Chapter 10: Alkyl Halides

Complete notes for Chapter 10. Covers Classification, Nomenclature, Preparation, Nucleophilic Substitution (SN1 vs SN2), Elimination Reactions (E1 vs E2), and Grignard Reagents.

Introduction & Classification

Alkyl Halides (Haloalkanes): Monohalo derivatives of alkanes. General formula CnH2n+1X or R-X.

Primary (1°): Carbon attached to halogen is attached to only one other carbon (e.g., Ethyl chloride).
Secondary (2°): Attached to two other carbons (e.g., Isopropyl chloride).
Tertiary (3°): Attached to three other carbons (e.g., tert-Butyl chloride).

Preparation of Alkyl Halides

From Alkanes (Halogenation): Free radical mechanism. Poor method due to mixture of products.
From Alkenes: Addition of HX (Markownikoff's Rule). Reactivity: HI > HBr > HCl.
From Alcohols:
- With HX (ZnCl2 catalyst for HCl).
- With SOCl2 (Pyridine). Best method (gaseous byproducts).
- With PX3 or PX5.
Preparation of Alkyl Iodides: Halogen exchange (NaI in Acetone).

Reactivity of Alkyl Halides

Depends on Bond Polarity and Bond Energy.

Bond Polarity: C-F > C-Cl > C-Br > C-I. would suggest R-F is most reactive.
Bond Energy: C-F has highest bond energy (strongest), C-I lowest (weakest). This is the controlling factor.
Order of Reactivity: R-I > R-Br > R-Cl > R-F. Alkyl Iodides are most reactive.

Nucleophilic Substitution Reactions (SN)

SN2 (Substitution Nucleophilic Bimolecular)

One step reaction. No intermediate.
Nucleophile attacks from backside. Inversion of configuration (Walden Inversion).
Rate = k[C-X][Nu]. Second order kinetics.
Favoured by Primary Alkyl Halides and Non-polar solvents.

SN1 (Substitution Nucleophilic Unimolecular)

Two step reaction. Carbocation intermediate formed.
Step 1: Slow ionization (Rate determining). Step 2: Fast attack of Nu.
Rate = k[C-X]. First order kinetics.
Favoured by Tertiary Alkyl Halides and Polar solvents.
Product shows 50% Inversion and 50% Retention (Racemization).

Elimination Reactions (Beta-Elimination)

E2 (Elimination Bimolecular)

Single step. Bimolecular (Rate depends on substrate and base).
Similarities with SN2 mechanism. Primary Alkyl Halides.

E1 (Elimination Unimolecular)

Two steps. Unimolecular (Rate depends on substrate only).
Carbocation intermediate. Tertiary Alkyl Halides.

Grignard Reagents (R-Mg-X)

Organometallic compounds. Prepared by reacting Alkyl Halide with Mg in dry ether.

Reactions

With Active Hydrogen (H2O, NH3, ROH): Forms Alkanes.
With CO2: Forms Carboxylic Acids.
With HCHO (Formaldehyde): Forms Primary Alcohols.
With CH3CHO (Acetaldehyde): Forms Secondary Alcohols.
With Ketones (Acetone): Forms Tertiary Alcohols.
With Epoxides: Forms Primary Alcohols (chain lengthened by 2 carbons).

Other Reactions

Wurtz Reaction: R-X + Na + X-R -> R-R (Alkane).
Reduction: Zn/HCl -> Alkane.
Sodium Lead Alloy: 4R-Cl + 4Na/Pb -> R4Pb (Tetraethyl Lead - Anti-knocking agent).

Download PDFPDF

Chapter 10

chemistry • intermediate 12th