What NEET Asks
- Predict major/minor products in free radical halogenation, considering reactivity vs. selectivity.
- Mechanism of free radical substitution.
- Stoichiometric calculations for alkane combustion.
- Identifying suitable alkanes for Wurtz reaction synthesis and its limitations.
Key Points
- Alkanes are saturated hydrocarbons; their characteristic reaction is Free Radical Substitution (FRS).
- Halogenation (Cl2, Br2 in UV light): Proceeds via a free radical mechanism (initiation, propagation, termination).
- Reactivity Order of Halogens: F2 > Cl2 > Br2 > I2. F2 is explosive, I2 is reversible.
- Free Radical Stability: Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl. Dictates regioselectivity.
- Bromination vs. Chlorination: Bromination is more selective (favors 3° H) but less reactive. Chlorination is less selective but more reactive, often yielding product mixtures.
- Combustion: Alkanes burn completely in excess O2 to yield CO2 and H2O, releasing heat.
- Wurtz Reaction: Synthesizes higher, symmetrical alkanes (R-R) from alkyl halides (RX) using Na in dry ether. Not suitable for unsymmetrical alkanes (R-R') due to product mixtures.
Must-Know Formula / Reaction
- General Combustion of Alkanes: CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O
- n: Number of carbon atoms.
- (3n+1)/2 O2: Moles of oxygen required.
- Wurtz Reaction: 2 RX + 2 Na --(dry ether)--> R-R + 2 NaX
- RX: Alkyl halide.
- Na: Sodium metal.
- dry ether: Solvent to prevent side reactions.
Common Mistakes
- Students often omit dry ether in Wurtz reaction, leading to Na reacting with protic solvents.
- Don't confuse free radical mechanism (homolytic cleavage) with ionic mechanisms.
- Misinterpreting selectivity vs. reactivity: Bromination is selective; chlorination is reactive.
Rapid Revision
- Alkanes = FRS. Mechanism has 3 steps.
- Free radical stability (3° > 2° > 1°) guides product formation.
- Wurtz reaction for symmetrical alkanes (R-R).
- Master complete combustion stoichiometry.