Unit 5: Alkyl and Aryl Halides
Course Code: CHM-DSM-252
Paper Name: Fundamentals of Chemistry - II
Table of Contents
1. Alkyl Halides: Preparation and Mechanisms
Alkyl halides (haloalkanes) are compounds where one or more hydrogen atoms in an alkane have been replaced by halogen atoms.
Methods of Preparation
- From Alkenes: Addition of hydrogen halides (HX) following Markownikoff's or anti-Markownikoff's rules.
- From Alcohols: Reaction with HX, PX3, PX5, or SOCl2.
2. Types of Nucleophilic Substitution
The reactivity of alkyl halides is dominated by nucleophilic substitution reactions.
| Mechanism | Kinetics | Stereochemistry | Preferred Substrate |
|---|---|---|---|
| SN1 | Unimolecular (First order) | Racemization | Tertiary (3°) halides |
| SN2 | Bimolecular (Second order) | Inversion (Walden Inversion) | Primary (1°) halides |
| SNi | Internal Nucleophilic Substitution | Retention of configuration | Alcohols + SOCl2 |
3. Williamson’s Ether Synthesis & Elimination
This is an important method for the preparation of symmetrical and unsymmetrical ethers.
- Reaction: An alkyl halide reacts with a sodium alkoxide (R-ONa).
- Competition: Elimination reactions often compete with substitution, especially with secondary and tertiary halides or at higher temperatures.
4. Aryl Halides: Synthesis and Reactions
Aryl halides are compounds where the halogen is directly attached to an aromatic ring.
Preparation Methods
- From Phenol: Replacement of the -OH group, though less common than in alcohols.
- Sandmeyer and Gattermann Reactions: Starting from diazonium salts to produce chloro, bromo, and iodo-benzene.
5. Benzyne Mechanism
Aryl halides are generally unreactive toward nucleophilic substitution but can react under vigorous conditions via the Benzyne Mechanism.
- Reagents: Use of very strong bases like KNH2/NH3 or NaNH2/NH3.
- Intermediate: Formation of a highly reactive "benzyne" intermediate (a benzene ring with a formal triple bond).
6. Reactivity and Bond Strength Comparisons
The strength of the Carbon-Halogen (C-X) bond varies significantly across different types of halides.
- Aryl and Vinyl Halides: These are less reactive toward nucleophilic substitution due to the partial double-bond character of the C-X bond (resonance).
- Alkyl, Allyl, and Benzyl Halides: Allyl and benzyl halides are exceptionally reactive in SN1 reactions because the resulting carbocations are stabilized by resonance.
7. Exam Focus: Tips and FAQs
- Stereochemistry: Always specify "Inversion" for SN2 and "Racemization" for SN1 in descriptive answers.
- Effect of Substituents: Remember that electron-withdrawing groups (like -NO2) at ortho and para positions significantly increase the reactivity of aryl halides toward nucleophilic substitution.
- Mechanism Identification: Primary halides + strong nucleophiles usually follow SN2; Tertiary halides + polar protic solvents follow SN1.
Frequently Asked Questions
Q: Why are aryl halides less reactive than alkyl halides toward nucleophilic substitution?
A: Due to the resonance effect which gives the C-X bond partial double-bond character, making it shorter and stronger.
Q: What is the Walden Inversion?
A: It is the inversion of optical configuration that occurs during an SN2 reaction.
Q: When does elimination predominate over substitution?
A: With bulky bases, tertiary substrates, or at elevated temperatures.