Hydrocarbons

JKBOSE/CBSE
Hydrocarbons Class 11 Chemistry Notes
Hydrocarbons - Class 11 Chemistry

Hydrocarbons

Overview: Alkanes, Alkenes, Alkynes, and Aromatic Hydrocarbons. Preparation, Properties, and Reactions.

1. Alkanes (Paraffins)

Saturated. CnH2n+2. Low reactivity.

Preparation

  • Wurtz Reaction: 2RX + 2Na -> R-R + 2NaX (Symmetrical alkanes).
  • Decarboxylation: RCOONa + NaOH/CaO -> RH + Na2CO3.
  • Kolbe's Electrolysis: 2RCOOK -> R-R + 2CO2 + H2 + 2KOH.

Properties

  • Free Radical Substitution (Halogenation).
  • Combustion, Isomerisation, Aromatization.

2. Alkenes (Olefins)

Unsaturated. Double bond. CnH2n.

Preparation

  • Dehydration of Alcohols (Acid catalyst).
  • Dehydrohalogenation of Alkyl Halides (Alc. KOH, Saytzeff Rule).

Properties

  • Electrophilic Addition: Markovnikov's Rule (H+ adds to C with more H). Anti-Markovnikov (Peroxide Effect apply to HBr only).
  • Ozonolysis: Reductive ozonolysis gives carbonyls.

3. Alkynes (Acetylenes)

Unsaturated. Triple bond. CnH2n-2. Acidic H.

  • CaC2 + H2O -> C2H2 + Ca(OH)2.
  • Acidic nature: React with Na to give H2 gas.
  • Polymerization to Benzene (Red hot Cu tube).

4. Aromatic Hydrocarbons (Arenes)

Benzene and derivatives. Electrophilic Substitution.

  • Friedel-Crafts Alkylation: Benzene + RX (AlCl3) -> Alkylbenzene.
  • Friedel-Crafts Acylation: Benzene + RCOCl (AlCl3) -> Acylbenzene.
  • Nitration: Conc HNO3 + Conc H2SO4 -> Nitrobenzene.
  • Sulphonation: Fuming H2SO4 -> Benzenesulphonic acid.

Directive Influence: o,p-directing (Activators usually, Halogens deactivating but o,p). m-directing (Deactivators).

Numericals - Hydrocarbons

Numericals

Wurtz Reaction
Q1. Product of Wurtz reaction of Ethyl chloride?
2 C2H5Cl + 2Na -> C2H5-C2H5 + 2NaCl
Product is Butane (n-Butane).
Ozonolysis
Q2. Alkene gives Propanone and Formaldehyde on Ozonolysis. Identify Alkene.
Remove Oxygen and join double bonds.
Propanone: (CH3)2C=O. Formaldehyde: O=CH2.
Join C=C: (CH3)2C=CH2.
Name: 2-Methylpropene (Isobutylene).
Markovnikov Addition
Q3. Major product of Propene + HBr.
H+ goes to C with more H (CH2). Br- goes to C with less H (CH).
CH3-CH=CH2 + HBr -> CH3-CH(Br)-CH3.
Product: 2-Bromopropane.
Anti-Markovnikov
Q4. Propene + HBr (Peroxide).
Kharasch Effect. Anti-Markovnikov.
Br goes to C with MORE H (Terminal).
Product: 1-Bromopropane.
Dehydrohalogenation
Q5. 2-Chlorobutane + Alc KOH. Major product?
Saytzeff Rule: More substituted alkene is major.
But-2-ene (80%) and But-1-ene (20%).
Major: But-2-ene.
Aromaticity
Q6. Is Cyclopentadienyl anion aromatic?
Ring Planar? Yes. sp2 carbons.
Pi electrons? 2 double bonds (4e) + 1 lone pair (2e) = 6 pi e-.
Huckel rule 4n+2? n=1 gives 6.
Yes, Aromatic.
Electrophilic Substitution
Q7. Nitrobenzene + Cl2 (AlCl3). Product?
Nitro group is meta-directing (Deactivating).
Electrophile Cl+ attacks meta position.
Product: m-Chloronitrobenzene.
Combustion
Q8. Volume of O2 needed for combustion of 1 L Ethyne (STP).
C2H2 + 2.5 O2 -> 2CO2 + H2O.
1 mol C2H2 needs 2.5 mol O2.
1 L needs 2.5 L O2.
Acidic H of Alkyne
Q9. Which reacts with NaNH2? Ethane, Ethene, Ethyne.
Only terminal alkynes have acidic hydrogen (sp carbon is electronegative).
Ethyne reacts to form Sodium Acetylide. Others don't.
Conformation
Q10. Most stable conformation of n-Butane?
Anti-staggered (Methyl groups 180 deg apart).
Least steric hindrance and torsional strain.
Formulas & Facts - Hydrocarbons

Equations & Formulas

ClassGeneral Formula
AlkaneCnH2n+2
AlkeneCnH2n
AlkyneCnH2n-2
CycloalkaneCnH2n
BenzeneC6H6
Huckel Rule(4n+2)π electrons
CombustionCnHx + (n+x/4)O2 -> nCO2 + x/2H2O

50 NEET Facts

Key points for Hydrocarbons.

1. Marsh Gas Methane (CH4). Simplest alkane.
2. LPG Liquefied Petroleum Gas. Mainly Butane and Isobutane.
3. CNG Compressed Natural Gas. Mainly Methane.
4. Wurtz Reaction Limit Good for even carbon alkanes. Odd gives mixture (Meth, Eth, Prop). Methane cannot be prepared.
5. Soda Lime NaOH + CaO (3:1). Used for decarboxylation. CaO keeps NaOH dry.
6. Frankland Reaction Uses Zn instead of Na in Wurtz type rxn.
7. Boiling Point Trend Increases with Mol. Wt. Decreases with branching (Surface area decreases). n-Pentane > Iso > Neo.
8. Halogenation Reactivity F2 > Cl2 > Br2 > I2. Fluorination explosive. Iodination reversible.
9. Iodination Requires Oxidizing agent (HIO3, HNO3) to consume HI formed.
10. Selectivity of Halogenation Br2 is highly selective (3 degree > 2 > 1). Cl2 is less selective.
11. Baeyer's Reagent Cold dilute alkaline KMnO4. Test for unsaturation (Purple to Brown/Colorless). Forms Glycols.
12. Bromine Water Test Reddish brown Br2/CCl4 decolorizes with Alkenes/Alkynes. Test for unsaturation.
13. Lindlar's Catalyst Pd/CaCO3 + S/Quinoline. Partial reduction of Alkyne to cis-Alkene.
14. Birch Reduction Na/Li in liq NH3. Reduction of Alkyne to trans-Alkene.
15. Ozonolysis (Reductive) Zn/H2O. Gives Aldehydes/Ketones.
16. Ozonolysis (Oxidative) H2O2. Gives Carboxylic acids / Ketones. (HCOOH becomes CO2+H2O).
17. Hydration of Alkynes HgSO4 / Dil H2SO4. Ethyne gives Acetaldehyde. All others give Ketones.
18. Acidity of Hydrocarbons Ethyne > Ethene > Ethane. (sp > sp2 > sp3 s-character).
19. Benzene Resonance Energy 36 kcal/mol (150 kJ/mol). Highly stable.
20. Bond Order in Benzene 1.5. Bond length 139 pm.
21. Nitration Electrophile NO2+ (Nitronium ion).
22. Chlorination Electrophile Cl+ (Chloronium ion).
23. Sulphonation Electrophile SO3 (Neutral electrophile).
24. Friedel Craft Reagent Anhydrous AlCl3 (Lewis Acid). Generates Carbocation.
25. Limitation of FC Alkylation Polyalkylation occurs. Rearrangement of carbocation occurs. Deactivated rings (Nitrobenzene) don't react.
26. FC Acylation No rearrangement. Mono product (Deactivating group introduced first). Clemmensen reduction gives alkylbenzene.
27. Gammexane BHC (Benzene Hexachloride). C6H6Cl6. Insecticide (Lindane). From Benzene + 3Cl2 (UV). Addn rxn.
28. Oxidation of Alkyl Benzene KMnO4/KOH. Side chain oxidizes to -COOH irrespective of length. (Must have benzylic H).
29. Tert-butyl benzene oxidation Resists KMnO4 oxidation (No benzylic H).
30. Polynuclear Hydrocarbons Naphthalene, Anthracene, Phenanthrene. Carcinogenic.
31. Order of reactivity towards E+ Toluene > Benzene > Chlorobenzene > Nitrobenzene.
32. Ethylene Glycol Antifreeze.
33. Mustang Gas S(CH2CH2Cl)2. War gas.
34. Teflon Polytetrafluoroethylene. (CF2=CF2 monomer).
35. PVC Polyvinyl Chloride. (CH2=CHCl monomer).
36. Peroxide Effect Kharasch effect. Only for HBr. Free radical mechanism.
37. Allylic Halogenation NBS (N-Bromosuccinimide) or Cl2/500C. Substitutes at allylic position.
38. Oxymercuration-Demercuration Markovnikov hydration without rearrangement.
39. Hydroboration-Oxidation Anti-Markovnikov hydration. Syn addition.
40. Syn Addition Both groups add from same side (H2/Ni, Cold KMnO4).
41. Anti Addition Groups add from opposite side (Br2/CCl4).
42. Meso form formation cis-alkene + syn-addition -> Meso. trans-alkene + anti-addition -> Meso.
43. Racemic mixture formation cis-alkene + anti-addition -> Racemic. trans-alkene + syn-addition -> Racemic. (CAR / TAM).
44. Acetylene lamp Uses CaC2 + H2O.
45. Fruit Ripening Ethylene (Ethene).
46. Welding Oxy-acetylene flame (high temp).
47. Aromatization agents V2O5, Cr2O3, Mo2O3. Hexane -> Benzene.
48. Controlled Oxidation CH4 Ni/Heat -> Syn Gas. Cu/Heat -> Methanol. Mo2O3/Heat -> Methanal.
49. Conformational Isomers Energy Eclipsed > Gauche > Staggered (Anti).
50. Chair vs Boat (Cyclohexane) Chair form is more stable (staggered) than Boat form (eclipsed/flagpole).
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