Chapter 6: Haloalkanes and Haloarenes
Key Topics: Classification, Methods of Preparation, Nucleophilic Substitution ($S_N1$ and $S_N2$), Elimination Reactions, Organometallic Compounds, and Stereochemistry.
1. Classification & Nomenclature
Haloalkanes (alkyl halides) and haloarenes (aryl halides) are organic compounds formed by replacing hydrogen atoms in aliphatic or aromatic hydrocarbons with halogen atoms ($X = \text{F, Cl, Br, I}$).
- Primary ($1^\circ$): Halogen bonded to a carbon attached to one carbon.
- Secondary ($2^\circ$): Halogen bonded to a carbon attached to two carbons.
- Tertiary ($3^\circ$): Halogen bonded to a carbon attached to three carbons.
- Allylic Halides: Halogen bonded to $sp^3$-hybridized carbon next to a $\text{C=C}$ double bond.
- Vinylic / Aryl Halides: Halogen bonded directly to an $sp^2$-hybridized carbon.
2. Methods of Preparation
From Alcohols using Thionyl Chloride (Darzens Process - Best Method):
$$\text{R-OH} + \text{SOCl}_2 \rightarrow \text{R-Cl} + \text{SO}_2\text{(g)} \uparrow + \text{HCl(g)} \uparrow$$
Finkelstein Reaction (Halogen Exchange for Alkyl Iodides):
$$\text{R-X} + \text{NaI} \xrightarrow{\text{Acetone}} \text{R-I} + \text{NaX} \quad (X = \text{Cl, Br})$$
Swarts Reaction (Preparation of Alkyl Fluorides):
$$\text{R-Br} + \text{AgF} \rightarrow \text{R-F} + \text{AgBr}$$
Sandmeyer Reaction (Preparation of Aryl Halides):
$$\text{Ar-NH}_2 \xrightarrow[\text{273-278 K}]{\text{NaNO}_2 + \text{HCl}} \text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{Cu}_2\text{X}_2} \text{Ar-X} + \text{N}_2$$
3. Nucleophilic Substitution Reactions ($S_N1$ vs $S_N2$)
| Feature | $S_N1$ Mechanism | $S_N2$ Mechanism |
|---|---|---|
| Kinetics & Order | First-order reaction: $\text{Rate} = k[\text{R-X}]$ | Second-order reaction: $\text{Rate} = k[\text{R-X}][\text{Nu}^-]$ |
| Number of Steps | 2 Steps (Carbocation intermediate) | 1 Concerted Step (Transition state) |
| Reactivity Order | $3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X}$ | $\text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ$ |
| Stereochemistry | Racemization occurs (attack from both sides) | Walden Inversion occurs (backside attack) |
| Solvent Preferred | Polar Protic Solvents (e.g., $\text{H}_2\text{O, EtOH, AcOH}$) | Polar Aprotic Solvents (e.g., DMSO, DMF, Acetone) |
4. Key Equations & Named Reactions
$S_N1$ Rate Expression:
$$\text{Rate} = k [\text{R-X}]$$
$S_N2$ Rate Expression:
$$\text{Rate} = k [\text{R-X}] [\text{Nu}^-]$$
Dehydrohalogenation / Saytzeff's Rule ($\beta$-Elimination):
$$\text{R-CH}_2\text{-CH(X)-CH}_3 + \text{alc. KOH} \xrightarrow{\Delta} \text{R-CH=CH-CH}_3 + \text{KX} + \text{H}_2\text{O}$$
Grignard Reagent Formation:
$$\text{R-X} + \text{Mg} \xrightarrow{\text{Dry Ether}} \text{R-Mg-X}$$
Hydrolysis of Grignard Reagent:
$$\text{R-Mg-X} + \text{H}_2\text{O} \rightarrow \text{R-H} + \text{Mg(OH)X}$$
Wurtz Reaction:
$$2\text{R-X} + 2\text{Na} \xrightarrow{\text{Dry Ether}} \text{R-R} + 2\text{NaX}$$
Fittig Reaction:
$$2\text{Ar-X} + 2\text{Na} \xrightarrow{\text{Dry Ether}} \text{Ar-Ar} + 2\text{NaX}$$
Wurtz-Fittig Reaction:
$$\text{Ar-X} + \text{R-X} + 2\text{Na} \xrightarrow{\text{Dry Ether}} \text{Ar-R} + 2\text{NaX}$$