Convert Ethyne to Benzene and name the process.

1. Hydrocarbons:

(i) Hydrocarbons are the compounds which are of carbon and hydrogen only.

(ii) Hydrocarbons are the major sources of energy. LPG (liquefied petroleum gas) and CNG (compressed natural gas), the main sources of energy for domestic fuels and the automobile industry, are obtained from petroleum.

2. Classification:

Hydrocarbons are classified as open chain saturated (alkanes) and unsaturated (alkenes and alkynes), cyclic (alicyclic) and aromatic, according to their structure.

3. Alkanes

(i) Alkanes are saturated hydrocarbons having general formula CnH2n+2. They contain only $\mathrm{C}—\mathrm{C}$ and $\mathrm{C}—\mathrm{H}$ sigma bonds.

(ii) Alkanes show conformational isomerism due to free rotation along the $\mathrm{C}—\mathrm{C}$ sigma bonds.

(iii) Out of staggered and the eclipsed conformations of ethane, staggered conformation is more stable as hydrogen atoms are farthest apart.

4. Preparation of alkanes:

(i) Alkanes can be prepared from alkyl halides by Wurtz reaction or by reduction or through Grignard reagents.

R–X+2Na →EtherR–R+2NaX

R–X+Mg→RMgX →H2OR–H

(ii) Alkanes can also be prepared from unsaturated hydrocarbons by hydrogenation.

R–X+2H→R–H+HX

(iii) Kolbe’s electrolysis of Na or K salts of fatty acids gives alkanes.

5. Chemical properties of alkanes:

(i) Halogenation takes place either at higher temperature $(573-773 \mathrm{K})$ or in the presence of diffused sunlight or ultraviolet light.

(ii) Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.

(iii) Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.

(iv) n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.

(v) n-Alkanes having six or more carbon atoms on heating to $773\mathrm{K}$ at $10-20$ atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as aromatization or reforming.

(vi) Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

6. Alkenes

(i) Alkenes are unsaturated hydrocarbons having general formula CnH2n. They contain a carbon-carbon double bond.

(ii) Alkenes exhibit geometric isomerism due to restricted rotation about carbon-carbon double bond.

(iii) The simplest alkene that can exhibit geometric isomerism is but$-2-$ene.

(iv) The cis isomer is more polar and has higher boiling point than the trans isomer. On the other hand, the melting point of trans isomer is higher.

7. Preparation of alkenes:

Alkenes can be obtained by dehydrohalogenation of alkyl halides, by dehydration of alcohols or by partial hydrogenation of alkynes using Lindlar’s catalyst.

8. Chemical properties of alkenes:

(i) Markovn ikov’s Rule: During electrophilic addition across unsymmetrical double bond, the negative part of the adding molecule goes to that carbon which has less number of hydrogen atoms.

(ii) Kharasch Effect: Anti-Markownikov addition of HBr to alkenes in the presence of organic peroxides.

9. Alkynes:

Alkynes are unsaturated hydrocarbons having general formula CnH2n–2_. They contain a carbon-carbon triple bond.

10. Preparation of alkynes:

Alkynes are prepared by dehydrohalogenation of dihalides or dehalogenation of tetrahalides. Calcium carbide on reaction with water liberates ethyne.

11. Chemical properties of alkynes:

(i) The important reactions of alkynes are free radical substitution, combustion, oxidation and aromatization.

(ii) Alkenes and alkynes undergo addition reactions, which are mainly electrophilic additions.

(iii) Terminal alkynes are acidic in character.

12. Arenes:

(i) Arenes are aromatic hydrocarbons. They contain at least one benzene ring.

(ii) Aromatic hydrocarbons undergo mainly electrophilic substitution reactions inspite of high degree of unsaturation present in them.

(iii) Aromaticity in aromatic compounds is due to the presence cyclic, delocalized system of $(4\mathrm{n} + 2)\mathrm{\pi }\mathrm{ }$electrons.

(iv) Electron releasing groups increase the reactivity of benzene ring towards electrophilic substitution reactions. Examples of such groups are —R,—OH,—NH2,—OR,—NHCOCH3, etc.

(v) Electron withdrawing groups decrease the reactivity of benzene ring towards electrophilic substitution reactions.

(vi) Benzene and many polynuclear hydrocarbons containing fused benzene rings are known to possess carcinogenic (cancer causing) properties.