Hydrocarbons are compounds of carbon and hydrogen only, obtained from coal and petroleum mainly which are major sources of energy. Hydrocarbons are classified as open chain, saturated (alkanes), unsaturated (alkenes and alkynes), cyclic (alicyclic) and aromatic based on structure.
Alkanes show conformational isomerism due to free rotation along C-C bond leading to staggered and eclipsed conformations of ethane. Staggered conformation is more stable. Alkenes show geometrical (Cis-trans) isomerism due to restricted rotation around carbon-carbon double bond. Benzene and benzenoid show aromatic character. They follow Huckel rule \((4 n+2) \pi\) electrons which must be delocalised. The presence of activating and deactivating groups decide the position of electrophile after electrophilic substitution. Polynuclear fused aromatic hydrocarbons have carcinogenic property. Benzene is prepared by polymerisation of ethyne and by heating sodium benzoate with soda lime.
(a) Why is staggered form of ethane more stable than eclipsed form?
(b) Out of T-butene and 2-butene which will show geometrical isomerism.
(c) Why is cis-2.-butene has higher boiling point than trans-2-butene?
(d) Why is cyclopentadienyl anion is aromatic?
(e) Why is -NO₂ group m-directing towards electrophilic substitution?
(f) Convert acetylene to benzene .
(g) Sodium benzoate, on heating with soda lime gives benzene, name the reaction.

Observe the table of variation of melting and boiling point in Alkanes. Study the table and answer the questions that follow based on table and related studied concepts.
Variation of Melting Point and Boiling Point in Alkanes

(a) How does boiling point varies with the increase in carbon chain? Why?
(b) Why does 2, 2-dimethyl propane has lower boiling point than 2-methyl butane and n-pentane?
(c) Why is melting point of 2, 2-dimethyl propane is higher than n-pentane and 2-methyl butane?
(d) What is physical state of heptane?
(e) How does density vary with the increase in molar mass in hydrocarbons?
(f) What is physical state of C₂₀H₄₂(Eicosane)?

Organic reactions can be classified into four main categories. Substitution reactions, addition reactions, elimination reactions and rearrangement reactions. Substitution reactions can be further classified into free radical, nucleophilic and electrophilic substitution reactions. Addition reactions can be nucleophilic as well as electrophilic addition reactions. Dehydration, dehydrohalogenation, dehalogenation are examples of elimination reactions. Conversion by ammonium cyanate to urea is an example of rearrangement reactions. Reactions are classified on the basis to nature of intermediate species formed. Mechanism of reaction is exact path followed by the reaction involving all steps showing intermediates and slowest steps of the reaction which is rate determining step. Oxidation, reduction, combustion reactions are also important in hydrocarbons.
(a) Halogenation of alkane is an example of which type of reaction?
(b) What happens when 2-methyl propane is heated with KMnO₄?
(c) What type of reaction takes place when n-hexane is heated in presence of AICI₃ (anhy.) and HCI?CH2 =CH2 + KCI + HzO
(d) What happens when but-2-yne reacts with H₂ in presence of Lindlar's catalyst.
(e) CH₃CH₂C1 + KOH(alc) \(\longrightarrow\) CH₂ =CH₂ + KCI + H₂O 
What is type of reaction?
(f) Why do aromatic hydrocarbons undergo electrophilic substitution reaction?