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

4.2.1 · Chemistry › Hydrocarbons

1. Preparation

1.1 Wurtz Reaction

WHY ye kaam karta hai (mechanism, derived): Sodium ek strong reducing agent hai (low ionization energy). Ye C–X bond ko ek electron donate karta hai:

$R - X + N a \to R^{∙} + N a^{+} X^{-}$ (ek alkyl free radical / organosodium $R - N a$ banta hai).
Organosodium $R - N a$ ek carbanion nucleophile ki tarah act karta hai aur doosre $R - X$ par attack karta hai: $R^{-} + R - X \to R - R + X^{-}$ .

1.2 Kolbe Electrolysis

HOW (anode par step-by-step):

Salt dissociate hota hai: $R C O O N a \to R C O O^{-} + N a^{+}$ .
Anode par (oxidation): $R C O O^{-} \to R C O O^{∙} + e^{-}$ .
Carboxyl radical $C O_{2}$ lose karta hai (bahut stable molecule = driving force): $R C O O^{∙} \to R^{∙} + C O_{2}$ .
Do $R^{∙}$ combine karte hain: $R^{∙} + R^{∙} \to R - R$ .

1.3 Hydrogenation (Sabatier–Senderens)

WHY ek catalyst chahiye: $H_{2}$ ka bond strong aur non-polar hai; metal surface $H_{2}$ ko adsorb karke split karta hai reactive H atoms mein aur alkene ko paas rakhta hai, activation energy kam karta hai. (Syn addition — dono H same face se.)

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

2. Physical Properties

3. Free-Radical Halogenation

Mechanism (teen phases — derive karo, memorize mat karo):

(i) Initiation — light se homolysis: $C l_{2} h ν 2 C l^{∙}$ Kyun? Cl–Cl sabse weak bond present hai; UV bilkul iski bond energy supply karta hai → evenly split hota hai (har atom ek electron rakhta hai).

(ii) Propagation — radical in, radical out: $C l^{∙} + C H_{4} \to C H_{3}^{∙} + H C l$ $C H_{3}^{∙} + C l_{2} \to C H_{3} C l + C l^{∙}$ Kyun? Regenerated $C l^{∙}$ chain ko zinda rakhta hai.

(iii) Termination — do radicals milte hain (chain khatam): $C l^{∙} + C l^{∙} \to C l_{2}, C H_{3}^{∙} + C H_{3}^{∙} \to C_{2} H_{6}, C H_{3}^{∙} + C l^{∙} \to C H_{3} C l$

Wurtz reaction mein kaun se reagents hote hain?

Alkyl halide + Na metal dry ether mein → symmetrical alkane.

Wurtz methane kyun nahi bana sakta?

Ye do R groups couple karta hai; methane (1C) ke do hisse nahi hote join karne ke liye.

Mixed Wurtz poor yields kyun deta hai?

Radicals cross-couple hote hain → teen alkanes ka mixture, separate karna mushkil.

Kolbe electrolysis mein alkane kis electrode par banta hai?

Anode par (carboxylate ka oxidation).

Kolbe electrolysis mein kaun si gas release hoti hai?

CO₂ (RCOO• ke decarboxylation se).

Kolbe se Cₙ acid se kitne carbons milte hain?

2(n−1) — har side se ek C, CO₂ ke roop mein loss hota hai.

Hydrogenation mein catalyst kyun chahiye?

Metal surface H₂ ko adsorb aur split karta hai, activation energy kam karta hai; syn addition deta hai.

Chain length badhne se boiling point kyun badhta hai?

Zyada surface area → stronger van der Waals (London) forces.

Branching boiling point kyun kam karta hai?

Zyada spherical shape → kam surface contact → weaker dispersion forces.

Free-radical halogenation ke teen steps ke naam batao.

Initiation, propagation, termination.

Initiation UV light se kyun hoti hai?

Photon energy weak Cl–Cl/Br–Br bond ko homolyze karke radicals banati hai.

Halogenation mein halogens ki reactivity ka order?

F₂ > Cl₂ > Br₂ > I₂ (I₂ effectively react nahi karta).

Cl₂ ya Br₂ mein zyada selective kaun hai, aur kyun?

Br₂; kam reactive hone ki wajah se ye 3° radical dene wale H ko prefer karta hai.

Free-radical stability order?

3° > 2° > 1° > methyl (hyperconjugation + inductive donation).

Alkanes paani mein kyun insoluble hain?

Non-polar hain; paani ke saath hydrogen bond nahi bana sakte ("like dissolves like").

Recall Feynman: ek 12-saal ke bachche ko samjhao

Alkanes aisi Lego bricks ki tarah hain jo sirf strong, plain seedhi bonds se bani hain — inpe koi cheez aasani se nahi chipakti. Lambi brick chain banane ke liye aap kar sakte ho: (1) do barabar tukdon ko sodium metal se jodo (Wurtz), (2) acid-salt ko electricity se shock do taaki wo CO₂ gas "burp" kare aur bache hue tukde jud jaayein (Kolbe), ya (3) hydrogen ko metal helper ke saath double bond mein bhar do (hydrogenation). Alkane ko decorate karne ke liye chlorine ke saath uس par light chamo: light Cl₂ ko aadha tor deti hai, aur wo aadhe hisse ek domino chain shuru kar dete hain H's ko Cl's se badalne ki — lekin ye kabhi saaf nahi rukta, isliye ek messy mix milta hai.

Connections

Alkenes — addition reactions (hydrogenation ka reverse)
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)