Haloalkanes and Haloarenes - Definition, Method of preparation

Haloalkanes &Haloarenes

Haloalkanes and Haloarenes may be classified as follow :

On the basis number of halogen atoms:-

Monohaloalkanes further classified according to the hybridization of the carbon atom to which the halogen is bonded.

(a)Compound containing Sp3 C---X Bond.

Mono and Dihaloalkanes

Alkyl Halide or Haloarenes:

Allylic halide: Halogen attaches to the Sp2 carbon next to the Sp2 hybridized carbon.

Benzylic halide: These are the compounds in which the halogen atom is bonded to an Sp3 hybridized carbon next to the aromatic ring.

Vinyl halide: Halogen atom is bonded to Sp2 hybridized carbon atom of a carbon-carbon double bond (C=C).

Aryl halide: Compounds in which halogen is bounded by Sp2 carbon the aromatic ring.

Nature of C---X bond:

 Halogen is more electronegative than carbon, the carbon halide bond of alkyl halide is polarized, carbon bears partially positive charge and halide bears a negative charge

.

The size of halogen atom increases as we go down the group in the periodic table, the fluorine atom is the smallest and iodine atom, the largest.

Method of preparation : 

From Alcohols :

The reactions of primary and secondary alcohols with HX require the presence of a catalyst ZnCl2 , with tertiary alcohols the reaction is conducted by simply shaking with concentrated HCl at room temp.

Constant boiling of HBr (48%) is used for the preparation of alkyl bromide.

Alkyl iodide may be obtained by a heating alcoholic with sodium iodide at 95% phosphoric acid.

So the substitution reaction easily is shown by alkyl halide as compare to the aryl halide , because of double bond character in Carbon and halide in aryl halide.

(b) By electrophilic substitution :

This method helps in the preparation of haloarenes such as aryl bromides and aryl chlorides. Electrophilic substitution forms the aryl bromides and aryl chlorides by using halogens such as chlorine and bromine in the presence of Lewis acid. However, the reaction requires the following of certain specific conditions for the generation of the proper electrophile.

For example, the reaction should be conducted in the presence of Lewis acid. Additionally, the reaction must be carried out in the dark. The reactions to obtain the electrophiles are

Sandmayer reaction: when aniline dissolved in aqueous mineral acid , is treated with sodium nitrite, a diazonium salt is formed.

mixing of cuprous chloride with diazonium salt replacement of diazonium group and chlorobenzene form.

Haloalkanes & Haloarenes from Alkenes

Addition of HX :

Alkene can be converted to haloalkane by an electrophilic addition reaction. Alkene reacts with HX to form R-X. The order of reactivity of halides with respect to alkenes follows the order HI > HBr > HCl > HF. The general reaction will be.

Peroxide effect (Kharash effect) :

There is another possibility where the reaction contradicts Markovnikov’s rule. This effect is known as Peroxide effect/ Kharash effect/ anti-markonikov’s rule. In this reaction, alkene reacts with HBr in the presence of peroxide. The Br- or the negative part of the reagent will attach itself to the carbon having more number of hydrogen atoms. For example, Prop-1-ene reacts with hydrogen bromide to form 1-bromopropane as a major product in the presence of peroxide.

Halogen exchange reaction :

Finkelstein Reaction: The last method of preparation of haloalkane and haloarene is a halogen exchange reaction. In this reaction, an alkyl chloride or alkyl bromide reacts with sodium iodide in acetone to form alkyl iodides.