Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

PREPARATION

(i) From Alkanes: Alkanes react with halogens in the presence of light to give alkyl halides.

$RH+C{l}_2\stackrel{h\nu }{\to }RCl+HCl$

$C{H}_4+C{l}_2\stackrel{h\nu }{\to }C{H}_{3}Cl+HCl$

Alkyl halides formed further react with halogen to give di, tri and tetra halogen compounds.

(ii) From alkenes: Alkenes add halogen acids to give halides. For example

$C{H}_2=C{H}_2+HCl\to C{H}_3-C{H}_{2}Cl$

Markonikoff's rule: In the addition reactions of unsymmetrical alkenes the -ve part attaches to the carbon atom having lesser number of H-atoms. E.g.

In case of HBr if peroxide is added antimarkonikoff's addition takes place which is also called Kharash effect or peroxide effect e.g.

$C{H}_3-CH=C{H}_2+HBr\stackrel{Peroxide}{\to }C{H}_3-C{H}_2-C{H}_2-Br$

(iii) From silver salt of carboxylic acids:

$RCOOAg+B{r}_2\stackrel{CC{l}_4}{\to }R-Br+C{O}_2+AgBr$

This reaction is called Borodine Hunsdiecker reaction.

(iv) Finkelstein reaction: Alkyl chlorides and bromides reacts with NaI in acetone to give alkyl iodides.

$R-Cl+NaI\stackrel{Acetone}{\to }R-I+NaCl$

This reactions is possible because NaI is soluble in acetone but NaCl and NaBr are insoluble in acetone.

Illustration 1. Dry gaseous hydrohalogen acid and not their aqueous solutions are used to prepare alkyl halides from alkenes.

Solution: Dry gaseous halogen acids are better electrophiles than $H_3{O}^{+}$ formed in water solutions. Also in aqueous solution H₂O acting as nucleophile may produce alcohol.