Write short note on the following.

Darzen's process

1. Halides and Haloarenes:

2. Preparation of Haloalkanes:

(i) From Alcohols:

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

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

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

$\mathrm{R}-\mathrm{OH}\frac{\text{ red }\mathrm{P}/{\mathrm{X}}_2}{{\mathrm{x}}_2={\mathrm{Br}}_2,{\mathrm{I}}_2}⟶\mathrm{R}-\mathrm{X}$

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

(ii) From Hydrocarbons:

(iii) From alkanes by free radical halogenation

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3\underset{\mathrm{or} \mathrm{heat}}{\overset{{\mathrm{Cl}}_2/\mathrm{UV} \mathrm{light}}{\to }}{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Cl}+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CHClCH}}_3$

(iv) From Alkenes by addition of hydrogen halides -

$$\begin{gathered} {\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2+\mathrm{H}-\mathrm{I}⟶{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{I}+{\mathrm{CH}}_3{\mathrm{CHCH}}_3 \\ \mathrm{minor} \mathrm{major} \end{gathered}$$

(v) From Alkenes by addition of Halogens

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

(vi) By Halogen exchange:

(a) Finkelstein reaction:

$\mathrm{CH},{\mathrm{CH}}_2\mathrm{Br}+\mathrm{Nal}\frac{\text{ Acetone }}{\text{ heat }}{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{I}+\mathrm{NaB}$

(b) Swarts 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 carbonation. The order of reactivity is: $3° > 2° > 1° >$ methyl

5. Reactions with metals

Wurtz reaction: ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}+2\mathrm{Na}+{\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}⟶\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$

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

7. Elimination reactions: ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{5}}\mathrm{Br}\stackrel{\mathrm{a}\mathrm{l}\mathrm{c}.\mathrm{ }\mathrm{K}\mathrm{O}\mathrm{H}}{\to }\underset{\mathrm{a}\mathrm{l}\mathrm{k}\mathrm{e}\mathrm{n}\mathrm{e}}{\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2}$

8. Reaction of Haloarenes: