CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Haloalkanes
1. Nature of C-X Bond & Properties
Haloalkanes ($R-X$) contain a polarized covalent bond due to electronegativity difference.
Bond length: $C-I > C-Br > C-Cl > C-F$
Bond Enthalpy: $C-F > C-Cl > C-Br > C-I$
Reactivity $\propto$ 1/Bond Strength. $R-I$ is most reactive.
B.P. $\propto$ Molecular Mass & Surface Area.
- $R-I > R-Br > R-Cl > R-F$
- Straight chain > Branched chain (due to surface area).
2. Methods of Preparation
From Alcohols
• Lucas Reagent: $ROH + HCl/ZnCl_2 \rightarrow RCl$
• Thionyl Chloride: $ROH + SOCl_2 \rightarrow RCl + SO_2 \uparrow + HCl \uparrow$ (Best method as byproducts are gases).
• $PCl_5 / PCl_3$ reactions.
Halogen Exchange
• Finkelstein: $R-X + NaI \xrightarrow{Acetone} R-I + NaX$
• Swarts: $R-X + AgF \rightarrow R-F + AgX$ (For alkyl fluorides).
3. Nucleophilic Substitution Mechanisms
| Feature | SN1 Mechanism | SN2 Mechanism |
|---|---|---|
| Kinetics | Unimolecular ($1^{st}$ Order) | Bimolecular ($2^{nd}$ Order) |
| Intermediate | Carbocation (Rearranges) | Transition State (Pentacoordinate) |
| Reactivity | $3^\circ > 2^\circ > 1^\circ$ | $1^\circ > 2^\circ > 3^\circ$ (Steric hindrance) |
| Stereochemistry | Racemization | Walden Inversion |
| Solvent | Polar Protic ($H_2O, EtOH$) | Polar Aprotic ($DMSO, DMF, Acetone$) |
4. Elimination & Reactions with Metals
Reagent: Alcoholic KOH. Follows Saytzeff Rule (Highly substituted alkene is major).
Formation: $R-X + Mg \xrightarrow{Dry Ether} R-Mg-X$
Highly sensitive to moisture!
$RMgX + H_2O \rightarrow R-H + Mg(OH)X$
Coupling of two alkyl groups.
$2RX + 2Na \xrightarrow{Dry Ether} R-R + 2NaX$
*Methane cannot be prepared by this method.*
5. Essential Stereochemistry Terms
Chirality
Non-superimposable mirror image. Requires asymmetric carbon.
Enantiomers
Stereoisomers related as object and mirror image.
Racemic Mixture
50:50 mixture of $d$ and $l$ forms. Optically inactive.
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