NCERT Solutions for Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes
NCERT Solutions for Class 12 Chemistry Chapter 6 Haloalkanes and Haloarenes covers detailed concepts of halogenated organic compounds, classification, IUPAC nomenclature, methods of preparation, physical properties, nucleophilic substitution mechanisms (SN1 and SN2), and polyhalogen compounds.
Overview of Topics & Subtopics (Click to Jump)
1. Classification, Nomenclature & C-X Bond Nature
Short Note:
• Classification: Based on the number of halogen atoms (mono, di, polyhalo) and hybridization of carbon attached to halogen:
- sp3 C-X compounds: Alkyl halides (haloalkanes), Allylic halides (CH2=CH-CH2X), Benzylic halides (C6H5CH2X).
- sp2 C-X compounds: Vinylic halides (CH2=CHX), Aryl halides (haloarenes).
• Nature of C-X Bond: Highly polar due to the greater electronegativity of halogen compared to carbon (Cδ+-Xδ-). Bond enthalpy decreases down the group: C-F > C-Cl > C-Br > C-I.
2. Methods of Preparation & Physical Properties
Short Note:
• Methods of Preparation:
- From Alcohols: Reaction with HX, PX3, PX5, or SOCl2. Reaction with SOCl2 (Darzen's process) is preferred as by-products (SO2 and HCl) escape as gases.
- Halogen Exchange: Finkelstein reaction (R-Cl/Br + NaI in dry acetone → R-I) and Swarts reaction (R-Cl/Br + AgF → R-F).
- From Diazonium Salts: Sandmeyer's reaction using Cu2Cl2 or Cu2Br2 to yield chlorobenzene/bromobenzene.
• Physical Properties: Boiling points follow the order: R-I > R-Br > R-Cl > R-F (due to increasing van der Waals forces with molecular size). Parasubstituted isomers have higher melting points due to symmetrical crystal lattice packing.
3. Chemical Reactions (SN1 vs SN2) & Polyhalogen Compounds
Short Note:
• Nucleophilic Substitution Mechanisms:
- SN2 Mechanism: Bimolecular, single-step reaction with transition state leading to inversion of configuration (Walden inversion). Reactivity order: 1° > 2° > 3° haloalkanes.
- SN1 Mechanism: Unimolecular, two-step reaction proceeding via carbocation intermediate leading to racemisation. Reactivity order: 3° > 2° > 1° haloalkanes.
• Elimination & Metal Reactions: β-elimination yields alkenes (Saytzeff's rule: major product is highly alkylated alkene). Reaction with Mg forms Grignard reagents (RMgX).
• Polyhalogen Compounds: Includes Chloroform (CHCl3), Iodoform (CHI3), Carbon tetrachloride (CCl4), Freons (CFCs causing ozone depletion), and DDT (insecticide).
Intext & Conceptual Questions
(i) CH3CH(Cl)CH(Br)CH3
(ii) CHF2CBrClF
(iii) (CH3)3CCH2Br
1. CH3CH(Cl)CH(Br)CH3: 2-Bromo-3-chlorobutane
2. CHF2CBrClF: 1-Bromo-1-chloro-1,2,2-trifluoroethane
3. (CH3)3CCH2Br: 1-Bromo-2,2-dimethylpropane
| Feature | SN1 Mechanism | SN2 Mechanism |
|---|---|---|
| Kinetics / Order | Unimolecular (First order reaction). | Bimolecular (Second order reaction). |
| Steps Involved | Two steps (Carbocation intermediate formed). | Single concerted step (Transition state formed). |
| Stereochemistry | Racemisation (50% retention + 50% inversion). | Complete inversion of configuration. |
| Reactivity Order | 3° > 2° > 1° alkyl halides. | 1° > 2° > 3° alkyl halides. |
Textbook Chapter End Exercise Questions
Thionyl chloride (SOCl2) is preferred for preparing alkyl chlorides from alcohols because the by-products formed during the reaction, namely sulfur dioxide (SO2) and hydrogen chloride (HCl), are both gaseous in nature.
Chemical Equation:
R-OH + SOCl2 → R-Cl + SO2↑ + HCl↑
Since both by-products escape into the atmosphere easily, the reaction yields pure alkyl chloride (R-Cl) in the reaction mixture without requiring complex purification processes.
1-bromobutane, 2-bromobutane, 2-bromo-2-methylpropane.
In SN2 reactions, steric hindrance around the carbon bearing the halogen atom plays the key role. Less steric hindrance leads to a faster SN2 attack by the incoming nucleophile.
• 1-bromobutane: 1° alkyl halide (least steric hindrance, most reactive).
• 2-bromobutane: 2° alkyl halide (intermediate steric hindrance).
• 2-bromo-2-methylpropane: 3° alkyl halide (maximum steric hindrance, least reactive).
Increasing Order of Reactivity:
2-bromo-2-methylpropane < 2-bromobutane < 1-bromobutane
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.