UNIT PROCESS
-
HALOGENATIO
N
R.AYSVARYAH
2nd SEMESTER M.PHARM
(PHARMACEUTICAL CHEMISTRY)
CONTENTS
INTRODUCTION
TYPES OF HALOGENATION
KINETICS OF
HALOGENATION
CATALYTIC HALOGENATION
CASE STUDY OF
INDUSTRIAL PROCESS OF
HALOGENATION
CONCLUSION
Introduction-HALOGENATION
o Halogenation is a chemical reaction in organic chemistry where one or more halogen
atoms are introduced into an organic compound.
o The halogens, which include Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I), are
elements from Group 17 of the periodic table.
o They are characterized as non-metals and possess an electronic configuration that is one
electron short of a stable noble gas configuration, meaning they have 7 valence electrons
in their outermost shell.
o This electronic configuration makes them highly reactive and prone to gaining an electron
to achieve a stable octet.
GROUP 17 ELEMENTS - HALOGENS
TYPES OF HALOGENATION
Free radical halogenation: This type of halogenation typically occurs with alkanes and involves
the replacement of a hydrogen atom with a halogen atom via a free radical mechanism, often
initiated by light or heat. This reaction proceeds through a chain reaction involving initiation,
propagation, and termination steps.
Electrophilic halogenation: This reaction is characteristic of aromatic compounds, where an
electrophilic halogen species (e.g., Br+ from Br2 and a Lewis acid catalyst) attacks the electron-
rich aromatic ring, leading to the substitution of a hydrogen atom with a halogen
Addition reaction halogenation: This type of halogenation occurs with unsaturated compounds like
alkenes and alkynes, where a halogen molecule (e.g., Br2 or Cl2) adds across the multiple bond,
breaking the pi bond and forming new sigma bonds with the halogen atoms. This is an electrophilic
addition reaction.
FREE RADICAL HALOGENATION
The halogenation of saturated hydrocarbon is a substituition reaction where halogen atom
replaces a hydrogen atom in an organic compound.
Free radicals are highly reactive species with unpaired electrons. The reaction occurs in the
presence of a halogen , chlorine , bromine and a source of energy such as heat or light.
There are three steps :-
• Chain initiation step
• Chain propagation step
• Chain terminating step
1. Chain Initiation
Requires UV light (hν) or heat.
The halogen molecule undergoes homolytic bond cleavage (equal splitting of electrons).
Example:
Result: two chlorine radicals are generated (highly reactive).
2. Chain Propagation
The radicals attack the substrate (methane), producing new radicals and continuing the chain reaction.
Step 1:
(A chlorine radical abstracts a hydrogen from methane, forming HCl and a methyl radical.)
Step 2:
(The methyl radical reacts with chlorine, producing chloromethane and regenerating another chlorine radical.)
The regenerated chlorine radical can attack another methane molecule → the chain continues.
3. Chain Termination
Occurs when two radicals combine and neutralize each other (removing radicals
from the system).
Examples:
Electrophilic aromatic substitution (EAS) halogenation
Electrophilic aromatic substitution (EAS) halogenation is a type of reaction where an electrophile (an
electron-deficient species) replaces a hydrogen atom on an aromatic ring, specifically with a halogen .
Electrophile: In halogenation, the electrophile is a positively charged halogen species (e.g., Cl ⁺, Br ⁺) generated
from a halogen molecule (Cl₂ or Br₂) in the presence of a Lewis acid catalyst (e.g., FeCl₃, AlCl₃, FeBr₃).
Attack on Aromatic System: The electron-rich aromatic ring acts as a nucleophile and attacks the electrophilic
halogen. This leads to the formation of a sigma complex (also known as an arenium ion or Wheland
intermediate), which is resonance-stabilized.
Formation of New Aromatic Compound: A base (often the conjugate base of the Lewis acid catalyst) then
deprotonates the sigma complex, restoring the aromaticity of the ring and forming the halogenated aromatic
compound.
Step 1: Formation of the chloronium ion initiates the process.
Step 2: This is the rate-determining step where a resonance-stabilized
carbocation intermediate forms, with the positive charge delocalized over
three atoms within the ring.
ADDITION REACTION
HALOGENATION
The addition of halogens to alkenes to form alkynes is known as
halogenation of alkyne.
It involves the addition of two halogen atom across a C-C triple
bond resulting in conversion of the alkyne into tetrahaloalkane ,
this reaction is usually carried out in the presence of halogens
such as chlorine and bromine in the presence of suitable solvent
such as carbon tetrachloride (CCl4), dichloromethane (CH2Cl2),
or benzene or a catalyst if needed.
Examples of addition halogenation:-
Addition of Br₂ to propene (an alkene) yields 1,2-dibromopropane.
Addition of Br₂ to propyne (an alkyne) can yield 1,2-dibromoalkene initially, and
with excess bromine, the product can be 1,1,2,2-tetrabromopropane.
KINETICS OF HALOGENATION
Chlorination and bromination both cause radical substitution in alkanes. Key
differences:
Chlorination is faster.
Chlorination is unselective, producing mixed products; bromination is
selective, giving mainly one product.
CHLORINATION BROMINATION
Chlorination occurs rapidly because chlorine Bromination proceeds more slowly due to the
radicals are highly reactive. lower reactivity of bromine radicals.
This speed leads to less control over which This slower pace allows the reaction to favor
hydrogen atoms are replaced, resulting in a the formation of the most stable radical
mixture of products. intermediate, making bromination more
The reaction is less selective because selective.
chlorine radicals can abstract hydrogen As a result, it predominantly yields one major
atoms from various positions on the alkane. product, often at the most substituted
carbon.
CHLORINATION BROMINATION
CASE STUDY OF INDUSTRIAL PROCESS OF
HALOGENATION
Halogenation is a chemical process where a halogen is introduced into an organic compound. It is
widely used in the chemical industry to create a variety of useful products such as
polymers,pharmaceuticals and agrochemicals.
1. Feedstock Preparation: The process begins with preparing and purifying the feedstock, which
is typically a hydrocarbon or other organic compound.
2. Halogenation Reaction: The prepared feedstock then reacts with a halogen, such as chlorine or
bromine, in the presence of a catalyst to introduce the halogen into the organic compound.
3. Product Separation: After the reaction, the resulting halogenated product is separated from the
other components of the reaction mixture using methods like distillation or crystallization.
4. Purification: Finally, the separated halogenated product undergoes purification through
methods such as recrystallization or chromatography to achieve the desired purity.
THE PROCESS CONDITIONS FOR THE INDUSTRIAL HALOGENATION
PROCESS:-
Temperature: 100-300°c
Pressure: 1-10 bar
Catalyst: various catalysts like lewis acids or transition metal complexes are used to promote
the reaction.
Solvent: solvents such as water, organic solvents, or ionic liquids are used to facilitate the
reaction.
Case study on industrial Halogenation process
1) Preparation of chloroacetic acid
• Raw material
• Glacial acetic acid- 365Ib
• Phosphorus trichloride- 121b
• Phosphorus trichloride (PC13) as catalyst
It is prepared by passing chlorine gas through glacial acetic acid heated
to 100°C
The vessel's top section features connections for
acetic acid, chlorine, air, effluent gases, condensate,
discharge outlets, and a thermometer well.
The chlorinator's jacket is linked to both water and
steam supply lines.
The condensation setup includes a vertical reflux
cooler that allows vapor to ascend, paired with a
return condenser to finalize the condensation
process.
Non-condensable gases are removed, while the
condensate is returned beneath the surface of the
reaction mixture inside the chlorinator.
Acetic acid (CH3COOH) is stored in aluminum tanks.
The chlorinator itself is a large, steam-jacketed
vessel coated with enamel for durability.
The brine flow to the jacketed reflux condenser of the adjacent finishing chlorinator is
stopped. Chlorine gas is introduced into the freshly charged chlorinator to absorb
hydrogen chloride, acetyl chloride, and to vent acetic acid.
Once absorption is complete, the chlorinator is heated to 100°C, and the chlorine
feed rate is increased.
To prevent acetic acid from crystallizing, brine circulation through the reflux
condenser must be avoided initially.
As chlorination progresses and the amount of distilled acetic acid decreases, the
brine flow can be gradually increased.
The reflux condenser effectively blocks a significant portion of acetic acid vapors
from reaching the return condenser.
Maintain the temperature at 0℃ to promote the condensation of acetyl chloride.
To obtain high-purity monochloroacetic acid, stop the chlorination process once half a
mole of chlorine has reacted with each mole of acetic acid.
For producing a crystallized commercial-grade product, continue chlorination until a
full mole of chlorine is added per mole of acetic acid.
Raw materials
• Basis 1 ton chlorobenzene
• Benzene 1,900 lb
• Chlorine- 1,750 lb
• Anhydrous ferric chloride(0.1-0.5%)
Reaction:-
Benzene chlorination is performed in a tall
tank made of cast iron or steel, lined
internally with lead for protection.
The setup includes a reflux condenser and
an external cooling system to circulate the
fluid.
Chlorine gas is introduced near the bottom
of the reactor through a specially designed
iron distributor pipe.
The chlorinator is charged with dry
benzene along with a small amount of
anhydrous ferric chloride catalyst.
Chlorine is bubbled through the mixture
while maintaining the temperature
between 40°C and 60°C.
Once a sample reaches the target density,
the chlorine flow is halted.
The reaction continues until all benzene is
SAFETY CONSIDERATIONS IN INDUSTRIAL HALOGENATION PROCESSES:-
Corrosion: Halogens such as chlorine, bromine, and fluorine are highly reactive and can
aggressively corrode standard metals and materials used in industrial equipment. This necessitates
the use of specialized materials of construction, such as corrosion-resistant alloys, glass-lined
reactors, or certain plastics, to ensure the longevity and safety of the equipment. Proper material
selection helps prevent leaks, equipment failure, and costly downtime.
Explosion Risk: Halogenation reactions are often highly exothermic, meaning they release
significant amounts of heat rapidly. Without adequate temperature control and reaction monitoring,
this can lead to runaway reactions, increasing the risk of explosions. Industrial setups must
incorporate robust cooling systems, pressure relief devices, and automated controls to maintain safe
operating conditions and prevent hazardous incidents.
Toxicity: Both elemental halogens and many halogenated compounds pose serious health hazards
due to their toxicity. Exposure can cause respiratory issues, skin burns, and other acute or chronic
health effects. Therefore, strict handling protocols, including the use of personal protective
equipment (PPE), proper ventilation, and secure storage facilities, are essential to protect workers
and the environment from accidental releases or contamination.
KEY ENVIRONMENTAL FACTORS:
Raw Material Expenses: The prices of essential inputs like halogens and base
chemicals fluctuate with market dynamics, influencing overall costs.
Energy Consumption: Halogenation processes often demand substantial energy,
making operational efficiency crucial.
Workforce Requirements: Skilled technicians are necessary to manage and
maintain halogenation equipment, impacting labor costs.
FOR EX:-
• Methane Halogenation: In producing chloromethane industrially, methane reacts with
chlorine gas under heat or light. This substitution reaction replaces a hydrogen atom in
methane with chlorine.
• The process releases significant heat, necessitating precise temperature control to
prevent overheating and minimize unwanted byproducts.
OTHER EXAMPLES OF CASE STUDIES:-
Production of Polyvinyl Chloride (PVC)
PVC, a versatile and widely utilized plastic, is manufactured through the halogenation of ethylene. This
involves reacting ethylene with chlorine gas to produce dichloroethane. Subsequently, dichloroethane
undergoes cracking to yield vinyl chloride monomer, which is then polymerized to create PVC.
Methane Halogenation Process:
In the commercial synthesis of chloromethane, methane undergoes a reaction with chlorine gas activated by
heat or light. This substitution reaction involves replacing a hydrogen atom in methane with chlorine.The
reaction is highly exothermic, necessitating careful temperature regulation to prevent overheating and
reduce the formation of undesired byproducts.
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