Questions
4 questions per board paper
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Haloalkanes and Haloarenes form the foundation of organic chemistry for Class 12, focusing on the substitution and elimination reactions of carbon-halogen bonds. Mastering the reactivity trends and mechanisms is essential as these reactions appear in almost every organic conversion question in board exams.
Nomenclature and Classification
Haloalkanes are classified based on the number of halogen atoms and the hybridization of the carbon atom to which the halogen is attached. IUPAC nomenclature rules prioritize halogen substituents, which must be addressed to correctly identify isomers.
- Primary (1°), secondary (2°), and tertiary (3°) classification
- Allylic and benzylic halides possess sp3 hybridized carbon bonded to X
- Vinylic and aryl halides possess sp2 hybridized carbon bonded to X
- Alphabetical order for substituents in IUPAC naming
SN1 and SN2 Reaction Mechanisms
The SN1 mechanism proceeds via a carbocation intermediate and is favored by polar protic solvents and tertiary substrates. Conversely, SN2 involves a concerted backside attack with an inversion of configuration, favored by polar aprotic solvents and unhindered substrates.
- SN1: Racemization occurs due to planar carbocation
- SN2: Walden inversion occurs at the chiral center
- SN1 order of reactivity: 3° > 2° > 1°
- SN2 order of reactivity: Methyl > 1° > 2° > 3°
- Nucleophilicity strength impacts SN2 rates significantly
Chemical Reactions and Conversions
Beyond substitution, haloalkanes undergo elimination reactions (dehydrohalogenation) and organometallic formation. These reactions are the building blocks for multi-step organic conversions required in the CBSE exam.
- Saytzeff's Rule: Major product is the highly substituted alkene
- Wurtz Reaction: Coupling of two alkyl halides with Na/dry ether
- Finkelstein Reaction: Halogen exchange for alkyl iodides
- Swarts Reaction: Synthesis of alkyl fluorides
- Grignard Reagent: R-Mg-X formation using dry ether
Formula Sheet
R-X + OH- -> R-OH + X- (Nucleophilic Substitution)
R-CH2-CH2-X + alc. KOH -> R-CH=CH2 + KX + H2O (Elimination)
2RX + 2Na -> R-R + 2NaX (Wurtz Reaction)
Exam Tip
Always identify the carbocation stability first for SN1 and the steric hindrance level for SN2 before attempting any conversion sequence.
Common Mistakes
- Confusing the order of reactivity for SN1 and SN2 pathways during substitution problems.
- Forgetting to mention the formation of a racemic mixture in SN1 reactions due to planar carbocation.
- Neglecting stereochemical outcomes like Walden inversion or retention of configuration.
More Revision Notes
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