CBSE Class 12 Chemistry Revision Notes Chapter 6 Haloalkanes and Haloarenes

Haloalkanes and haloarenes are organic compounds formed when hydrogen atoms in hydrocarbons are replaced by halogen atoms. In CBSE Class 12 Chemistry, this chapter explains classification, preparation, reactions, stereochemistry and environmental effects of halogen compounds.

Haloalkanes and Haloarenes is an important organic chemistry chapter in Class 12 Chemistry. It explains how halogen atoms like fluorine, chlorine, bromine and iodine are attached to aliphatic and aromatic compounds. Haloalkanes contain halogen attached to sp³ carbon, while haloarenes contain halogen attached to sp² carbon of an aromatic ring.

Use these CBSE Class 12 Chemistry Revision Notes Chapter 6 to revise the chapter in a simple way. The notes cover classification, nomenclature, preparation methods, SN1 and SN2 reactions, elimination, stereochemistry, named reactions and environmental effects.

Key Takeaways

• Haloalkanes: Halogen is attached to sp³ hybridised carbon of an alkyl group.
• Haloarenes: Halogen is attached to sp² hybridised carbon of an aromatic ring.
• SN1 and SN2: These are important nucleophilic substitution mechanisms of haloalkanes.
• Polyhalogen Compounds: Some compounds like CFCs, DDT and carbon tetrachloride cause environmental problems.

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Access 30 Minutes Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes Notes

Start with classification and nomenclature. Then revise preparation methods of haloalkanes and haloarenes.

After that, focus on chemical reactions. Give more time to SN1, SN2, elimination, stereochemistry and named reactions like Finkelstein, Swarts, Wurtz, Fittig and Grignard reactions.

Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes Notes Overview

Haloalkanes and haloarenes are also called organohalogen compounds.

They are formed when one or more hydrogen atoms of hydrocarbons are replaced by halogen atoms.

Examples:

• CH₃Cl is chloromethane.
• C₂H₅Br is bromoethane.
• C₆H₅Cl is chlorobenzene.
• C₆H₅CH₂Cl is benzyl chloride.

Many halogen compounds are useful in medicine, industry and organic synthesis. But some are harmful because they persist in the environment.

CBSE Class 12 Chemistry Chapter 6 Notes on Classification of Haloalkanes and Haloarenes

Haloalkanes and haloarenes are classified on the basis of number of halogen atoms and type of carbon-halogen bond.

Classification Based on Number of Halogen Atoms

Classification Based on Carbon-Halogen Bond

Primary, Secondary and Tertiary Haloalkanes

Haloalkanes are also classified according to the carbon atom attached to halogen.

Haloalkanes and Haloarenes Class 12 Notes on Nomenclature

In common names, alkyl halides are named by writing the alkyl group followed by halide.

Example:

CH₃Cl = methyl chloride

In IUPAC names, haloalkanes are named as halo-substituted hydrocarbons.

Example:

CH₃Cl = chloromethane

Common and IUPAC Names

Dihalogen Compounds

Dihalogen compounds may be geminal or vicinal.

Nature of C-X Bond in Haloalkanes and Haloarenes

The carbon-halogen bond is polar because halogens are more electronegative than carbon.

Carbon gets partial positive charge.

Halogen gets partial negative charge.

R-X can be shown as:

Rδ⁺ - Xδ⁻

C-X Bond Length and Bond Strength

As the size of halogen increases from F to I, the C-X bond length increases and bond strength decreases.

Order of bond length:

C-F < C-Cl < C-Br < C-I

Order of bond strength:

C-F > C-Cl > C-Br > C-I

This affects the reactivity of alkyl halides.

Preparation of Haloalkanes

Haloalkanes can be prepared from alcohols, alkanes, alkenes and by halogen exchange.

Preparation of Haloalkanes from Alcohols

Alcohols are commonly used to prepare haloalkanes.

The -OH group of alcohol is replaced by halogen.

General reaction:

R-OH + HX → R-X + H₂O

Reagents used:

Why is Thionyl Chloride Preferred?

Thionyl chloride, SOCl₂, is preferred for preparing alkyl chlorides because the by-products are gases.

R-OH + SOCl₂ → R-Cl + SO₂ + HCl

SO₂ and HCl escape, so the alkyl chloride is obtained in purer form.

Preparation of Haloalkanes from Hydrocarbons

From Alkanes by Free Radical Halogenation

Alkanes react with chlorine or bromine in the presence of sunlight or heat.

Example:

CH₄ + Cl₂ → CH₃Cl + HCl

This method may give a mixture of products, so it is not always suitable for preparing one pure haloalkane.

From Alkenes by Addition of Hydrogen Halides

Alkenes react with HCl, HBr or HI to form haloalkanes.

Example:

CH₂=CH₂ + HBr → CH₃CH₂Br

In unsymmetrical alkenes, addition follows Markovnikov’s rule.

Example:

CH₃CH=CH₂ + HBr → CH₃CHBrCH₃

The major product is 2-bromopropane.

From Alkenes by Addition of Halogens

Alkenes react with Br₂ or Cl₂ to form vicinal dihalides.

Example:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

The reddish brown colour of bromine disappears, so this reaction is used to test unsaturation.

Halogen Exchange Reactions

Finkelstein Reaction

Alkyl chlorides or bromides react with sodium iodide in dry acetone to form alkyl iodides.

R-Cl + NaI → R-I + NaCl

R-Br + NaI → R-I + NaBr

NaCl or NaBr precipitates in dry acetone, so the reaction moves forward.

Swarts Reaction

Alkyl chlorides or bromides are heated with metallic fluorides to form alkyl fluorides.

Common reagents:

• AgF
• Hg₂F₂
• CoF₂
• SbF₃

Example:

R-Cl + AgF → R-F + AgCl

Preparation of Haloarenes

Haloarenes can be prepared from hydrocarbons and aromatic amines.

From Arenes by Electrophilic Substitution

Benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst.

Example:

C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Catalyst used:

• FeCl₃
• FeBr₃
• Anhydrous AlCl₃

For substituted benzene, ortho and para products are usually formed.

From Amines by Sandmeyer Reaction

Primary aromatic amines react with sodium nitrite and mineral acid at low temperature to form diazonium salts.

The diazonium group is then replaced by Cl, Br or I.

Example:

C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂

For iodine, potassium iodide is used.

C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂

Physical Properties of Haloalkanes and Haloarenes

Pure alkyl halides are generally colourless. Some bromides and iodides develop colour when exposed to light.

Many volatile halogen compounds have a sweet smell.

Boiling Points

Haloalkanes have higher boiling points than hydrocarbons of similar molecular mass.

This is because they have stronger dipole-dipole and van der Waals forces.

For the same alkyl group, boiling point order is:

RI > RBr > RCl > RF

This is because size and mass of halogen increase from F to I.

Effect of Branching

Boiling point decreases with increase in branching.

Example:

1-bromobutane has a higher boiling point than 2-bromo-2-methylpropane.

Solubility

Haloalkanes are very slightly soluble in water.

Reason:

• Water-water hydrogen bonds are strong.
• Haloalkane-water attraction is not strong enough.
• So haloalkanes do not dissolve much in water.

But haloalkanes dissolve in organic solvents.

Density

Bromo, iodo and polychloro derivatives are usually heavier than water.

Density increases with:

• increase in carbon atoms
• increase in halogen atoms
• increase in atomic mass of halogen

Chemical Reactions of Haloalkanes

Haloalkanes mainly undergo three types of reactions:

Nucleophilic Substitution Reactions of Haloalkanes

Haloalkanes undergo nucleophilic substitution because the carbon attached to halogen is electron deficient.

A nucleophile attacks this carbon and replaces the halide ion.

General reaction:

R-X + Nu⁻ → R-Nu + X⁻

Common Nucleophilic Substitution Products

KCN and AgCN Difference

KCN gives alkyl cyanide.

R-X + KCN → R-CN

AgCN gives alkyl isocyanide.

R-X + AgCN → R-NC

Reason:

KCN is mainly ionic, so cyanide attacks through carbon.

AgCN is mainly covalent, so nitrogen is free to donate electron pair.

Class 12 Haloalkanes and Haloarenes Revision Notes on SN1 and SN2 Reactions

SN1 and SN2 are the two main mechanisms of nucleophilic substitution in haloalkanes.

SN2 Reaction Mechanism

SN2 means substitution nucleophilic bimolecular.

It is a one-step reaction.

The nucleophile attacks from the back side, and the leaving group leaves at the same time.

Important points:

• Rate depends on both alkyl halide and nucleophile.
• No carbocation intermediate is formed.
• Backside attack takes place.
• Inversion of configuration occurs.

Rate law:

Rate = k[R-X][Nu⁻]

Order of reactivity:

Methyl halide > 1° halide > 2° halide > 3° halide

Tertiary halides are least reactive in SN2 because bulky groups block the nucleophile.

SN1 Reaction Mechanism

SN1 means substitution nucleophilic unimolecular.

It takes place in two steps.

Step 1: Alkyl halide forms carbocation.

R-X → R⁺ + X⁻

Step 2: Nucleophile attacks carbocation.

R⁺ + Nu⁻ → R-Nu

Important points:

• Rate depends only on alkyl halide.
• Carbocation intermediate is formed.
• It is common in polar protic solvents.
• Racemisation may occur if the substrate is optically active.

Rate law:

Rate = k[R-X]

Order of reactivity:

3° halide > 2° halide > 1° halide > methyl halide

Tertiary halides react faster in SN1 because tertiary carbocations are more stable.

SN1 and SN2 Difference

Leaving Group Ability in Haloalkanes

For the same alkyl group, reactivity of alkyl halides follows:

R-I > R-Br > R-Cl > R-F

Iodide is the best leaving group because C-I bond is weakest.

Fluoride is the poorest leaving group because C-F bond is strongest.

Stereochemistry in Substitution Reactions

Stereochemistry helps explain the 3D arrangement of atoms in reactions.

Optical Activity

A compound is optically active if it rotates plane-polarised light.

Chirality

A molecule is chiral if it is non-superimposable on its mirror image.

A carbon atom attached to four different groups is called an asymmetric carbon or chiral carbon.

Example:

2-bromobutane is chiral.

Retention of Configuration

Retention means the spatial arrangement around the chiral carbon remains the same after reaction.

Inversion of Configuration

Inversion means the arrangement around the chiral carbon becomes opposite.

SN2 reactions show inversion of configuration due to backside attack.

Racemisation

Racemisation means formation of a 50:50 mixture of two enantiomers.

SN1 reactions often show racemisation because the carbocation formed is planar and nucleophile can attack from either side.

Elimination Reactions of Haloalkanes

When a haloalkane with beta hydrogen is heated with alcoholic KOH, hydrogen and halogen are removed.

An alkene is formed.

This is called beta-elimination or dehydrohalogenation.

Example:

CH₃CH₂Br + alc. KOH → CH₂=CH₂ + KBr + H₂O

Saytzeff Rule

If more than one alkene can form, the major product is the alkene with more alkyl groups attached to the double-bonded carbon atoms.

Example:

2-bromopentane gives pent-2-ene as the major product.

Substitution vs Elimination

Haloalkanes may undergo substitution or elimination depending on conditions.

Primary alkyl halides usually prefer SN2.

Tertiary alkyl halides usually prefer SN1 or elimination.

Reaction of Haloalkanes with Metals

Haloalkanes react with metals to form organometallic compounds or higher alkanes.

Grignard Reagent

Haloalkanes react with magnesium in dry ether to form Grignard reagents.

General reaction:

R-X + Mg → R-MgX

Example:

CH₃Br + Mg → CH₃MgBr

Grignard reagents are highly reactive. They react with water, alcohols and amines to form hydrocarbons.

That is why Grignard reagents are prepared in dry ether.

Wurtz Reaction

Alkyl halides react with sodium metal in dry ether to form higher alkanes.

General reaction:

2R-X + 2Na → R-R + 2NaX

Example:

2CH₃Br + 2Na → C₂H₆ + 2NaBr

Wurtz reaction is useful for preparing symmetrical alkanes.

Chemical Reactions of Haloarenes

Haloarenes are less reactive than haloalkanes towards nucleophilic substitution.

Why are Haloarenes Less Reactive?

Nucleophilic Substitution in Haloarenes

Chlorobenzene reacts with NaOH only under harsh conditions.

C₆H₅Cl + NaOH → C₆H₅OH

Conditions:

• high temperature
• high pressure

Electrophilic Substitution in Haloarenes

Halogens are deactivating but ortho-para directing.

This means haloarenes react slower than benzene, but incoming groups mainly go to ortho and para positions.

Examples:

Chlorobenzene gives ortho and para products in nitration, sulphonation and halogenation.

Important Named Reactions in Haloalkanes and Haloarenes

Polyhalogen Compounds and Environmental Effects

Polyhalogen compounds contain more than one halogen atom.

Important examples:

• Dichloromethane
• Chloroform
• Iodoform
• Carbon tetrachloride
• Freons
• DDT

Dichloromethane

Dichloromethane is also called methylene chloride.

It is used as a solvent, paint remover and cleaning agent.

It can affect the nervous system if inhaled in high amounts.

Chloroform

Chloroform is trichloromethane.

It was earlier used as an anaesthetic.

On exposure to air and light, chloroform can form poisonous phosgene gas.

So it is stored in dark bottles filled to the top.

Iodoform

Iodoform is triiodomethane.

It is a yellow solid with a characteristic smell.

It was earlier used as an antiseptic because it releases iodine slowly.

Carbon Tetrachloride

Carbon tetrachloride is tetrachloromethane.

It was used as a cleaning solvent and fire extinguisher.

It is harmful to the liver and can also damage the ozone layer.

Freons and Environmental Effects

Freons are chlorofluorocarbon compounds.

They were used as refrigerants and aerosol propellants.

CFCs are harmful because they release chlorine radicals in the upper atmosphere and damage the ozone layer.

Ozone layer depletion allows more ultraviolet radiation to reach Earth.

DDT

DDT is a chlorinated pesticide.

It was used to control mosquitoes and agricultural pests.

But DDT is not easily decomposed and can accumulate in living organisms. This makes it an environmental hazard.

Quick Revision Table for Haloalkanes and Haloarenes

Important Terms in Haloalkanes and Haloarenes

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