Noesis
4.2.1Hydrocarbons

Alkanes — preparation (Wurtz, Kolbe electrolysis, hydrogenation), properties, free-radical halogenation (Cl₂ - Br₂)

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1. Preparation

1.1 Wurtz Reaction

WHY it works (mechanism, derived): Sodium is a strong reducing agent (low ionization energy). It donates one electron to the C–X bond:

  1. R–X+Na→R∙+Na+X−R\text{–}X + Na \rightarrow R^{\bullet} + Na^+X^- (an alkyl free radical / organosodium R–NaR\text{–}Na forms).
  2. The organosodium R–NaR\text{–}Na acts as a carbanion nucleophile and attacks a second R–XR\text{–}X: R−+R–X→R–R+X−R^- + R\text{–}X \rightarrow R\text{–}R + X^-.

1.2 Kolbe Electrolysis

HOW (step-by-step at the anode):

  1. Salt dissociates: RCOONa→RCOO−+Na+RCOONa \rightarrow RCOO^- + Na^+.
  2. At anode (oxidation): RCOO−→RCOO∙+e−RCOO^- \rightarrow RCOO^{\bullet} + e^-.
  3. The carboxyl radical loses CO2CO_2 (very stable molecule = driving force): RCOO∙→R∙+CO2RCOO^{\bullet} \rightarrow R^{\bullet} + CO_2.
  4. Two R∙R^{\bullet} combine: R∙+R∙→R–RR^{\bullet} + R^{\bullet} \rightarrow R\text{–}R.

1.3 Hydrogenation (Sabatier–Senderens)

WHY a catalyst: H2H_2 has a strong, non-polar bond; the metal surface adsorbs and splits H2H_2 into reactive H atoms and holds the alkene close, lowering the activation energy. (Syn addition — both H on same face.)

Figure — Alkanes — preparation (Wurtz, Kolbe electrolysis, hydrogenation), properties, free-radical halogenation (Cl₂ - Br₂)

2. Physical Properties

3. Free-Radical Halogenation

Mechanism (the three phases — derive, don't memorize):

(i) Initiation — homolysis by light: Cl2→hν2 Cl∙Cl_2 \xrightarrow{h\nu} 2\,Cl^{\bullet} Why? Cl–Cl is the weakest bond present; UV supplies exactly its bond energy → splits evenly (each atom keeps one electron).

(ii) Propagation — radical in, radical out: Cl∙+CH4→CH3∙+HClCl^{\bullet} + CH_4 \rightarrow CH_3^{\bullet} + HCl CH3∙+Cl2→CH3Cl+Cl∙CH_3^{\bullet} + Cl_2 \rightarrow CH_3Cl + Cl^{\bullet} Why? The regenerated Cl∙Cl^{\bullet} keeps the chain alive.

(iii) Termination — two radicals meet (chain dies): Cl∙+Cl∙→Cl2,CH3∙+CH3∙→C2H6,CH3∙+Cl∙→CH3ClCl^{\bullet}+Cl^{\bullet}\rightarrow Cl_2,\quad CH_3^{\bullet}+CH_3^{\bullet}\rightarrow C_2H_6,\quad CH_3^{\bullet}+Cl^{\bullet}\rightarrow CH_3Cl

What reagents define the Wurtz reaction?
Alkyl halide + Na metal in dry ether → symmetrical alkane.
Why can't Wurtz make methane?
It couples two R groups; methane (1C) has no two halves to join.
Why does mixed Wurtz give poor yields?
Radicals cross-couple → mixture of three alkanes, hard to separate.
At which electrode does the alkane form in Kolbe electrolysis?
At the anode (oxidation of carboxylate).
What gas is released in Kolbe electrolysis?
CO₂ (from decarboxylation of RCOO•).
Number of carbons from a Cₙ acid via Kolbe?
2(n−1) — one C lost as CO₂ per side.
Why is a catalyst needed in hydrogenation?
Metal surface adsorbs and splits H₂, lowers activation energy; gives syn addition.
Why does boiling point increase with chain length?
Larger surface area → stronger van der Waals (London) forces.
Why does branching lower boiling point?
More spherical shape → less surface contact → weaker dispersion forces.
Name the three steps of free-radical halogenation.
Initiation, propagation, termination.
Why is initiation done with UV light?
Photon energy homolyzes the weak Cl–Cl/Br–Br bond into radicals.
Order of halogen reactivity in halogenation?
F₂ > Cl₂ > Br₂ > I₂ (I₂ effectively doesn't react).
Which is more selective, Cl₂ or Br₂, and why?
Br₂; being less reactive it prefers H giving the most stable (3°) radical.
Free-radical stability order?
3° > 2° > 1° > methyl (hyperconjugation + inductive donation).
Why are alkanes insoluble in water?
Non-polar; cannot hydrogen-bond with water ("like dissolves like").
Recall Feynman: explain to a 12-year-old

Alkanes are like Lego bricks made only of strong, plain straight bonds — nothing sticky sticks to them easily. To build a longer brick chain you can: (1) glue two equal pieces with sodium metal (Wurtz), (2) shock acid-salt with electricity so it burps out CO₂ gas and the leftovers join (Kolbe), or (3) squeeze in hydrogen with a metal helper to fill a double bond (hydrogenation). To decorate an alkane you shine light on it with chlorine: the light snaps the Cl₂ in half, and those halves start a domino chain swapping H's for Cl's — but it never stops cleanly, so you get a messy mix.

Connections

  • Alkenes — addition reactions (reverse of hydrogenation)
  • Free radicals and reaction intermediates
  • Carbanions and organometallic reagents (Wurtz intermediate)
  • Van der Waals forces (boiling point trends)
  • Electrolysis and redox at electrodes (Kolbe)
  • Hyperconjugation (radical stability)

Concept Map

strong nonpolar bonds cause

only attacked by

prepared by

prepared by

prepared by

alkyl halide plus Na in ether

anode decarboxylation loses CO2

reduces unsaturation

enables under heat or light

uses Cl2 or Br2

Alkanes CnH2n+2 sp3 saturated

Unreactive stable

Free radical attack

Wurtz reaction

Kolbe electrolysis

Hydrogenation

Symmetrical alkanes

Free-radical halogenation

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, alkanes (CnH2n+2C_nH_{2n+2}) bahut "shaant" molecules hote hain — sirf single bonds, koi double bond ya lone pair nahi, isliye normal ionic reagents inhe chhedte nahi. Lambi chain banane ke liye teen famous tareeke hain. Wurtz mein alkyl halide ko dry ether mein sodium ke saath milao — do barabar tukde jud jaate hain. Yaad rakho: alag-alag tukde mat lo, warna mixture ban jaata hai, aur methane to bana hi nahi sakte. Kolbe electrolysis mein carboxylic acid ka sodium salt ka aqueous solution electrolyse karo — anode par CO2CO_2 nikalta hai aur bache hue radicals jud kar symmetrical alkane dete hain. Hydrogenation mein alkene par Ni/Pt catalyst ke saath H2H_2 add karke alkane banate hain.

Physical properties simple logic se: chain jitni lambi, surface area utna zyada, van der Waals force utna strong, isliye boiling point badhta hai. Branching karne se molecule gol ho jaata hai, contact kam, isliye boiling point gir jaata hai (n-pentane vs neopentane).

Sabse important exam topic hai free-radical halogenation. UV light Cl2Cl_2 ko do radicals mein tod deti hai (initiation). Phir propagation mein chain chalti rehti hai — ek radical andar, ek radical bahar. Jab do radical aapas mein mil jaayein to chain khatam (termination). Yaad rakho: yeh reaction clean nahi hoti, CH3ClCH_3Cl se aage CH2Cl2,CHCl3,CCl4CH_2Cl_2, CHCl_3, CCl_4 tak ban jaata hai, isliye excess alkane use karo. Aur reactivity F2>Cl2>Br2>I2F_2>Cl_2>Br_2>I_2, par selectivity ulti — bromine picky hota hai aur 3° hydrogen prefer karta hai kyunki 3° radical sabse stable hai.

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Connections

van der Waals forces — London dispersion, dipole-dipole, dipole-induced dipoleChemistry · 2.3.17