Master Alkane Reactions for NEET Chemistry: Your Go-To Guide

Introduction

Alkanes, often called paraffins (from Latin 'parum affinis' meaning little affinity), are saturated hydrocarbons with only carbon-carbon and carbon-hydrogen single bonds. Due to the strong, non-polar nature of these sigma bonds, alkanes are generally quite unreactive under normal conditions. However, under specific conditions, they undergo several important reactions that are crucial for your NEET Chemistry preparation. Let's explore these key reactions.

Core Concept

1. Halogenation (Free Radical Substitution)

This is the most characteristic reaction of alkanes. Hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I).

• Conditions: Presence of UV light or high temperature (300-500°C).
• Mechanism: Proceeds via a free radical chain mechanism (initiation, propagation, termination).
• Reactivity Order: For halogens: F2 > Cl2 > Br2 > I2. For hydrogen abstraction: Tertiary H > Secondary H > Primary H (most pronounced for bromination).
• Example: CH4 + Cl2 --(UV light)--> CH3Cl + HCl (monochlorination).

2. Combustion

Alkanes burn readily in the presence of sufficient oxygen.

• Conditions: Presence of oxygen/air, ignition source.
• Products (Complete Combustion): Carbon dioxide (CO2) and water (H2O), releasing a large amount of heat.
• General Equation: CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O.
• Incomplete Combustion: If oxygen supply is limited, carbon monoxide (CO) or even carbon (soot) is formed.

3. Pyrolysis (Cracking)

This involves heating higher alkanes to high temperatures, leading to the breaking of C-C and C-H bonds.

• Conditions: High temperature (400-700°C), absence of air, sometimes with catalysts (e.g., Al2O3, SiO2).
• Products: Smaller alkanes, alkenes, and hydrogen gas.
• Significance: Used industrially to produce gasoline and other valuable lighter hydrocarbons from heavier crude oil fractions.

4. Isomerization

Straight-chain alkanes can be converted into branched-chain alkanes.

• Conditions: Anhydrous Aluminium Chloride (AlCl3) and Hydrogen Chloride (HCl) at moderate temperature and pressure.
• Example: n-Butane --(anhyd. AlCl3/HCl, heat)--> Isobutane (2-methylpropane).

5. Aromatization (Dehydrocyclization)

Straight-chain alkanes with six or more carbon atoms can be converted into aromatic hydrocarbons.

• Conditions: Heating to ~500°C under high pressure in the presence of Cr2O3, V2O5, or Mo2O3 supported on Al2O3.
• Example: n-Hexane --(Cr2O3/Al2O3, 500°C, high pressure)--> Benzene + 4H2.

Solved Example

Question: Write the reaction mechanism for the monochlorination of methane.

Solution:

• Reaction: CH4 + Cl2 --(UV light)--> CH3Cl + HCl
• Mechanism (Free Radical Substitution):
  1. Initiation: Cl₂ --(UV light)--> 2Cl• (Homolytic cleavage of the Cl-Cl bond)
  2. Propagation: a. Cl• + CH₄ → •CH₃ + HCl (Chlorine radical abstracts H from methane) b. •CH₃ + Cl₂ → CH₃Cl + Cl• (Methyl radical abstracts Cl from chlorine molecule)
  3. Termination: Combination of any two radicals: a. Cl• + Cl• → Cl₂ b. •CH₃ + •CH₃ → CH₃CH₃ (Ethane) c. •CH₃ + Cl• → CH₃Cl (Chloromethane)

NEET Trick

• Reactivity vs. Selectivity in Halogenation: F > Cl > Br > I in reactivity. But for selectivity towards H-abstraction (3° > 2° > 1°), Bromination is much more selective than Chlorination. Fluorination is too violent, and iodination is reversible.
• Catalysts: Remember AlCl3/HCl for Isomerization and Cr2O3/Al2O3 for Aromatization.

Quick Recap

• Alkanes undergo free radical substitution (e.g., halogenation) due to strong C-H bonds.
• Combustion of alkanes yields CO2 and H2O (complete) or CO/C (incomplete).
• Pyrolysis (cracking) breaks larger alkanes into smaller ones at high temperatures.
• Isomerization converts straight-chain alkanes to branched ones using AlCl3/HCl.
• Aromatization converts n-alkanes (C6+) to aromatic compounds using Cr2O3/Al2O3 catalysts at high temperatures.