What are the major limitations of the Wurtz Reaction for preparing alkanes?

The primary limitations of the Wurtz reaction are: (1) Methane (CH4) cannot be prepared since the reaction couples two alkyl radicals to form an alkane with at least two carbons; (2) It gives good yields only for symmetrical alkanes with an even number of carbon atoms (e.g., ethane, butane); (3) When two different alkyl halides are used, a mixture of three different alkanes is obtained which is difficult to separate due to close boiling points.

Why is the staggered conformation of ethane more stable than the eclipsed conformation?

In the staggered conformation of ethane, C-H bonds on adjacent carbons are at a maximum dihedral angle of 60 degrees, resulting in minimum repulsion between bonding electron pairs (zero torsional strain). In the eclipsed conformation (dihedral angle 0 degrees), C-H bonds directly align, creating maximum torsional strain. Ethane's staggered form is more stable by about 12.5 kJ/mol.

What are the anode and cathode products during Kolbe's electrolytic synthesis of sodium acetate?

During Kolbe's electrolysis of aqueous sodium acetate (CH3COONa): At the Anode (oxidation), acetate ions lose electrons to form methyl radicals which couple into Ethane (C2H6) and Carbon Dioxide (CO2) gas. At the Cathode (reduction), water is reduced to produce Hydrogen gas (H2) and sodium hydroxide (NaOH).

What is the order of reactivity and selectivity in free-radical halogenation of alkanes?

Reactivity order of halogens: F2 > Cl2 > Br2 > I2. Fluorination is explosively violent, while iodination is reversible requiring an oxidizing agent (HNO3 or HIO3). Selectivity for hydrogen abstraction follows: 3 deg C-H > 2 deg C-H > 1 deg C-H due to the relative thermodynamic stability of the intermediate alkyl free radicals (tertiary > secondary > primary).

§ 9.1NCERT Class 11 · Chemistry · Chapter 9

Alkanes: Preparation, Reactions & Conformations – Complete Guide

QCC Notes · Class 11 Chemistry NCERT-aligned Updated 26 Sep 2026 5 min read

Hydrocarbons composed exclusively of carbon and hydrogen form the bedrock of organic chemistry. Alkanes are saturated aliphatic open-chain hydrocarbons possessing only carbon-carbon and carbon-hydrogen single sigma (σ) bonds, with the general molecular formula C_nH_2n+2. Formerly known as paraffins (Latin: parum affinis = little affinity) due to their inertness under ordinary laboratory conditions.

1. Structure & Isomerism

In all alkanes, every carbon atom is sp³ hybridized with a tetrahedral geometry (bond angle ≈ 109.5°), and C−C and C−H bond lengths are approximately 154 pm and 112 pm, respectively.

Methane (CH₄), Ethane (C₂H₆), and Propane (C₃H₈) have only one structural arrangement.
Butane (C₄H₁₀) exhibits 2 chain isomers: n-butane and isobutane (2-methylpropane).
Pentane (C₅H₁₂) exhibits 3 structural isomers: n-pentane, isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane).
Boiling Point Trend: Boiling points increase with increasing molecular weight (greater van der Waals surface area), but decrease with branching (branched isomers become more spherical and compact, reducing contact surface area: n-pentane > isopentane > neopentane).

2. Methods of Preparation of Alkanes

A. Catalytic Hydrogenation of Unsaturated Hydrocarbons (Sabatier-Senderens Reaction)

Alkenes and alkynes add dihydrogen in the presence of finely divided catalysts like Nickel (Raney Ni at 523 K) or Platinum / Palladium at room temperature:

CH₂=CH₂ + H₂ → (Pt/Pd/Ni) → CH₃−CH₃
CH₃−C≡CH + 2 H₂ → (Pt/Pd/Ni) → CH₃−CH₂−CH₃

B. Wurtz Reaction

Alkyl halides on treatment with sodium metal in dry ethereal solution give higher symmetrical alkanes:

2 R−X + 2 Na → (dry ether) → R−R + 2 NaX
2 CH₃−Br + 2 Na → CH₃−CH₃ + 2 NaBr

Key Limitations:

Methane (CH₄) cannot be prepared.
Produces good yields only for symmetrical alkanes with an even number of carbon atoms.
Cross-Wurtz (R−X + R'−X) yields a mixture of R−R, R'−R', and R−R', making separation difficult.

C. Decarboxylation with Soda Lime

Sodium salts of carboxylic acids heated with soda lime (mixture of NaOH and CaO in 3:1 ratio) lose carbon dioxide to yield an alkane with one carbon atom less than the parent acid:

CH₃COONa + NaOH → (CaO, Δ) → CH₄ + Na₂CO₃

CaO keeps the NaOH dry (hygroscopic protection) and raises the fusion temperature.

D. Kolbe's Electrolytic Synthesis

Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid produces an alkane containing an even number of carbon atoms at the anode:

2 CH₃COO⁻Na⁺ + 2 H₂O → (Electrolysis) → C₂H₆ + 2 CO₂ + H₂ + 2 NaOH

Electrode Mechanisms:

At Anode (Oxidation): 2 CH₃COO⁻ → 2 CH₃COO^• + 2 e⁻ → 2 CH₃^• + 2 CO₂ ↑ ⇒ CH₃^• + CH₃^• → C₂H₆ ↑
At Cathode (Reduction): 2 H₂O + 2 e⁻ → 2 OH⁻ + H₂ ↑

3. Chemical Reactions of Alkanes

A. Free Radical Halogenation

Alkanes react with halogens in the presence of ultraviolet light or heat (573–773 K). The chlorination of methane proceeds via a three-step free radical chain mechanism:

Initiation: Homolytic cleavage of Cl−Cl bond by light quanta: Cl₂ → (hν) → 2 Cl^•.
Propagation:
• Cl^• + CH₄ → ^•CH₃ + HCl
• ^•CH₃ + Cl₂ → CH₃Cl + Cl^• (regenerates chain carrier).
Termination: Combination of radicals to consume chain carriers (Cl^• + Cl^• → Cl₂; ^•CH₃ + Cl^• → CH₃Cl; ^•CH₃ + ^•CH₃ → C₂H₆ [explains traces of ethane formed during chlorination of methane!]).

B. Controlled Oxidation

2 CH₄ + O₂ → (Cu/523 K/100 atm) → 2 CH₃OH (Methanol)
CH₄ + O₂ → (Mo₂O₃, Δ) → HCHO (Methanal) + H₂O
(CH₃)₃CH + [O] → (KMnO₄) → (CH₃)₃C−OH (tert-butyl alcohol) (selective for tertiary hydrogen).

C. Aromatisation & Isomerisation

Aromatisation: n-Hexane passed over Cr₂O₃ / V₂O₅ / Mo₂O₃ supported over alumina at 773 K and 10–20 atm undergoes simultaneous cyclisation and dehydrogenation to yield Benzene.
Isomerisation: n-Alkanes heated with anhydrous AlCl₃ and HCl gas isomerise to branched chain alkanes.

4. Conformations of Ethane

Alkanes possess carbon-carbon single bonds. Due to cylindrical symmetry of the σ molecular orbital, free rotation around the C−C single bond is possible. This rotation results in different spatial arrangements of atoms that can be converted into one another by rotation around a single bond, called conformations (or conformers / rotamers).

Conformation Property	Staggered Conformation	Eclipsed Conformation
Dihedral (Torsion) Angle (θ)	θ = 60°	θ = 0°
Proximity of C−H bonds	Maximum distance apart	Closest possible alignment
Torsional Strain	Minimum (virtually zero)	Maximum (repulsion between C−H electron clouds)
Potential Energy Difference	Lowest energy (Most stable)	Higher by 12.5 kJ mol⁻¹ (Less stable)

Sawhorse vs Newman Projections:

Sawhorse Projection: Viewed from an oblique angle showing the central C−C bond as a projected diagonal line.
Newman Projection: Viewed end-on along the C−C bond axis. The front carbon is represented by a central point (dot) with radiating bonds, while the rear carbon is represented by a large circle.

Because the energy barrier of 12.5 kJ/mol is easily overcome by thermal kinetic energy at room temperature (which is ~ 2.5 kJ/mol collision energy), the two conformations interconvert millions of times per second and cannot be isolated under ordinary conditions.

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