Summary of "HaloAlkanes and HaloArenes 01 : Preparation Of HaloAlkanes 1 - From Alkanes and Alkene : JEE/NEET"

Summary of the Video

HaloAlkanes and HaloArenes 01: Preparation of HaloAlkanes from Alkanes and Alkenes (JEE/NEET)

Main Ideas and Concepts

1. Introduction to Class 12 Organic Chemistry

The video begins with motivational advice for Class 12 organic chemistry students, emphasizing:
- The importance of thorough note-making.
- Repeated revision and consistent practice.
Suggests maintaining separate notes for important reactions and name reactions to facilitate easier revision.
Class 12 organic chemistry builds on Class 11 concepts; some Class 11 reactions and mechanisms will be briefly covered, with detailed videos available separately.

2. Introduction to Haloalkanes and Haloarenes

Haloalkanes (Alkyl Halides): Formed by replacing hydrogen atoms in alkanes with halogen atoms (F, Cl, Br, I).
Classification based on number and position of halogen atoms:
- Mono haloalkane: One halogen atom.
- Di haloalkane: Two halogen atoms; can be:
  - Geminal: Both halogens on the same carbon.
  - Vicinal: Halogens on adjacent carbons.
- Tri haloalkane: Three halogen atoms, e.g., chloroform (CHCl₃), bromoform (CHBr₃), iodoform (CHI₃).
- Tetra haloalkane: Four halogen atoms, e.g., carbon tetrachloride (CCl₄).
Special types:
- Allylic halide: Halogen on carbon next to a double bond.
- Vinylic halide: Halogen directly attached to a carbon of the double bond.
- Benzylic halide: Halogen on carbon attached to a benzene ring.
- Aryl halide (Haloarene): Halogen directly attached to the benzene ring.

3. General Formula of Haloalkanes

Mono haloalkane: CnH2n+1X (one hydrogen replaced by halogen).

4. Preparation of Haloalkanes

Method 1: From Alkanes (Direct Halogenation)

Reaction: Alkane + Halogen (Cl₂ or Br₂) in presence of sunlight (UV light).
Mechanism: Free radical substitution.
Example: CH₄ + Cl₂ → CH₃Cl (chloromethane) + HCl.
Issues:
- Produces a mixture of products (mono-, di-, tri-, tetra-substituted haloalkanes).
- Not preferred in laboratory due to difficulty in controlling reaction and product mixture.
- Preferred in industry where reaction conditions can be controlled to obtain mainly mono-substituted products.
Selectivity:
- Radical stability order: 3° > 2° > 1°.
- Major product formation depends on both stability of radical intermediate and number of available hydrogens.
- Example: Propane chlorination illustrates selectivity and statistical factors.
Additional Notes:
- Bromination follows a similar mechanism but with different selectivity and reactivity details.
- Iodination is reversible due to formation of strong reducing agent HI; strong oxidizing agents (e.g., HNO₃, HIO₃) are used to drive the reaction forward.
- Fluorination is dangerous and explosive; special methods (halide exchange, Swarts reaction) are used for fluoroalkanes.

Method 2: From Alkenes (Electrophilic Addition of HX)

Reaction: Alkene + HX → Haloalkane.
Mechanism: Electrophilic addition.
Markovnikov’s Rule:
- Hydrogen adds to the carbon with more hydrogens.
- Halogen adds to the carbon with fewer hydrogens.
Detailed Mechanism:
- Protonation of double bond forms carbocation intermediate.
- Carbocation stability governs regioselectivity (3° > 2° > 1°).
- Carbocation rearrangement (hydride or alkyl shifts) can occur to form more stable carbocation.
- Halide ion attacks carbocation to form product.
Exceptions:
- Peroxide effect (Kharasch effect or anti-Markovnikov addition) occurs with HBr in presence of peroxides (ROOR).
- In such cases, addition proceeds via free radical mechanism and hydrogen adds to carbon with fewer hydrogens.
Stereochemistry:
- Carbocation intermediate is planar.
- Halide can attack from either side → syn or anti addition possible.

Allylic Substitution

Halogen substitution occurs at the allylic position (carbon adjacent to double bond) rather than addition across double bond.
Reagents: Halogens at high temperature (400–500°C), NBS (N-bromosuccinimide) with heat or light.
Mechanism: Free radical substitution at allylic carbon.
No addition to double bond; double bond remains intact.

5. Summary of Preparation Methods Covered

Direct halogenation of alkanes (free radical substitution).
Electrophilic addition of HX to alkenes (Markovnikov and anti-Markovnikov addition).
Allylic substitution (free radical substitution at allylic position).

6. Future Topics

Preparation of haloalkanes from alcohols.
New reactions of Class 12 (not studied in Class 11).
Emphasis on revising old concepts and practicing problems.

Detailed Bullet Points of Methodologies and Important Concepts

Note-taking and Revision Strategy:
- Make proper notes and revise repeatedly by writing.
- Maintain a separate copy for name reactions.
- Revise Class 11 reactions superficially; study Class 12 reactions thoroughly.
Classification of Haloalkanes:
- Mono haloalkane: One halogen.
- Di haloalkane: Two halogens (geminal or vicinal).
- Tri haloalkane: Three halogens (e.g., chloroform).
- Tetra haloalkane: Four halogens (e.g., carbon tetrachloride).
- Special types: allylic, vinylic, benzylic, aryl halides.
Direct Halogenation of Alkanes:
- Requires UV light or sunlight.
- Free radical mechanism: initiation, propagation, termination.
- Produces mixture of products.
- Industrially useful but not preferred in lab.
- Selectivity influenced by radical stability and number of hydrogens.
- Bromination is more selective than chlorination.
- Iodination is reversible; use oxidizing agents to drive forward.
- Fluorination dangerous; special methods needed.
Electrophilic Addition to Alkenes:
- HX adds across double bond.
- Markovnikov’s rule: H adds to carbon with more H.
- Mechanism involves carbocation intermediate.
- Carbocation rearrangement possible (hydride/alkyl shifts).
- Peroxide effect causes anti-Markovnikov addition (only with HBr + peroxides).
- Stereochemistry: planar carbocation → syn and anti addition possible.
Allylic Substitution:
- Halogen replaces hydrogen on allylic carbon.
- Conditions: high temperature halogenation or NBS with light/heat.
- Free radical substitution mechanism.
- Double bond remains intact.

Speaker/Source Featured

Alak Pandey The instructor and presenter of the video, explaining organic chemistry concepts, mechanisms, and examples in detail.

This summary captures the key concepts, methodologies, and teaching approach presented in the video on the preparation of haloalkanes from alkanes and alkenes, along with important mechanistic insights and practical considerations for students preparing for competitive exams like JEE and NEET.