Preparation & Properties of Haloalkanes
The gateway to organic synthesis. Master Darzen's procedure, specific Halogen Exchange reactions, allylic halogenation with NBS, and the physical property traps.
Module Focus: The Polar C-X Bond
Haloalkanes ($R-X$) are extraordinarily versatile intermediates. The high electronegativity of the halogen creates a permanent dipole ($C^{\delta+}-X^{\delta-}$), rendering the carbon susceptible to nucleophilic attack. Synthesizing them cleanly is paramount. In this module, we dissect the exact reagents needed to swap an $-OH$ group for an $-X$ group, and how to exchange one halogen for another via Le Chatelier's principle.
1. Preparation from Alcohols ($-OH \rightarrow -X$)
The hydroxyl group of an alcohol is a poor leaving group. It must be converted into a good leaving group using specific acidic reagents or phosphorus halides.
Requires a catalyst (like Anhydrous $ZnCl_2$, Lucas Reagent) for $1^\circ$ and $2^\circ$ alcohols to break the C-O bond.
Trap: $3^\circ$ alcohols are so reactive they don't need $ZnCl_2$, they react by merely shaking with conc. $HCl$ at room temp.
You must memorize the distinct by-products for $PCl_3$ vs $PCl_5$.
Reacting an alcohol with Thionyl Chloride ($SOCl_2$) is universally preferred for preparing alkyl chlorides.
Why is it the best?
Both by-products ($SO_2$ and $HCl$) are escapable gases. They leave the reaction mixture entirely on their own, resulting in pure alkyl chloride without the need for complex separation techniques.
2. Preparation from Hydrocarbons
A. Free Radical Halogenation (Alkanes)
Yields a complex mixture of isomeric mono- and polyhaloalkanes which are difficult to separate. Not ideal for lab synthesis, but critical for industrial processes. Reactivity order: $3^\circ > 2^\circ > 1^\circ$.
B. Electrophilic Addition (Alkenes)
Addition of hydrogen halides ($HX$) follows Markovnikov's Rule, while addition of $HBr$ in the presence of peroxides follows the Anti-Markovnikov (Kharasch) Effect.
If an alkene is treated with halogens at high temperature (500°C) or with a specific reagent like N-Bromosuccinimide (NBS) in the presence of light/peroxide, addition to the double bond does NOT occur. Instead, free radical substitution happens at the allylic carbon.
3. Halogen Exchange Reactions
Direct fluorination is explosive, and direct iodination is reversible. To prepare alkyl iodides and fluorides cleanly, we synthesize an alkyl chloride/bromide first, and then simply swap the halogen.
(X = Cl, Br)
$NaI$ is soluble in dry acetone, but $NaCl$ and $NaBr$ are NOT. They precipitate out of the solution as solids. According to Le Chatelier's Principle, the continuous removal of products forces the equilibrium strongly in the forward direction.
Heating an alkyl chloride/bromide in the presence of heavy metallic fluorides.
Permitted Reagents:
$AgF, Hg_2F_2, CoF_2, SbF_3$
4. Physical Properties
Haloalkanes have higher boiling points than their parent alkanes due to greater polarity (dipole-dipole interactions) and higher molecular mass.
- For same alkyl group ($R$): BP increases with the mass of the halogen.
$R-I > R-Br > R-Cl > R-F$ - For isomeric haloalkanes: BP decreases as branching increases. Branching makes the molecule spherical, decreasing surface area and weakening van der Waals forces.
$1^\circ > 2^\circ > 3^\circ$
Bromo, iodo, and polychloro derivatives are heavier than water (unlike normal alkanes).
To dissolve in water, they must break the strong hydrogen bonds between water molecules. The energy released by forming new, weaker ion-dipole/dipole-dipole interactions between the haloalkane and water is not sufficient to overcome the energy required to break water's hydrogen bonding network.
NEET Grand Test: Haloalkanes Prep
15 High-Yield Questions testing specific reagents, the Finkelstein logic, NBS specificity, and boiling point trends.
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