CHEMCA
EXAM MASTER REVIEW SHEET
Alkanes (Paraffins)
Strategic Summary for Hydrocarbons (JEE & NEET)
1. Preparation of Alkanes
Wurtz Reaction
Coupling of alkyl halides with Sodium.
\[ \ce{2R-X + 2Na ->[\text{dry ether}] R-R + 2NaX} \]*Methane cannot be prepared.*
Soda Lime Decarboxylation
Loss of \(\ce{CO2}\) from sodium salt of acid.
\[ \ce{R-COONa + NaOH ->[CaO][\Delta] R-H + Na2CO3} \]
Kolbe's Electrolysis
Aqueous electrolysis of fatty acid salts.
Anode: \(\ce{R-R + CO2}\)
Cathode: \(\ce{H2 + NaOH}\)
Clemmensen Reduction
\[ \ce{>C=O ->[Zn-Hg][\text{conc. } HCl] >CH2} \]
Corey-House Synthesis:
Reaction of lithium dialkyl cuprate (\(\ce{R'_2CuLi}\)) with alkyl halide (\(\ce{RX}\)).
Equation: \[ \ce{R'_2CuLi + R-X -> R-R' + R'Cu + LiX} \]
*Best method for unsymmetrical alkanes.*
2. Physical Property Trends
Boiling Point (B.P.)
- • \(\text{B.P.} \propto \text{Molecular Weight}\)
- • \(\text{B.P.} \propto \frac{1}{\text{Branching}}\) (Due to surface area)
- \( n\text{-pentane} > \text{isopentane} > \text{neopentane} \)
Melting Point (M.P.)
- • \(\text{M.P.} \propto \text{Packing Efficiency}\)
- • Alternation Effect: Even-numbered carbons pack better than odd-numbered counterparts.
3. Free Radical Halogenation
Reactivity Orders:
Halogens: \( \ce{F2 > Cl2 > Br2 > I2} \)
H-atoms: \( \ce{3^\circ > 2^\circ > 1^\circ > CH4} \)
| Feature | Chlorination (\(\ce{Cl2}\)) | Bromination (\(\ce{Br2}\)) |
|---|---|---|
| Selectivity | Non-selective | Highly Selective |
| Relative Rates | \(3^\circ : 2^\circ : 1^\circ = 5 : 3.8 : 1\) | \(3^\circ : 2^\circ : 1^\circ = 1600 : 82 : 1\) |
4. Rearrangement Reactions
Aromatization
\[ \ce{n-Hexane ->[Cr2O3 / Al2O3][773K, 10-20 atm] Benzene} \]
Isomerization
\[ \ce{n-Butane ->[anhyd. AlCl3 / HCl] Isobutane} \]
5. Miscellaneous Reactions
General Combustion
\[ \ce{C_{n}H_{2n+2} + \frac{3n+1}{2} O2 -> n CO2 + (n+1) H2O} \]
Nitration
Occurs at high temp (400-500°C) via C-C cleavage.
Pyrolysis
Cracking into lower hydrocarbons via radical mechanism.
No comments:
Post a Comment