Haloalkanes and haloarenes are important classes of organic compounds where hydrogen atoms in alkanes or arenes are replaced by halogen atoms (fluorine, chlorine, bromine, iodine). These compounds have significant applications in pharmaceuticals, agrochemicals, dyes, and many industrial processes. Studying their properties, reactions, and preparation methods is essential for understanding organic chemistry concepts at a deeper level.
Class 12 Chemistry Notes: Haloalkanes and Haloarenes | Complete Guide with Questions
Haloalkanes
Definition
Haloalkanes are organic compounds containing one or more halogen atoms (F, Cl, Br, I) attached to an aliphatic carbon chain.
Classification of Haloalkanes
Haloalkanes are classified based on:
1. Number of Halogen Atoms
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Monohaloalkanes – Contain one halogen atom (e.g., CH₃Cl).
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Dihaloalkanes – Contain two halogen atoms (e.g., CH₂Cl₂).
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Polyhaloalkanes – Contain three or more halogen atoms (e.g., CHCl₃).
2. Nature of Carbon Atom
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Primary (1°) Haloalkane – Halogen attached to a primary carbon (e.g., CH₃CH₂Cl).
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Secondary (2°) Haloalkane – Halogen attached to a secondary carbon (e.g., CH₃CHClCH₃).
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Tertiary (3°) Haloalkane – Halogen attached to a tertiary carbon (e.g., (CH₃)₃CCl).
3. Type of Carbon Chain
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Alkyl halides – Saturated carbon chain with halogen atoms.
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Allyl halides – Halogen attached to allylic carbon (CH₂=CH–CH₂Cl).
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Benzyl halides – Halogen attached to benzylic carbon (C₆H₅CH₂Cl).
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Vinyl halides – Halogen attached to sp² hybridized carbon (CH₂=CHCl).
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Aryl halides – Halogen directly attached to aromatic ring (C₆H₅Cl).
Nomenclature of Haloalkanes
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IUPAC system: Prefix: halo–, suffix: alkane
Example: CH₃CH₂Cl → Chloroethane -
Numbering begins from the end nearer to the halogen atom.
Preparation of Haloalkanes
1. From Alcohols
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React alcohol with HX (HCl, HBr, HI) or phosphorus halides (PX₃).
Example: CH₃CH₂OH + HCl → CH₃CH₂Cl + H₂O
2. From Hydrocarbons
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Free radical halogenation using Cl₂ or Br₂ in the presence of UV light.
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Addition of HX to alkenes (Markovnikov’s rule applies).
3. From Halogen Exchange Reactions
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Finkelstein reaction: CH₃CH₂Cl + NaI → CH₃CH₂I + NaCl
4. From Diazonium Salts
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Sandmeyer reaction: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂
Physical Properties
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State: Lower haloalkanes are gases; higher members are liquids/solids.
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Boiling points: Increase with molecular mass, decrease with branching.
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Solubility: Insoluble in water, soluble in organic solvents.
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Density: Generally heavier than water, especially bromides and iodides.
Chemical Reactions of Haloalkanes
1. Nucleophilic Substitution Reactions (SN1 and SN2)
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Example: CH₃CH₂Br + KOH → CH₃CH₂OH + KBr
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SN1: Unimolecular, tertiary halides.
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SN2: Bimolecular, primary halides.
2. Elimination Reactions (Dehydrohalogenation)
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Formation of alkenes: CH₃CH₂Br + alcoholic KOH → CH₂=CH₂ + HBr
3. Reaction with Metals
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Wurtz reaction: 2CH₃CH₂Br + 2Na → CH₃CH₂CH₂CH₃ + 2NaBr
4. Reduction
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CH₃CH₂Br + Zn/HCl → CH₃CH₃ + HBr
Uses of Haloalkanes
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Solvents (e.g., chloroform, carbon tetrachloride).
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Refrigerants (e.g., freons).
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Anesthetics (e.g., chloroform).
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Raw materials for synthesis of organic compounds.
Haloarenes
Definition
Haloarenes are aromatic compounds in which halogen atoms are directly attached to the benzene ring.
Example: Chlorobenzene (C₆H₅Cl)
Classification of Haloarenes
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Monohaloarenes: One halogen atom (e.g., C₆H₅Cl).
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Dihaloarenes: Two halogen atoms (e.g., p-dichlorobenzene).
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Polyhaloarenes: More than two halogens (e.g., hexachlorobenzene).
Nomenclature
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Prefix: halo–, suffix: benzene
Example: C₆H₅Br → Bromobenzene
Preparation of Haloarenes
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From Benzene:
C₆H₆ + Cl₂ → C₆H₅Cl + HCl (in presence of FeCl₃) -
From Diazonium Salts:
C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂ -
By Electrophilic Substitution:
Halogenation using halogens in presence of Lewis acids.
Physical Properties
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Insoluble in water, soluble in organic solvents.
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Higher melting and boiling points than benzene.
Chemical Properties
1. Electrophilic Substitution Reactions
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Halogens are ortho-para directing.
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Nitration: C₆H₅Cl + HNO₃ → o- & p-nitrochlorobenzene
2. Nucleophilic Substitution
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Requires strong conditions due to resonance stabilization.
3. Reaction with Metals
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Wurtz-Fittig reaction: C₆H₅Cl + CH₃Cl + 2Na → C₆H₅CH₃ + 2NaCl
Uses of Haloarenes
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Used in dyes, pesticides, drugs.
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Raw materials for synthesis of aromatic compounds.
Environmental Effects
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Some haloalkanes (e.g., CFCs) deplete the ozone layer.
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Toxicity and non-biodegradability pose environmental hazards.
Objective Questions-Haloalkanes and Haloarenes
Q1. Which of the following is a monohaloalkane?
a) CH₃Cl
b) CH₂Cl₂
c) CCl₄
d) CHCl₃
Answer: a) CH₃Cl
Q2. Which halogen has the highest electronegativity?
a) F
b) Cl
c) Br
d) I
Answer: a) F
Q3. Wurtz reaction involves:
a) Alcohol + Na
b) Alkyl halide + Na
c) Aryl halide + Cu
d) Alkene + H₂
Answer: b) Alkyl halide + Na
Q4. What is the hybridization of carbon in CH₃Cl?
a) sp³
b) sp²
c) sp
d) sp³d
Answer: a) sp³
Q5. Which of the following follows Markovnikov’s rule?
a) Addition of HBr to CH₂=CH₂
b) Addition of Cl₂ to CH₃CH=CH₂
c) Addition of H₂ to CH₃CH=CH₂
d) Addition of HBr to CH₃CH=CH₂ in presence of peroxide
Answer: a) Addition of HBr to CH₂=CH₂
Q6. Which of the following is a freon?
a) CCl₄
b) CF₂Cl₂
c) CHCl₃
d) C₆H₅Cl
Answer: b) CF₂Cl₂
Q7. Which reaction converts CH₃CH₂OH to CH₃CH₂Br?
a) Sandmeyer reaction
b) Wurtz reaction
c) Finkelstein reaction
d) Reaction with HBr
Answer: d) Reaction with HBr
Q8. Vinyl chloride is:
a) CH₃CH₂Cl
b) CH₂=CHCl
c) C₆H₅CH₂Cl
d) CH₃CHClCH₃
Answer: b) CH₂=CHCl
Q9. Which halide is most reactive towards SN1?
a) Primary
b) Secondary
c) Tertiary
d) Allylic
Answer: c) Tertiary
Q10. What is the hybridization of carbon in C₆H₅Cl?
a) sp³
b) sp²
c) sp
d) sp³d
Answer: b) sp²
Q11. Dehydrohalogenation of haloalkanes gives:
a) Alkenes
b) Alcohols
c) Alkynes
d) Ethers
Answer: a) Alkenes
Q12. In chlorobenzene, the C–Cl bond is:
a) Purely ionic
b) Purely covalent
c) Resonance-stabilized covalent
d) Hydrogen bond
Answer: c) Resonance-stabilized covalent
Q13. Which of the following halides is used as an anesthetic?
a) CHCl₃
b) CCl₄
c) CH₃Cl
d) C₆H₅Cl
Answer: a) CHCl₃
Q14. Allyl chloride is:
a) CH₂=CH–CH₂Cl
b) CH₃CH₂Cl
c) CH₃CHClCH₃
d) CH₂ClCH₂CH₃
Answer: a) CH₂=CH–CH₂Cl
Q15. Wurtz-Fittig reaction involves:
a) Alkyl halide + Aryl halide + Na
b) Alkyl halide + Alkyl halide + Cu
c) Aryl halide + Cu
d) Alcohol + Na
Answer: a) Alkyl halide + Aryl halide + Na
Q16. Which halogen compound causes ozone depletion?
a) Chloroform
b) CFCs
c) Benzyl chloride
d) Vinyl chloride
Answer: b) CFCs
Q17. Iodoform test is given by:
a) CH₃CH₂I
b) CH₃CH(OH)CH₃
c) CHI₃
d) CH₃CHO
Answer: b) CH₃CH(OH)CH₃
Q18. SN2 reactions proceed via:
a) Carbocation
b) Free radical
c) Transition state
d) Carbene
Answer: c) Transition state
Q19. Which halide is least reactive towards nucleophiles?
a) CH₃I
b) CH₃Br
c) CH₃Cl
d) CH₃F
Answer: d) CH₃F
Q20. Which reagent is used for bromination of benzene?
a) FeBr₃
b) AlCl₃
c) ZnCl₂
d) H₂SO₄
Answer: a) FeBr₃
Short Answer Questions
Q1. What are allyl halides? Give one example.
Answer:
Allyl halides are compounds where the halogen atom is attached to an sp³ hybridized carbon adjacent to a carbon–carbon double bond. For example, CH₂=CH–CH₂Cl is allyl chloride.
Q2. Explain Markovnikov’s rule with an example.
Answer:
Markovnikov’s rule states that when an unsymmetrical reagent like HX adds to an unsymmetrical alkene, the negative part (X⁻) attaches to the carbon with fewer hydrogen atoms, and H⁺ attaches to the carbon with more hydrogen atoms. For example:
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃
Q3. Define Finkelstein reaction.
Answer:
Finkelstein reaction involves halogen exchange, where an alkyl halide reacts with sodium iodide in acetone to form alkyl iodide. Example: CH₃CH₂Cl + NaI → CH₃CH₂I + NaCl.
Q4. Why is C–Cl bond in chlorobenzene stronger than in CH₃Cl?
Answer:
In chlorobenzene, the lone pair on chlorine interacts with the π-electrons of the benzene ring, giving resonance stabilization. This partial double bond character strengthens the C–Cl bond compared to CH₃Cl.
Q5. Write one use of chloroform.
Answer:
Chloroform (CHCl₃) is used as a solvent and was earlier used as an anesthetic in surgeries.
Long Answer Questions
Q1. Explain the classification of haloalkanes and haloarenes with examples.
Answer:
Haloalkanes are classified based on the number of halogens, nature of the carbon atom, and type of carbon chain. For example, monohaloalkanes like CH₃Cl contain one halogen, dihaloalkanes like CH₂Cl₂ contain two, and polyhaloalkanes like CCl₄ contain multiple halogens. Based on carbon atom, primary (CH₃CH₂Cl), secondary (CH₃CHClCH₃), and tertiary ((CH₃)₃CCl) halides exist. Similarly, haloarenes like chlorobenzene (C₆H₅Cl) are classified as mono-, di-, and polyhaloarenes.
Q2. Describe the methods of preparation of haloalkanes.
Answer:
Haloalkanes are prepared by:
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From Alcohols: ROH + HX → R–X + H₂O
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From Hydrocarbons: Free radical halogenation using halogens in UV light.
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Halogen Exchange: Finkelstein reaction.
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From Diazonium Salts: Sandmeyer reaction for haloarenes.
These methods help synthesize different types of haloalkanes with high efficiency.
Q3. Explain the chemical reactions of haloalkanes.
Answer:
Haloalkanes undergo:
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Nucleophilic substitution reactions (SN1/SN2): Halogen replaced by nucleophile.
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Elimination reactions: Alkene formation by dehydrohalogenation.
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Reaction with metals: Wurtz reaction forms higher alkanes.
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Reduction: Produces alkanes using Zn/HCl.
These reactions make haloalkanes versatile intermediates in organic synthesis.
Q4. Discuss the electrophilic substitution reactions of haloarenes.
Answer:
In haloarenes, halogens are ortho-para directing due to +R effect but deactivate the ring by –I effect. Reactions include nitration, sulphonation, halogenation, and Friedel–Crafts alkylation/acylation at ortho and para positions.
Q5. Write the environmental effects of halogenated compounds.
Answer:
Certain halogenated compounds like CFCs destroy the ozone layer, leading to global warming and increased UV radiation. Some compounds are toxic and non-biodegradable, causing water and soil pollution.
Conclusion
Haloalkanes and haloarenes are key organic compounds with versatile applications. Understanding their structure, classification, preparation methods, and reactions helps build a strong base in organic chemistry and supports further studies in industrial and environmental chemistry.