3 - chloro nitrobenzene :

Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules
(i) The molecule must be co-planar
(ii) Complete delocalization of \(\pi\) electron in the ring
(iii) Presence of (4n+2) \(\pi\) electrons in the ring where n is an integer (n = 0,1,2 .... ) This is known as Huckel's rule.
Eg:

(i) The benzene is a planar molecule
(ii) It has six deloclaised \(\pi\) electoms
(iii) 4n + 2 = 6
4n = 6-2
4n = 4
n = 1
it obeys Huckel's (4n+2) \(\pi\) electron rule with n = 1 hence, benzene is aromatic.

Benzene undergoes nitration reaction becasuse it is an elecron - rich system due to delocalised \(\pi\) electron. So it is easily attacked by electrophiles and gives substituted products.

Mechanism:
Step - I : Formafion of the electrophile:

Step - II : Formation of carbocation intermediate:
The electrophile attacks the aromatic ring to form a carbocation intermediate which is stabilised by resonance.

Step - III: Formation of nitro benzene:
The electrophile attacks the artomatic ring to fo

(i) Sodium Propionate:
\(\underset { Sodium\quad Propionate }{ C{ H }_{ 3 }C{ H }_{ 2 }CO{ O }^{ - }N{ A }^{ + } } +NaOH\overset { CaO }{ \underset { \Delta }{ \longrightarrow } } \underset { Ethane }{ C{ H }_{ 3 }-C{ H }_{ 3 } } +N{ a }_{ 2 }C{ O }_{ 3 }\)
(ii) Pentanoic acid:
\(C{ H }_{ 3 }C{ H }_{ 2 }C{ H }_{ 2 }C{ H }_{ 2 }COONa+NaOH+NaOH\overset { CaO }{ \underset { \Delta }{ \longrightarrow } } C{ H }_{ 3 }C{ H }_{ 2 }C{ H }_{ 2 }C{ H }_{ 3 }+N{ a }_{ 2 }C{ O }_{ 3 }\)

Ozone adds to carbon-carbon triple bond of alkynes to form ozonides. The ozonides are hydrolyzed by water to form carbonyl compounds. The hydrogen peroxide (H₂O₂) formed in the reaction may cxidise the carbonyl compound to carboxylic acid.

(i) Benzene contains delocalized \(\pi\)-electrons which make the ring to act as an electro rich centre. So electrophilic substitution reaction occurs in benzene.
(ii) Benzene ring is stabilized by delocalized \(\pi\) electrons. Though it is highly stable, it undergoes addition and oxidation reaction under specic conditions.

1) Acetylene is used in oxyacetylene torch used for welding and cutting metals.
2) It is used for manufacture of PVC, polyvinyl acetate, polyvinyl ether, orlon and neoprene rubbers.

1) Alkenes find many diverse applications in industry. They are used as starting materials in the synthesis of alcohols, plastics, liquors, detergents and fuels
2) Ethene is the most important organic feed stock in the polymer industry. E.g. PVC, Sarans and polyethylene. These polymer are used in the manufacture of floor tiles, shoe soles, synthetic fibres, raincoats, pipes etc.,

Isomerisation is a chemical process by which a compound is transformed into any its isomeric forms. Normal alkanes can be converted into branched alkanes in the presence of AICl₃ and HCI at 298 K.
CH₃ - CH₂ - CH₂ - CH₃ \(\overset { AIC{ I }_{ 3 }/HCI }{ \underset { 298\quad K }{ \longrightarrow } } \overset { { CH }_{ 3 }-CH-{ CH }_{ 3 } }{ \underset { { CH }_{ 3 } }{ | } } \)
This process is of great industrial importance. The quality of gasoline is improved by isomerising its components.

(i) Benzenoid compounds:
Benzene and its derivatives are called benzenoid compounds

(ii) Non-benzenoid compounds:
The aromatic compounds that are not have benzene ring in their structure are called non-benzenoid compounds.

Ethene reacts with cold alk KMno₄ (Balyer's regent to give ethane 1,2 - diol

Markovnikoff's rule: "When an unsymmetrical alkene reacts with hydrogen halide, the hydrogen adds to the carbon that has more number of hydrogen and halogen add to the carbon having fewer hydrogen".
Eg: Addition HBr to unsymmetrical alkene: In the addition of hydrogen halide to an unsymmetrical alkene, two products are obtained.

Propane burns in excess of oxygen to form water and carbon di oxide.
CH₂CH₂CH₃ + 50₂ \(\rightarrow\) 3CO₂ + 4H₂O
      Propane