or closed chain compounds are further classified as:
(a) Alicyclic compounds
Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring, Cyclopropane, Cyclohexane, Cyclohexene are the examples of homocyclic. Sometimes atoms other than carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound.

(b) Aromatic compounds
Aromatic compounds are special types of compounds.These include benzene and other related ring compounds (benzenoid). Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called hetrocyclic aromatic compounds. Some of the examples of various types of aromatic compounds are: Benzenoid aromatic compounds: Benzene, Aniline, Naphthalene.
Non-benzenoid compound: Tropone.
Heterocyclic aromatic compounds: Furan, Thiophene, Pyridine.

The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as isomerism. Such compounds are called as isomers.

Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. The common techniques used for purification are as follows:

• (i) Sublimation
• (ii) Crystallisation
• (iii) Distillation
• (iv) Differential extraction and
• (v) Chromatography