Resonance energy is the difference in energy between actual structure of compound and most stable resonating structure. The resonance energy of benzene is 150.325 J mol^-1.

For preparation of alkanes containing odd number of carbon atoms, a mixture of two alkyl halides has to be used. Since two alkyl halides can react in three different ways, therefore, a mixture of three alkanes instead of the desired alkane would be formed. For example, the Wurtz reaction between 1-bromopropane and 1-bromobutane gives a mixture of three alkanes i.e., hexane, heptane and octane as shown below:

Step 1. Write the structure of the products side by side with their oxygen atoms pointing towards each other.

Step 2. Remove the oxygen atoms and join the two ends by a double bond, the structure of the alkene 'A' is

Peroxide effect is not observed in addition of HCl and HI. This is due to fact that the H__Cl bond being stronger (430.5 kJ mol^-1) than H__Br bond (363.7 kJ mol^-1) is not cleaved by the free radical whreas the H___I bond is weaker (296.8 kJ mol^-1) and iodine free radicals combine to form iodine molecules instead of adding to the double bond.

CH₃CH₂C(CH₃) = CHCH₃

Boiling points of alkanes
1. The boiling points of alkanes rise as the number of carbons increases.
2. Since nonane has a longer carbon chain than octane, it will have a higher boiling point.
Effect of branching on the boiling point of alkane
1. As an alkane chain branch, the surface area of the molecule decreases.
2. The intermolecular force reduces as a result of this.
3. At a lower temperature, the forces can be readily overcome.
4. Therefore, as the branching increases, the boiling point of an alkane chain decreases.

The structures of cis- and trans-isomer of hex-2-ene are:

The boiling point of a molecule depends upon dipole-dipole interactions. Since cis-isomer has higher dipole moment, therefore, it has higher boiling point.

Iso - propylmagnesium bromide (CH₃)₂CHMgBr,
(CH₃)₂CHMgBr+H₂O⟶CH₃CH₂CH₃+Mg(OH)Br

Molecules with odd number of carbon atoms have lower melting points because they do not fit into crystal lattice easily whereas, hydrocarbons with even number of carbon atoms can fit into crystal lattice easily.

On converting benzene to toluene, there is a fall in the melting point although toluene has the higher molecular mass. This is because the planar molecules of benzene can pack more closely in the crystal lattice and the cohesive forces are strong, whereas the methyl group in toluene prevents such close packing.

Benzene is a planar molecule having delocalized electrons above and below the plane of ring. Hence, it is electron-rich. As a result, it is highly attractive to electron deficient species i.e., electrophiles.
Therefore, it undergoes electrophilic substitution reactions very easily.

Nucleophiles are electron-rich. Hence, they are repelled by benzene. Hence, benzene undergoes nucleophilic substitutions with difficulty.

For exhibiting cis-trans (or geometrical isomerism, a molecule must fulfil the following condition.
It must have atlest one double bond.

(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

(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