Briefly explain geometrical isomerism in alkene by considering 2-butene as an example.

1. Organic chemistry:

(i) Organic chemistry is the study of hydrocarbon and their derivatives.

(ii) Carbon in its compounds forms covalent bonds. The nature of the covalent bonding can be described in terms of orbitals hybridization concept. Carbon can undergo ${\mathrm{sp}}^3, {\mathrm{sp}}^2$ and $\mathrm{sp}$ hybridization in its compounds.

(iii) In a homologous series, any two successive members differ in their molecular mass by $14 \mathrm{u}$. and in their molecular formula by .

2. Nomenclature of organic compounds:

(i) IUPAC name of a compound is constituted from:

Secondary prefix + Primary prefix + Word root + Primary suffix + Secondary prefix.

(ii) The decreasing order of preferences of the secondary suffixes (functional groups) is :

Carboxylic acid > sulphonic acid > acid anhydride > ester > acid chloride > acid amide > nitrile > aldehyde > ketone > alcohol > amine.

3. Classification of organic compounds:

Aliphatic (Acyclic) compounds are open chain compounds which contain straight or branched chain of carbon atoms. Benzenoids are the aromatic compounds which contain one or more benzene ring.

4. Isomerism:

(i) Compounds having the same molecular formula but different physical and chemical properties are called isomers and the phenomenon is called isomerism.

(ii) Isomerism is broadly divided into two types.

(a) Constitutional isomerism

(b) stereoisomerism.

(iii) Chain or nuclear or skeletal isomerism: Isomers have similar molecular formula but differ in the nature of the carbon skeleton.

(iv) Position isomerism: If different compounds belonging to same homologous series with the same molecular formula and carbon skeleton, but differ in the position of substituent or functional group or an unsaturated linkage are said to exhibit position isomerism.

(v) Functional isomerism: Different compounds having same molecular formula but different functional groups are said to exhibit functional isomerism.

(vi) Metamerism: This type of isomerism is a special kind of structural isomerism arises due to the unequal distribution of carbon atoms on either side of the functional group or different alkyl groups attached to the either side of the same functional group and having same molecular formula.

(vii) Tautomerism: It is a special type of functional isomerism in which a single compound exists in two readily inter convertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers.

(viii) The isomers which have same bond connectivity but different arrangement of groups or atoms in space are known as stereoisomers.

(ix) Geometrical isomers are the stereoisomers which have different arrangement of groups or atoms around a rigid frame work of double bonds. This type of isomerism occurs due to restricted rotation of double bonds, or about single bonds in cyclic compounds.

(x) Compounds having same physical and chemical property but differ only in the rotation of plane of the polarized light are known as optical isomers and the phenomenon is known as optical isomerism.

5. Qualitative and quantitative analysis:

(i) Qualitative and quantitative analysis of organic compounds help in determining their structures.

(ii) Nitrogen, sulphur, halogens and phosphorus are detected by Lassaigne’s test.

(iii) Carbon and hydrogen are estimated by determining the amounts of carbon dioxide and water produced.

(iv) Nitrogen is estimated by Dumas or Kjeldahl’s method.

(v) Halogens are estimated by Carius method.

(vi) Sulphur and phosphorus are estimated by oxidizing them to sulphuric and phosphoric acids respectively.

(vii) The percentage of oxygen is usually determined by difference between the total percentage (100) and the sum of percentages of all other elements present.

6. Methods of purification of organic compounds:

(i) Crystallisation is used to purify organic solids by dissolving them in suitable solvents.

(ii) Fractional Crystallisation is used to separate two or more organic solids with different solubilities in the same solvent.

(iii) Camphor, benzoic acid, naphthalene etc. can be purified by sublimation process.

(iv) Simple distillation is used to purify liquids with non–volatile impurities or liquid mixture in which the components differ in boiling points by atleast $30 – 50 \mathrm{K}$.

(v) Fractional distillation is used to purify a liquid mixture in which the components differ in boiling points by $10 -20 \mathrm{K}$.

Steam distillation can be used to purify organic compounds which give sufficient vapours at the boiling point of water and are not miscible with water.

(vi) Chromatography is particularly useful to purify and separate the constituents from a sample if available in a very small amount.