Class 12 Chemistry: Haloalkanes and Haloarenes Revision for CUET 2027

Haloalkanes and Haloarenes can be challenging to revise because the chapter includes several classifications, IUPAC naming rules, preparation methods, and chemical reactions. Remembering named reactions, comparing SN1 and SN2 mechanisms, understanding reactivity orders, and identifying reaction conditions can make CUET 2027 Chemistry preparation more demanding.

For effective revision, Physics Wallah brings together key concepts, reaction equations, comparison tables, and memory tips in one place. The revision notes cover the classification, nomenclature, physical properties, nucleophilic substitution, elimination reactions, Grignard reagents, coupling reactions, and important polyhalogen compounds to help you revise the chapter for CUET 2027.

Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes can be classified according to the number of halogen atoms, the hybridisation of the carbon bonded to the halogen, and the degree of the carbon attached to the halogen.

Classification by number of halogen atoms

Dihaloalkanes are further classified according to the positions of their halogen atoms.

Classification by hybridisation

Key distinction: In an allylic halide, the halogen-bearing carbon is next to a double bond. In a vinylic halide, the halogen-bearing carbon is part of the double bond.

Classification by degree of alkyl halide

The degree depends on the number of carbon atoms directly attached to the carbon bearing the halogen.

Memory tip: Count the carbon atoms attached directly to the carbon bearing the halogen, not the total number of carbon atoms in the molecule.

IUPAC Nomenclature and Isomerism

IUPAC nomenclature helps you name haloalkanes and haloarenes correctly, while isomerism explains how compounds can have the same molecular formula but different structures.

Rules of IUPAC nomenclature

The word root indicates the number of carbon atoms in the parent chain.

Other naming elements include:

Choose the appropriate parent chain, number it to give substituents the lowest possible locants, and list substituents according to IUPAC rules.

Isomerism in haloalkanes

Isomers have the same molecular formula but different arrangements of atoms or different spatial arrangements.

For example, 1-chloropropane and 2-chloropropane are position isomers because the chlorine atom is attached to different carbon atoms.

Nature of the Carbon–Halogen Bond

The carbon–halogen (C–X) bond is polar because halogens are more electronegative than carbon. The halogen carries a partial negative charge (-), while the carbon carries a partial positive charge (+).

Trends in carbon–halogen bonds

Bond dissociation energy is the energy required to break a bond by homolytic cleavage in the gas phase.

Dipole moment is expressed as:

=qr

where q is the magnitude of the separated charge and r is the distance between the charges.

An important comparison is that the dipole moment of C–Cl is greater than that of C–F, despite fluorine being more electronegative.

Preparation of Haloalkanes

Haloalkanes can be prepared from alcohols, alkanes, and alkenes, as well as through halogen-exchange reactions. Remember the reagents and conditions because they help distinguish between different preparation methods.

From alcohols

Alcohols react with hydrogen halides, phosphorus halides, or thionyl chloride to form haloalkanes.

The mixture of concentrated HCl and anhydrous ZnCl₂ is called Lucas reagent. The reaction rate depends on the alcohol's structure; tertiary alcohols generally react faster than secondary alcohols, which react faster than primary alcohols under Lucas test conditions.

Thionyl chloride is often preferred because its by-products, SO₂ and HCl, are gases that can escape from the reaction mixture, helping obtain a purer alkyl chloride. This preparation is commonly associated with the Darzens method.

Other reagents include PCl₅ and PCl₃:

ROH+PCl5RCl+POCl3+HCl

3ROH+PCl33RCl+H3PO3

From hydrocarbons

Alkanes: Halogenation occurs through free-radical substitution in the presence of sunlight or UV light.

• Chlorination and bromination are common methods.

• Iodination is generally reversible and requires suitable conditions.

• Fluorination is highly vigorous and difficult to control.

Alkenes: Addition of hydrogen halides (HX) generally follows Markovnikov's rule. In the presence of peroxides, HBr can follow the anti-Markovnikov rule.

Halogen-exchange reactions

The Finkelstein reaction is driven by the precipitation of suitable sodium halide by-products in dry acetone.

Preparation of Haloarenes

Haloarenes can be prepared from aromatic diazonium salts or through direct halogenation of aromatic compounds.

In the Balz–Schiemann reaction, a diazonium tetrafluoroborate is heated to form the aryl fluoride.

Direct halogenation of toluene using a Lewis acid catalyst generally favours substitution at the ortho and para positions because the methyl group activates the aromatic ring and directs electrophilic substitution to these positions.

Physical Properties of Haloalkanes and Haloarenes

Physical properties depend on molecular mass, molecular shape, intermolecular forces, and symmetry.

For dichlorobenzene isomers, the melting-point order is generally:

p-isomer>m-isomer>o-isomer

The para isomer has a higher melting point because its symmetry allows more efficient crystal packing.

The boiling-point order is generally:

o-isomer>p-isomer>m-isomer

Chemical Reactions of Haloalkanes

Haloalkanes undergo nucleophilic substitution, elimination, and reactions with metals, depending on the reagent and reaction conditions.

Nucleophilic substitution: SN1 and SN2

A nucleophile is an electron-rich species that attacks an electron-deficient carbon atom. Haloalkanes undergo nucleophilic substitution through SN1 or SN2 mechanisms, depending on the substrate, nucleophile, solvent, and reaction conditions.

Remember: SN1 involves carbocation formation, whereas SN2 occurs in one step through backside attack. These trends are general; the actual mechanism also depends on the reaction conditions.

Elimination reactions

When haloalkanes react with alcoholic KOH under suitable conditions, elimination can occur. A hydrogen atom is removed from a β-carbon while the halogen leaves the adjacent α-carbon, forming a double bond.

Aqueous KOH generally favours nucleophilic substitution, while alcoholic KOH and heat favour elimination. The major product can depend on the substrate and reaction conditions.

Grignard reagents

Grignard reagents are organomagnesium compounds prepared by reacting a haloalkane with magnesium in dry ether.

The carbon bonded to magnesium has considerable carbanion-like character, making the reagent highly reactive towards electrophiles. Grignard reagents react with water and other proton donors, so moisture must be excluded.

For example:

RMgX+H2ORH+Mg(OH)X

The hydrocarbon formed depends on the original alkyl group, R.

Chemical Reactions of Haloarenes

Haloarenes show characteristic reactions such as electrophilic substitution and coupling reactions, while the halogen attached to the benzene ring affects their reactivity.

Why haloarenes resist nucleophilic substitution

Haloarenes are less reactive towards nucleophilic substitution than many haloalkanes under ordinary conditions.

The main reasons are:

• Resonance: The carbon–halogen bond acquires partial double-bond character.

• Hybridisation: The halogen-bearing carbon is sp²-hybridised, giving the C–X bond greater strength.

• Carbocation instability: Formation of a phenyl carbocation is highly unfavourable.

• Bond strength: The carbon–halogen bond is harder to break than a comparable bond in many haloalkanes.

Electrophilic substitution reactions

Halogens deactivate the benzene ring through their electron-withdrawing inductive effect but direct incoming electrophiles mainly to the ortho and para positions because of resonance.

Final Revision and CUET 2027 Practice

After revising the reactions and their conditions, use these resources to test your understanding and identify topics that need another review:

• Check syllabus coverage: Compare your revision with the Chemistry syllabus to confirm that you have covered Haloalkanes and Haloarenes along with the other prescribed Chemistry units.

• Review important topics: Use the Chemistry important topics and unit-wise weightage to plan revision across the Chemistry syllabus without relying only on predicted question counts.

• Practise actual exam questions: Solve previous year question papers to practise applying reaction mechanisms, named reactions, and reactivity trends to multiple-choice questions.

• Test your preparation: Attempt the Chemistry mock test under timed conditions and review any errors involving reaction conditions, products, or SN1 and SN2 reactivity orders.

Haloalkanes and haloarenes are important parts of Class 12 Chemistry and CUET 2027 preparation. Revising their classification, IUPAC nomenclature, preparation methods, physical properties, and chemical reactions can help you understand the chapter more clearly. Physics Wallah revision notes bring these concepts, reaction equations, and key comparisons together to support your final revision. Practise reaction-based questions and review important reagents and conditions to strengthen your understanding before the exam.