Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Halogenation Reactions of Benzene-Definition, Mechanism, Importance, FAQs

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Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Halogenation can be further described as the process of replacing any number of hydrogen atoms with these given elements in the group. The product that is formed after halogenation will possess more new and unique properties than the initial substance. Meanwhile, halogens are a group of elements, such as iodine, chlorine, fluorine and bromine. Most of the time, these elements exhibit similar behaviour, and are therefore categorised under the same group.

Types of Halogenation

Halogenation can occur in several ways for organic compounds as well as for inorganic compounds. Halogenations can occur in a particular way, depending on the substrate.

  • Saturated hydrocarbons halogenate via a free radical process.
  • Unsaturated organics halogenate via an addition reaction.
  • Aromatics halogenate via electrophilic substitution.

We will briefly discuss the types of halogenation below.

Halogen Addition

This type of reaction is common in unsaturated carbons. Typically, alkynes and alkenes follow this reaction where they add halogens. For example, the addition of bromine to ethane.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Halogen addition to alkenes is achieved through intermediate halonium ions.

Halogen Substitution or Free Radical Halogenation

Here, we consider saturated hydrocarbons, where the hydrogen atoms are usually replaced by halogens. The hydrocarbons basically undergo free radical halogenation. In this, saturated hydrocarbons do not add halogens. The halogenation regiochemistry of alkanes is usually determined by the relative weakness of the C-H bonds available.

We will take the example of halogens reacting with alkanes under the influence of heat to form alkyl halides.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

As you can see, the halogen atom replaces the hydrogen atom into an alkane hence called a substitution reaction.

Meanwhile, aromatic compounds also go through substitution reactions or electrophilic halogenation in the presence of Lewis acids.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Electrophilic Substitution Reaction or Halogenation of Aromatic Compounds

An electrophilic aromatic substitution reaction mainly involves chlorine and bromine compounds. This reaction is further conducted in the presence of a Lewis acid such as FeX3 (laboratory method), which is used mainly to polarise the halogen-halogen bond. This results in the halogen molecule being more electrophilic.

Benzene reacts with bromine or chlorine in an electrophilic substitution reaction only in the presence of a catalyst which is either chloride or iron. However, iron is not entirely a catalyst because it changes permanently during the reaction. It reacts with some bromine to form iron 3 chloride, FeCl3 or iron 3 bromide, FeBr3.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Other Important Halogenation Methods

If we look at the Hunsdiecker reaction, carboxylic acids are changed to the chain-shortened halide. The carboxylic acid converts into silver salt. This is then oxidised with halogen:

RCO2Ag + Br2 → RBr + CO2 + AgBr

Usually, the Sandmeyer reaction is used to produce diazonium salt aryl halides. These are mostly obtained from anilines.

Another reaction is known as Hell–Volhard–Zelinsky halogenation, where the carboxylic acids are alpha-halogenated.

Inorganic Chemistry

Elements apart from argon, neon, and helium can form fluorides when they react with fluorine. In the case of chlorine, it is quite selective but still can react with heavier nonmetals and metals. Bromine is less reactive, and if we take iodine, it is the least reactive of them all.

Nonetheless, chlorination of metals is usually considered not essential industrially. Chlorides can easily be made from oxides and hydrogen halides. On the contrary, chlorination of inorganic compounds is done on a large scale, mainly for the production of sulfur monochloride and phosphorus trichloride.

Halogenation of Benzene – Mechanism 

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Step 1: The bromine reacts with Lewis acid to create a complex that makes bromine more electrophilic.

Step 2: The π electrons of aromatic C=C behave as a nucleophile which attacks the electrophilic Br and displaces iron tetrabromide.

Step 3: The proton is removed from sp3 C and bears the Bromo group that reforms C=C, and the aromatic system generates HBr and regulates the active catalyst.

The compounds act as a catalyst and behave similarly to aluminium chloride in the following chemical reactions.

We will further look at how halogenation is influenced by each halogen or the halogenating agents.

Halogenation Reactions

Just to recap, a halogenation reaction mainly occurs when one or more chlorine fluorine, chlorine, bromine, or iodine atoms replace hydrogen atoms in an organic compound. As for the order of reactivity, it is as follows:

Fluorine > Chlorine > Bromine > Iodine

As for fluorine, it is slightly aggressive and reacts violently with organic materials. However, it tends to make the most stable of the organohalogens. A fluorine atom, once added, is difficult to remove. On the other hand, iodine is difficult to add to an organic molecule, but after the formation of iodoorganic forms, the iodine atom is easily removed. From this, we can say that halogenation reactions depend upon,

  • The electronegativity of the halogen atom.
  • The nature of the substrate molecule that is being halogenated.

Reaction with Chlorine

The reaction between chlorine and benzene gives chlorobenzene in the presence of either iron or aluminium chloride.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Reaction with Bromine

The reaction between bromine and benzene gives bromobenzene in the presence of either iron or aluminium bromide. Iron is used normally because it is readily available and cheaper.

Halogenation refers to a type of chemical reaction that involves the replacement of a halogen atom with another substance, wherein it ends up as a part of that substance or a compound. In general, during the halogenation reaction, there is usually an addition of one or more halogens to the substance.

Reaction with Fluorine

If we take any organic compounds, it readily reacts with fluorine (usually explosive in nature). However, in the case of elemental fluorine (F2), we will need some proper apparatus and fulfil certain conditions. A variety of fluorinating reagents, such as xenon difluoride and cobalt (III) fluoride, are also used sometimes for the reaction.

Reaction with Iodine

In comparison to the above three halogens, iodine is the least reactive halogen. It does not readily react with organic substances.

Simultaneously, when we study inorganic chemistry, almost every element, excluding helium,  argon and neon, forms fluorides when it is reacted with fluorine.

Importance of Halogenation Reactions

Halogenations reactions are very useful and have a broad scope of use in synthetic chemistry. Halogenation reactions are important in chemical synthesis, and the intermediates generated via this process are widely found in products, such as polymers and plastics, refrigerants, fire retardants, fuel additives, agro products, etc. In pharmaceuticals, fluorine or chlorine atoms are added to a molecule to increase the effectiveness of its therapeutic aspects.

Additionally, important commercial chemicals are produced from halogenation reactions. For instance, chloroform is fluorinated to produce chlorodifluoromethane. This is then converted to fluoroethylene and polymerised to yield PTFE. Another common example that we can look at is the addition halogenation of ethylene with chlorine. This is done to form dichloroethane which is further polymerised to yield PVC.

Frequently Asked Questions (FAQs)

Q1

What is meant by halogenation?

The addition or substitution of halogen to an organic substrate is called halogenation.

Q2

What is the product of the reaction between ethylene and bromine?

1,2 dibromoethane is formed on a reaction between ethylene and bromine.

Q3

Which reaction mechanism is followed when methane reacts with chlorine?

The free radical mechanism is followed when methane reacts with chlorine.

Q4

In the presence of which catalyst does the bromination of benzene take place?

FeCl3 or AlCl3 is used as a catalyst in the bromination of benzene.

Q5

What is the order of reactivity of halogens in halogenation reactions?

Fluorine > Chlorine > Bromine > Iodine is the order of reactivity of halogens in halogenation reactions.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

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CBSE Class 11 Chemistry Syllabus

CBSE Class 11 Chemistry Syllabus is a vast which needs a clear understanding of the concepts and topics. Knowing CBSE Class 11 Chemistry syllabus helps students to understand the course structure of Chemistry.

Unit-wise CBSE Class 11 Syllabus for Chemistry

Below is a list of detailed information on each unit for Class 11 Students.

UNIT I – Some Basic Concepts of Chemistry

General Introduction: Importance and scope of Chemistry.

Nature of matter, laws of chemical combination, Dalton’s atomic theory: concept of elements,
atoms and molecules.

Atomic and molecular masses, mole concept and molar mass, percentage composition, empirical and molecular formula, chemical reactions, stoichiometry and calculations based on stoichiometry.

UNIT II – Structure of Atom

Discovery of Electron, Proton and Neutron, atomic number, isotopes and isobars. Thomson’s model and its limitations. Rutherford’s model and its limitations, Bohr’s model and its limitations, concept of shells and subshells, dual nature of matter and light, de Broglie’s relationship, Heisenberg uncertainty principle, concept of orbitals, quantum numbers, shapes of s, p and d orbitals, rules for filling electrons in orbitals – Aufbau principle, Pauli’s exclusion principle and Hund’s rule, electronic configuration of atoms, stability of half-filled and completely filled orbitals.

UNIT III – Classification of Elements and Periodicity in Properties

Significance of classification, brief history of the development of periodic table, modern periodic law and the present form of periodic table, periodic trends in properties of elements -atomic radii, ionic radii, inert gas radii, Ionization enthalpy, electron gain enthalpy, electronegativity, valency. Nomenclature of elements with atomic number greater than 100.

UNIT IV – Chemical Bonding and Molecular Structure

Valence electrons, ionic bond, covalent bond, bond parameters, Lewis structure, polar character of covalent bond, covalent character of ionic bond, valence bond theory, resonance, geometry of covalent molecules, VSEPR theory, concept of hybridization, involving s, p and d orbitals and shapes of some simple molecules, molecular orbital theory of homonuclear diatomic molecules(qualitative idea only), Hydrogen bond.

UNIT V – Chemical Thermodynamics

Concepts of System and types of systems, surroundings, work, heat, energy, extensive and intensive properties, state functions. First law of thermodynamics – internal energy and enthalpy, measurement of U and H, Hess’s law of constant heat summation, enthalpy of bond dissociation, combustion, formation, atomization, sublimation, phase transition, ionization, solution and dilution. Second law of Thermodynamics (brief introduction)
Introduction of entropy as a state function, Gibb’s energy change for spontaneous and nonspontaneous processes.
Third law of thermodynamics (brief introduction).

UNIT VI – Equilibrium

Equilibrium in physical and chemical processes, dynamic nature of equilibrium, law of mass action, equilibrium constant, factors affecting equilibrium – Le Chatelier’s principle, ionic equilibrium- ionization of acids and bases, strong and weak electrolytes, degree of ionization,
ionization of poly basic acids, acid strength, concept of pH, hydrolysis of salts (elementary idea), buffer solution, Henderson Equation, solubility product, common ion effect (with illustrative examples).

UNIT VII – Redox Reactions

Concept of oxidation and reduction, redox reactions, oxidation number, balancing redox reactions, in terms of loss and gain of electrons and change in oxidation number, applications of redox reactions.

UNIT VIII – Organic Chemistry: Some basic Principles and Techniques

General introduction, classification and IUPAC nomenclature of organic compounds. Electronic displacements in a covalent bond: inductive effect, electromeric effect, resonance and hyper conjugation. Homolytic and heterolytic fission of a covalent bond: free radicals, carbocations, carbanions, electrophiles and nucleophiles, types of organic reactions.

UNIT IX – Hydrocarbons

Classification of Hydrocarbons
Aliphatic Hydrocarbons:
Alkanes – Nomenclature, isomerism, conformation (ethane only), physical properties, chemical reactions.
Alkenes – Nomenclature, structure of double bond (ethene), geometrical isomerism, physical properties, methods of preparation, chemical reactions: addition of hydrogen, halogen, water, hydrogen halides (Markovnikov’s addition and peroxide effect), ozonolysis, oxidation, mechanism of electrophilic addition.
Alkynes – Nomenclature, structure of triple bond (ethyne), physical properties, methods of preparation, chemical reactions: acidic character of alkynes, addition reaction of – hydrogen, halogens, hydrogen halides and water.

Aromatic Hydrocarbons:

Introduction, IUPAC nomenclature, benzene: resonance, aromaticity, chemical properties: mechanism of electrophilic substitution. Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and acylation, directive influence of functional group in monosubstituted benzene. Carcinogenicity and toxicity.

To know the CBSE Syllabus for all the classes from 1 to 12, visit the Syllabus page of CBSE. Meanwhile, to get the Practical Syllabus of Class 11 Chemistry, read on to find out more about the syllabus and related information in this page.

CBSE Class 11 Chemistry Practical Syllabus with Marking Scheme

In Chemistry subject, practical also plays a vital role in improving their academic scores in the subject. The overall weightage of Chemistry practical mentioned in the CBSE Class 11 Chemistry syllabus is 30 marks. So, students must try their best to score well in practicals along with theory. It will help in increasing their overall academic score.

CBSE Class 11 Chemistry Practical Syllabus

The experiments will be conducted under the supervision of subject teacher. CBSE Chemistry Practicals is for 30 marks. This contribute to the overall practical marks for the subject.

The table below consists of evaluation scheme of practical exams.

Evaluation SchemeMarks
Volumetric Analysis08
Salt Analysis08
Content Based Experiment06
Project Work04
Class record and viva04
Total30

CBSE Syllabus for Class 11 Chemistry Practical

Micro-chemical methods are available for several of the practical experiments. Wherever possible such techniques should be used.

A. Basic Laboratory Techniques
1. Cutting glass tube and glass rod
2. Bending a glass tube
3. Drawing out a glass jet
4. Boring a cork

B. Characterization and Purification of Chemical Substances
1. Determination of melting point of an organic compound.
2. Determination of boiling point of an organic compound.
3. Crystallization of impure sample of any one of the following: Alum, Copper Sulphate, Benzoic Acid.

C. Experiments based on pH

1. Any one of the following experiments:

  • Determination of pH of some solutions obtained from fruit juices, solution of known and varied concentrations of acids, bases and salts using pH paper or universal indicator.
  • Comparing the pH of solutions of strong and weak acids of same concentration.
  • Study the pH change in the titration of a strong base using universal indicator.

2. Study the pH change by common-ion in case of weak acids and weak bases.

D. Chemical Equilibrium
One of the following experiments:

1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of either of the ions.
2. Study the shift in equilibrium between [Co(H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.

E. Quantitative Estimation
i. Using a mechanical balance/electronic balance.
ii. Preparation of standard solution of Oxalic acid.
iii. Determination of strength of a given solution of Sodium hydroxide by titrating it against standard solution of Oxalic acid.
iv. Preparation of standard solution of Sodium carbonate.
v. Determination of strength of a given solution of hydrochloric acid by titrating it against standard Sodium Carbonatesolution.

F. Qualitative Analysis
1) Determination of one anion and one cation in a given salt
Cations‐ Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH4 +
Anions – (CO3)2‐ , S2‐, NO2‐ , SO32‐, SO2‐ , NO ‐ , Cl‐ , Br‐, I‐, PO43‐ , C2O2‐ ,CH3COO‐
(Note: Insoluble salts excluded)

2) Detection of ‐ Nitrogen, Sulphur, Chlorine in organic compounds.

G) PROJECTS
Scientific investigations involving laboratory testing and collecting information from other sources.

A few suggested projects are as follows:

  • Checking the bacterial contamination in drinking water by testing sulphide ion
  • Study of the methods of purification of water.
  • Testing the hardness, presence of Iron, Fluoride, Chloride, etc., depending upon the regional
    variation in drinking water and study of causes of presence of these ions above permissible
    limit (if any).
  • Investigation of the foaming capacity of different washing soaps and the effect of addition of
    Sodium carbonate on it.
  • Study the acidity of different samples of tea leaves.
  • Determination of the rate of evaporation of different liquids Study the effect of acids and
    bases on the tensile strength of fibres.
  • Study of acidity of fruit and vegetable juices.

Note: Any other investigatory project, which involves about 10 periods of work, can be chosen with the approval of the teacher.

Practical Examination for Visually Impaired Students of Class 11

Below is a list of practicals for the visually impaired students.

A. List of apparatus for identification for assessment in practicals (All experiments)
Beaker, tripod stand, wire gauze, glass rod, funnel, filter paper, Bunsen burner, test tube, test tube stand,
dropper, test tube holder, ignition tube, china dish, tongs, standard flask, pipette, burette, conical flask, clamp
stand, dropper, wash bottle
• Odour detection in qualitative analysis
• Procedure/Setup of the apparatus

B. List of Experiments A. Characterization and Purification of Chemical Substances
1. Crystallization of an impure sample of any one of the following: copper sulphate, benzoic acid
B. Experiments based on pH
1. Determination of pH of some solutions obtained from fruit juices, solutions of known and varied
concentrations of acids, bases and salts using pH paper
2. Comparing the pH of solutions of strong and weak acids of same concentration.

C. Chemical Equilibrium
1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing
the concentration of eitherions.
2. Study the shift in equilibrium between [Co(H2O)6]2+ and chloride ions by changing the
concentration of either of the ions.

D. Quantitative estimation
1. Preparation of standard solution of oxalic acid.
2. Determination of molarity of a given solution of sodium hydroxide by titrating it against standard
solution of oxalic acid.

E. Qualitative Analysis
1. Determination of one anion and one cation in a given salt
2. Cations – NH+4
Anions – (CO3)2-, S2-, (SO3)2-, Cl-, CH3COO-
(Note: insoluble salts excluded)
3. Detection of Nitrogen in the given organic compound.
4. Detection of Halogen in the given organic compound.

Note: The above practicals may be carried out in an experiential manner rather than recording observations.

We hope students must have found this information on CBSE Syllabus useful for their studying Chemistry. Learn Maths & Science in interactive and fun loving ways with ANAND CLASSES (A School Of Competitions) App/Tablet.

Frequently Asked Questions on CBSE Class 11 Chemistry Syllabus

Q1

How many units are in the CBSE Class 11 Chemistry Syllabus?

There are 9 units in the CBSE Class 11 Chemistry Syllabus. Students can access various study materials for the chapters mentioned in this article for free at ANAND CLASSES (A School Of Competitions).

Q2

What is the total marks for practicals examination as per the CBSE Class 11 Chemistry Syllabus?

The total marks for the practicals as per the CBSE Class 11 Chemistry Syllabus is 30. It includes volumetric analysis, content-based experiment, salt analysis, class record, project work and viva.

Q3

Which chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry?

The organic chemistry chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry.

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