Haloalkanes: Preparation, Trends & Chemical Reactivity
Haloalkanes (Alkyl Halides) are versatile organic compounds where one or more hydrogen atoms in an alkane are replaced by halogens. This guide explores their synthesis, physical trends, and substitution-elimination mechanisms.
1. Preparation of Haloalkanes
Preparation methods range from the direct halogenation of hydrocarbons to the more efficient conversion of alcohols and halogen exchange reactions.
Key Synthesis Highlights
- From Alcohols: Reaction with $\text{SOCl}_2$ (Darzen's process) is preferred as side products ($\text{SO}_2, \text{HCl}$) are gases.
- Halogen Exchange:
- Finkelstein Reaction: $R-X + \text{NaI} \rightarrow R-I + \text{NaX}$ (in dry acetone).
- Swarts Reaction: $R-X + \text{AgF/Hg}_2\text{F}_2 \rightarrow R-F$ (Best for Alkyl Fluorides).
- From Alkenes: Addition of $\text{HX}$ following Markovnikov's Rule (or Anti-Markovnikov with peroxides).
2. Physical Properties of Haloalkanes
The polar nature of the $\text{C-X}$ bond and increasing molecular mass lead to distinct trends in boiling points and solubility.
Physical Trends for Exams
- Boiling Point: Increases with molecular mass ($R-I > R-Br > R-Cl > R-F$).
- Branching Effect: For isomeric haloalkanes, boiling point decreases with branching due to lower surface area.
- Solubility: Though polar, they are insoluble in water as they cannot form hydrogen bonds with $\text{H}_2\text{O}$.
3. Chemical Properties & Mechanisms
Haloalkanes are the primary focus for studying Nucleophilic Substitution reactions, which are fundamental to organic synthesis.
Mechanism Comparison: $S_N1$ vs $S_N2$
- $S_N1$ (Unimolecular): 2-step process, Carbocation intermediate, Racemization occurs. Rate: $3^\circ > 2^\circ > 1^\circ$.
- $S_N2$ (Bimolecular): 1-step process, Transition state, Walden Inversion. Rate: $1^\circ > 2^\circ > 3^\circ$.
- Wurtz Reaction: $2R-X + 2\text{Na} \rightarrow R-R + 2\text{NaX}$ (Coupling to form higher alkanes).
- Elimination: Reaction with Alcoholic $\text{KOH}$ yields Alkenes (Saytzeff’s product is major).
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