Methods of Preparation of Haloalkanes

Formation of Alkyl Halide from Alcohols:

From Alkenes

(a) Addition of halogen acids:

(b) Allylic halogenation:

\[\ce{\underset{Propene}{CH3 - CH} = CH2 + Cl2 ->[775K] \underset{\underset{(Allyl chloride)}{3-Chloro-1-propene}}{ClCH2 - CH = CH2 + HCl}}\]

From alkanes (Swarts reaction)

Alkyl chloride or bromide react with AgF, SbF₃, or Hg₂F₂ give alkyl fluoride. This reaction is known Swarts reaction. Antimony trifluoride (SbF₂) is commonly used in this reaction.

2CH₃CH₃CI + Hg₂F₂→ 2CH₃CH₂F+ Hg₂Cl₂

From the halide exchange method (Finkelstein reaction)

Alkyl chloride or bromide react with Nal or KI in presence of acetone give alkyl iodide. It is halide exchange reaction or Finkelstein reaction. It is a S_N2 reaction.

\[\ce{R - Cl + Nal ->[Acetone or Methanol] R - I + NaCl}\]

\[\begin{array}{cc}
\phantom{..........}\ce{Cl}\phantom{................................................}\ce{I}\phantom{..}\\
\phantom{..........}|\phantom{..................................................}|\phantom{.}\\
\phantom{}\ce{CH3 - CH2 - CH - CH3 ->[Nal][Acetone] CH3 - CH2 - CH - CH3}\phantom{}
\end{array}\]

Borodine-Hunsdiecker reaction

Silver salt of carboxylic acid on heating with Br₂ + carbon tetrachloride give alkyl bromide

\[\ce{\underset{Silver acetate}{CH3COOAg} + Br2 ->[CCl4/Reflux] \underset{Methyl bromide}{CH3 - Br} + AgBr + CO2}\]

Note: If silver salt of carboxylic acid is heated with I₂ in presence of CCI₄ ester is obtained. This reaction is known as simonini reaction.

\[\ce{2RCOOAg + I2 ->[CCl4] \underset{Ester}{RCOOR} + 2CO2 + 2AgI}\]

If silver salt of carboxylic acid is heated with I₂ in presence of HgO or lead tetra acetate than alkyl iodide is obtained

\[\ce{2RCOOAg ->[I2/HgO][Δ] 2R - X + 2CO2 + HgI2 + H2O}\]