The treatment of benzene with isobutene in the presence of sulphuric acid gives

1. Classification of Haloalkanes and Haloarenes:

2. Preparation of Haloalkanes:

(i) From Alcohols:

$\mathrm{R}-\mathrm{OH}+\mathrm{HCl}\stackrel{{\mathrm{ZnCl}}_2}{⟶}\mathrm{R}-\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$  

$\mathrm{R}-\mathrm{OH}+\mathrm{NaBr}+{\mathrm{H}}_2{\mathrm{SO}}_4⟶\mathrm{R}-\mathrm{Br}+{\mathrm{NaHSO}}_4+{\mathrm{H}}_2\mathrm{O}$

$3\mathrm{R}-\mathrm{OH}+{\mathrm{PX}}_3⟶3\mathrm{R}-\mathrm{X}+{\mathrm{H}}_3{\mathrm{PO}}_3(\mathrm{X}=\mathrm{Cl},\mathrm{Br})$

$\mathrm{R}-\mathrm{OH}+{\mathrm{PCl}}_{\mathrm{s}}⟶\mathrm{R}-\mathrm{Cl}+{\mathrm{POCl}}_3+\mathrm{HCl}$

$\mathrm{R}-\mathrm{OH}\underset{{\mathrm{x}}_2={\mathrm{Br}}_2,{\mathrm{I}}_2}{\overset{\mathrm{red} \mathrm{P}/{\mathrm{X}}_2}{\to }}\mathrm{R}-\mathrm{X}$

$\mathrm{R}-\mathrm{OH}+{\mathrm{SOCl}}_2⟶\mathrm{R}-\mathrm{Cl}+{\mathrm{SO}}_2+\mathrm{HCl}$

(ii) From alkanes by free radical halogenation :

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3\underset{\mathrm{or} \mathrm{heat}}{\overset{{\mathrm{Cl}}_2/\mathrm{UV} \mathrm{light}}{\to }}{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Cl}+{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}\left(\mathrm{Cl}\right){\mathrm{CH}}_3$

(iii) From Alkenes by addition of hydrogen halides -

${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2+\mathrm{H}-\mathrm{I}⟶\underset{\mathrm{Minor}}{{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{I}}+\underset{\mathrm{Major}}{{\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{I}\right){\mathrm{CH}}_3}$

(iv) From Alkenes by addition of Halogens

${\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{Br}}_2\stackrel{{\mathrm{CCl}}_4}{⟶}{\mathrm{BrCH}}_2-{\mathrm{CH}}_2\mathrm{Br}$

(v) By Halogen exchange method:

(a) Finkelstein reaction:
${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br}+\mathrm{Nal}\underset{\mathrm{heat} }{\overset{\mathrm{Acetone}}{\to }}{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{I}+\mathrm{NaBr}$

(b) Swart's reaction:
${\mathrm{CH}}_3\mathrm{Br}+\mathrm{AgF}\to {\mathrm{CH}}_3 \mathrm{F}+\mathrm{AgBr}$

3. Preparation of Haloarenes:

(i) From hydrocarbons by electrophilic substitution :

(ii) From amines by Sandmeyer’s reaction :

4. Nucleophilic substitution reactions :

(i) The reactivity of alkyl halides towards ${\mathrm{S}}_{\mathrm{N}}2$ mechanism is: methyl $> 1° > 2° > 3°$.

(ii) The ${\mathrm{S}}_{\mathrm{N}}1$ mechanism occurs through the formation of carbocation. The order of reactivity is: $3° > 2° > 1° >$ methyl

5. Reaction with metals (Wurtz reaction):

 ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}+2\mathrm{Na}+{\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}\stackrel{\mathrm{Dry} \mathrm{ether}}{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3+2\mathrm{NaBr}$

6. Reduction: ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}\underset{\mathrm{Zn}/\mathrm{Cu}\text{ couple or }\mathrm{Sn}/\mathrm{HCl}}{\overset{{\mathrm{H}}_{\mathrm{2}},\mathrm{Ni}\text{ or} \mathrm{HI},\text{ red} \mathrm{P}\text{ or }}{\to }}{\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}+\mathrm{HBr}$

7. Elimination reactions: ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}\stackrel{\mathrm{alc}.\mathrm{ }\mathrm{KOH}}{\to }\underset{\mathrm{alkene}}{{\mathrm{CH}}_2={\mathrm{CH}}_2}+\mathrm{HBr}$

8. Reaction of Haloarenes: