PREPARATION OF HALOALKANES & HALOARENES || PART-2

Описание: From Hydrocarbons (Alkenes & Alkanes)
This is a major part of the synthesis, focusing on how halogens are added to carbon chains.
From Alkanes (Free Radical Halogenation): Alkanes react with
or
in the presence of UV light or heat. This follows a free radical mechanism and often results in a mixture of mono-, di-, and polyhaloalkanes, making it difficult to isolate a single pure product.
From Alkenes (Addition of Hydrogen Halides): Alkenes react with
(
) to form haloalkanes.
Markovnikov's Rule: In unsymmetrical alkenes, the halogen attaches to the carbon with fewer hydrogen atoms.
Anti-Markovnikov (Peroxide Effect): When
is added in the presence of peroxides, the halogen attaches to the carbon with more hydrogen atoms.
From Alkenes (Addition of Halogens): Adding
in
to an alkene results in vic-dibromides. This reaction is also used as a laboratory test for unsaturation, as the reddish-brown colour of bromine disappears.
Methods of preparation of Haloalkanes and Haloarenes ...
Haloalkanes and Haloarenes Class 12 Notes Chemistry Chapter ...
2. Halogen Exchange Reactions
These specific "named reactions" are used when a particular halogen is difficult to introduce directly.
Finkelstein Reaction: Used primarily to prepare alkyl iodides. Alkyl chlorides or bromides are treated with Sodium Iodide (
) in dry acetone.
Reaction:
(where
)
Swarts Reaction: The best method for preparing alkyl fluorides. Alkyl chlorides or bromides are heated with metallic fluorides like
or
.
3. Preparation of Haloarenes (Aryl Halides)
Part 2 also distinguishes how aromatic halides are made differently from aliphatic ones.
Electrophilic Substitution: Benzene or its derivatives react with
or
in the presence of a Lewis acid catalyst like
or
.
Sandmeyer’s Reaction: This is a key two-step process:
Aniline is treated with
at low temperature (273–278 K) to form a diazonium salt.
The diazonium salt is mixed with Cuprous chloride (
) or Cuprous bromide (
) to replace the diazonium group with
or
.