Science Class 12 - Haloalkanes And-Haloarenes Notes

Haloalkanes and Haloarenes

Welcome to the chapter on Haloalkanes and Haloarenes for Class 12. In this chapter, you will learn about the classification, nomenclature, preparation, properties, and uses of haloalkanes and haloarenes. By the end of this chapter, you will understand their structure, reactions, and importance in daily life and industry.

Introduction

Haloalkanes and haloarenes are organic compounds containing halogen atoms (F, Cl, Br, I) attached to aliphatic (alkane) or aromatic (arene) carbon atoms. They are important intermediates in organic synthesis and have many industrial and pharmaceutical applications.

Classification

• Haloalkanes (Alkyl halides): Halogen atom attached to an aliphatic carbon.
• Haloarenes (Aryl halides): Halogen atom attached to an aromatic ring.

Nomenclature

• IUPAC: Prefix the halogen name to the parent hydrocarbon (e.g., chloromethane, bromobenzene).
• Common: Use alkyl or aryl group followed by halide (e.g., methyl chloride, phenyl chloride).

Preparation of Haloalkanes

• From alcohols (by reaction with halogen acids, PCl₃, PCl₅, SOCl₂)
• By halogenation of alkanes (free radical substitution)
• From alkenes (addition of HX, halogenation)

Preparation of Haloarenes

• By direct halogenation of benzene (in presence of Lewis acid like FeCl₃)
• From diazonium salts (Sandmeyer and Gattermann reactions)

Physical Properties

• Generally colorless, pleasant-smelling liquids or solids.
• Insoluble in water, soluble in organic solvents.
• Higher boiling points than corresponding hydrocarbons.

Chemical Properties

• Nucleophilic Substitution: Replacement of halogen by nucleophile (SN₁ and SN₂ mechanisms).
• Elimination Reactions: Formation of alkenes (dehydrohalogenation).
• Reactions of Haloarenes: Less reactive towards nucleophilic substitution due to resonance and partial double bond character.
• Electrophilic Substitution: Halogen is ortho-para directing in aromatic rings.

Uses

• Used as solvents, refrigerants, and in the manufacture of pharmaceuticals and pesticides.
• Chloroform and carbon tetrachloride are important laboratory reagents.
• Haloarenes are used in the synthesis of dyes and drugs.

Practice Questions

1. Write the IUPAC name of CH₃CH₂Cl.
2. How is chlorobenzene prepared from benzene?
3. What is the difference between SN₁ and SN₂ reactions?
4. Why are haloarenes less reactive towards nucleophilic substitution than haloalkanes?
5. List two uses of haloalkanes.

Challenge Yourself

• Explain the mechanism of the SN₂ reaction with a suitable example.
• Describe the Sandmeyer reaction and its importance in organic synthesis.

Did You Know?

• Chlorofluorocarbons (CFCs), a type of haloalkane, were once widely used in refrigerators but are now banned because they harm the ozone layer.
• Iodoform (CHI₃) is a yellow solid with a distinct smell and is used as an antiseptic.

Glossary

• Haloalkane: An alkane in which one or more hydrogen atoms are replaced by halogen atoms.
• Haloarene: An aromatic compound in which one or more hydrogen atoms are replaced by halogen atoms.
• Nucleophile: A chemical species that donates an electron pair to form a chemical bond.
• Electrophile: A chemical species that accepts an electron pair.
• Sandmeyer Reaction: A method for preparing haloarenes from diazonium salts using copper salts.

Answers to Practice Questions

1. Chloroethane
2. By direct chlorination of benzene in the presence of FeCl₃ (Lewis acid catalyst).
3. SN₁ is a two-step mechanism involving a carbocation intermediate; SN₂ is a one-step mechanism with a transition state.
4. Due to resonance and partial double bond character of the C–X bond in haloarenes.
5. As solvents, in pharmaceuticals, as refrigerants, and in pesticides (any two).

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Understanding haloalkanes and haloarenes is key to mastering organic chemistry and its applications!