a) Alkenes contain two hydrogen atoms less than alkanes and thus they contain C - C double bond (C = C) in their molecule. Thus they are called unsatujated hydrocarbons.
(b) Alkenes react with cold dilute KMnO₄ solution to form gycols. Since bright purple colour of KMnO₄ disappears during the reaction it is used as a test for the presence of double bond.
(c) A mixture of acetaldehyde and formaldehyde is formed.

Wurtz synthesis: Higher alkanes are prepared by heating an alkyl halide (RX) with sodium metal in dry ether solution.
R - X + 2Na + XR \(\overset { ether }{ \longrightarrow } \) R - R + 2NaX
CH₃Br + 2Na + BrCH_{3\(\longrightarrow \)} CH₃-CH₃ + 2NaBr
Limitations: Use of two different alkyl halides in Wurtz reaction always leads to a mixture of alkanes. The separation of these alkanes is difficult because there is only a little difference in their boiling points. Thus only symmetrical alkanes can be prepared by this method.

(a) Functional isomerism.
(b) (i) \(CH\equiv CH+{ 2H }_{ 2 }\overset { Ni }{ \underset { 573\ K }{ \longrightarrow } } \ { CH }_{ 3 }-{ CH }_{ 3 }\)
            Acetylene                               Ethane

(iii) CH₃CH₂OH \(\overset { Conc.{ H }_{ 2 }{ SO }_{ 4 } }{ \underset { heat }{ \longrightarrow } } \) CH₂ = CH₂ + H₂O

(a) Substitution reactions are those reactions in which an atom or group of atoms directly attached to carbon in the substrate molecule is replaced by another atom or group of atoms, for example,

A hydrogen atom of the methane molecule is replaced by chlorine atom. Benzene undergoes electrophilic substitution reactions because benzene ring has delocalized electrons is an electron-rich system. It is attacked by electrophiles giving substitution products.

The aromatic compounds apparently contain alternate double and single bonds in a cyclic structure, and resemble benzene in chemical behaviour. They undergo substitution reactions rather addition reactions. This characteristic behaviour is called Aromatic character or Aromaticity.
Conditions for Aromaticity:
(i) An aromatic compound is cyclic and planar.
(ii) Each atom in an aromatic ring has a p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around a ring.
(iii) The cyclic \(\pi\)-molecular orbital formed by the overlap of p-orbitals must contain (4n + 2) \(\pi\) electrons, where (n = 0, 1, 2, 3, 4 etc.)

(ii) Ethene is first converted into ethyne and then to benzene as shown above.

(iii) When vapours of hexane are passed over heated catalyst consisting of Cr₂ O₃, MO₂O₃ and V₂O₅ at 773 K under 10-20 atm pressure, cyclization and aromatization occurs simultaneously to afford benzene

Addition of HBr to propene is an ionic electrophilic addition reaction in which the electrophile, i.e., H+ first adds to give a more stable 2° carbocation. In the 2nd step, the carbocation is rapidly attacked by the nucleophile Br^- ion to give 2-bromopropane.

In presence of benzoyl peroxide, the reaction is still electrophilic but the electrophile here is a Br free radical which is obtained by the action of benzoyl peroxide on HBr

In the first step, Br radical adds to propene in such a way so as to generate the more stable 2° free radical. In the second step, the free radical thus obtained rapidly abstracts a hydrogen atom from HBr to give 1-bromopropane.

From the above discussion, it is evident that although both reactions are electrophilic addition reactions but it is due to different order of addition of H and Br atoms which gives different products.

(i) The two substituents in the benzene ring are present at p-positions. Therefore, the sequence of reactions should be such that first an o, p-directing group, i.e., Br atom should be introduced in the benzene ring and this should be followed by nitration. Thus,

(ii) Here since the two substituents are at m-position w.r.t. each other, therefore, the first substituent in the benzene ring should be a m-directing group (i.e., NO₂) and then other group (i.e., CI) should be introduced. Therefore, the sequence of reactions is:

(iii) Here since the two substituents are at p-position w.r.t. each other, therefore, the first substituent in the benzene ring should be a o, p-directing group (i.e., CH₃) and then the other group (i.e., NO₂) should be introduced.

(iv) Acetophenone can be prepared by F.C. acylation using either acetyl chloride or acetic anhydride.

(i) \(\overset { 5 }{ C } { H }_{ 3 }-\overset { 4 }{ C } H_{ 2 }-\overset { 3 }{ C } H=\overset { 2 }{ C } H\overset { 1 }{ C } { H }_{ 3 }\overset { (i){ O }_{ 3 }/{ CH }_{ 2 }{ CI }_{ 2 },196\quad K }{ \underset { (ii)\quad Zn/{ H }_{ 2 }O }{ \longrightarrow } } { CH }_{ 3 }-{ CH }_{ 2 }-CH=O+O=CH-{ CH }_{ 3 }\)
                       Pent-2-ene                                                                                 Propanal                                Ethanal

(iv) \({ \overset { 4 }{ C } }H_{ 3 }\overset { 3 }{ C } { H }_{ 2 }-\overset { 2 }{ C } H=\overset { 1 }{ C } { H }_{ 2 }-{ C }_{ 6 }{ CH }_{ 5 }\overset { (i){ O }_{ 3 },{ CH }_{ 2 }{ C1 }_{ 2 },196\quad K }{ \underset { (ii)Zn/{ H }_{ 2 }O }{ \longrightarrow } } { CH }_{ 3 }{ CH }_{ 2 }CH=0+0=CH-{ C }_{ 6 }{ H }_{ 5 }\)
                   1-Phenylbut-1-ene                                                               Propanal                              Benzaldehyde

Presence of electron releasing group(or activating group) increases the electron density in benzene nucleus. Therefore, electrophile will attack
benzene nucleus easily. But, the presence of electron withdrawing group like -NO₂ decrease the electron density in benzene ring. Therefore, electrophile will attack
benzene nucleus with difficulty. The order of reactivity towards electrophile, E+ in order of their decreasing relative reactivity is
(i) Chlorobenzene > p-nitrochlorobenzene > 2, 4- dinitrochlorobenzene
(ii) Toluene > \(p-{ CH }_{ 3 }-{ C }_{ 6 }{ H }_{ 4 }-{ NO }_{ 2 }>p-{ O }_{ 2 }N-{ C }_{ 6 }{ H }_{ 4 }-{ NO }_{ 2 }\)