class 11 notes

Class 11 Chemistry Notes Chapter 13 (Chapter 13) – Examplar Problems (English) Book

Philoid

Philoid

— 9 min read

Examplar Problems (English)
Alright class, let's begin our detailed study of Chapter 13, Hydrocarbons, focusing on the concepts crucial for your government exam preparations, drawing insights from the NCERT Exemplar problems.

Chapter 13: Hydrocarbons - Detailed Notes

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are the simplest organic compounds and form the backbone of many others.

I. Classification

Hydrocarbons are broadly classified into:

  1. Open Chain (Acyclic) Hydrocarbons: Also called aliphatic hydrocarbons.
    • Saturated: Contain only carbon-carbon single bonds (Alkanes). General formula: CnH2n+2.
    • Unsaturated: Contain carbon-carbon multiple bonds (double or triple bonds).
      • Alkenes: Contain at least one C=C double bond. General formula: CnH2n.
      • Alkynes: Contain at least one C≡C triple bond. General formula: CnH2n-2.
  2. Closed Chain (Cyclic) Hydrocarbons:
    • Alicyclic: Properties similar to aliphatic hydrocarbons (e.g., cyclopropane, cyclohexane, cyclohexene).
    • Aromatic: Contain at least one benzene ring or exhibit aromatic character (e.g., benzene, toluene, naphthalene).

II. Alkanes (Paraffins)

  • Structure: C atoms are sp3 hybridized, forming sigma (σ) bonds. Tetrahedral geometry around each carbon (bond angle ~109.5°). C-C bond length ~154 pm, C-H bond length ~112 pm.
  • Nomenclature: Follows IUPAC rules (Root word + suffix '-ane').
  • Isomerism:
    • Chain Isomerism: Same molecular formula but different carbon skeleton arrangements (e.g., n-butane and isobutane).
    • Conformational Isomerism: Different spatial arrangements arising due to rotation around C-C single bonds. Studied using Newman and Sawhorse projections.
      • Ethane: Staggered (most stable, minimum repulsion/torsional strain) and Eclipsed (least stable, maximum repulsion/torsional strain). Infinite intermediate forms called skew or gauche forms. Energy difference ~12.5 kJ/mol.
      • Butane: Anti (most stable), Gauche (less stable than anti), Eclipsed, Fully Eclipsed (least stable).
  • Preparation:
    • From Unsaturated Hydrocarbons (Hydrogenation): Alkenes/Alkynes + H2 (in presence of Ni/Pt/Pd) → Alkanes (Sabatier-Senderens reaction).
    • From Alkyl Halides:
      • Reduction: R-X + Zn + H+ → R-H + ZnX+ OR R-X + H2 (Ni/Pt/Pd) → R-H + HX OR R-X + LiAlH4/NaBH4 → R-H.
      • Wurtz Reaction: 2R-X + 2Na (dry ether) → R-R + 2NaX. Used for preparing symmetrical alkanes with an even number of carbons. Methane cannot be prepared. Free radical mechanism. Side reactions possible.
    • From Carboxylic Acids:
      • Decarboxylation: R-COONa + NaOH (CaO, Δ) → R-H + Na2CO3 (Soda-lime decarboxylation). Alkane formed has one carbon less than the carboxylic acid salt.
      • Kolbe's Electrolytic Method: 2R-COONa (aq) --Electrolysis--> R-R (at anode) + 2CO2 (at anode) + H2 (at cathode) + 2NaOH. Forms symmetrical alkanes with even number of carbons. Radical mechanism.
  • Physical Properties:
    • Non-polar, insoluble in water, soluble in non-polar solvents.
    • Boiling Point (BP): Increases with molecular mass. Decreases with branching (due to decreased surface area and weaker van der Waals forces).
    • Melting Point (MP): General increase with molecular mass. Alkanes with even number of carbons have higher MP than preceding/succeeding odd-carbon alkanes (better packing in crystal lattice).
    • Density: Less dense than water.
  • Chemical Properties: Relatively inert due to strong C-C and C-H sigma bonds and non-polar nature.
    • Substitution Reactions (Free Radical Mechanism):
      • Halogenation: R-H + X2 (UV light or heat) → R-X + HX. Reactivity order: F2 > Cl2 > Br2 > I2. Reactivity of H atoms: 3° > 2° > 1°.
      • Mechanism (e.g., Chlorination of Methane):
        • Initiation: Cl2 --UV light--> 2Cl• (Chlorine free radicals)
        • Propagation: Cl• + CH4 → •CH3 + HCl ; •CH3 + Cl2 → CH3Cl + Cl•
        • Termination: Cl• + Cl• → Cl2 ; •CH3 + •CH3 → C2H6 ; •CH3 + Cl• → CH3Cl
      • Further substitution can occur, leading to mixtures (CH3Cl, CH2Cl2, CHCl3, CCl4). Iodination is reversible; carried out in presence of oxidizing agents (HNO3, HIO3). Fluorination is explosive.
    • Combustion: Alkane + O2 (excess) → CO2 + H2O + Heat (Exothermic). Used as fuels. Incomplete combustion yields CO (poisonous) or Carbon black. CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O.
    • Controlled Oxidation: Depends on catalyst and conditions.
      • CH4 + O2 (Cu/523K/100atm) → 2CH3OH (Methanol)
      • CH4 + O2 (Mo2O3/Δ) → HCHO (Methanal) + H2O
      • (CH3)3CH + KMnO4 (alkaline) → (CH3)3COH (tert-Butyl alcohol)
    • Isomerisation: n-Alkanes (Anhydrous AlCl3/HCl, Δ) → Branched alkanes.
    • Aromatization (Reforming): n-Alkanes (≥ 6 carbons) (Cr2O3 or V2O5 or Mo2O3 supported on Al2O3, 773K, 10-20 atm) → Benzene or its derivatives + H2. (e.g., n-hexane → benzene).
    • Pyrolysis (Cracking): Decomposition of higher alkanes into smaller alkanes, alkenes by heat. Free radical mechanism. Used in petroleum industry.

III. Alkenes (Olefins)

  • Structure: At least one C=C double bond. Carbon atoms in double bond are sp2 hybridized. Trigonal planar geometry around C=C (bond angle ~120°). C=C bond consists of one strong σ bond and one weak π bond. C=C bond length ~134 pm.
  • Nomenclature: Root word + suffix '-ene'. Position of double bond indicated by lowest number.
  • Isomerism:
    • Structural Isomerism: Chain and Position isomerism.
    • Geometrical (cis-trans) Isomerism: Due to restricted rotation around C=C bond. Requires each doubly bonded carbon to be attached to two different groups.
      • cis: Similar groups on the same side of the double bond.
      • trans: Similar groups on opposite sides of the double bond.
      • Trans isomers are generally more stable than cis isomers due to less steric hindrance. They usually have higher melting points (better packing) and lower boiling points (less polar).
  • Preparation:
    • From Alkynes (Partial Hydrogenation):
      • Alkyne + H2 (Pd/C, quinoline or sulphur - Lindlar's catalyst) → cis-Alkene.
      • Alkyne + Na/Liq. NH3 (Birch reduction) → trans-Alkene.
    • From Alkyl Halides (Dehydrohalogenation): R-CH2-CHX-R' + Alc. KOH (Δ) → Alkenes + KX + H2O. Follows Saytzeff's Rule: More substituted alkene is the major product (elimination favours removal of H from β-carbon having fewer H atoms). E2 mechanism common.
    • From Vicinal Dihalides: CH2X-CH2X + Zn (Methanol, Δ) → CH2=CH2 + ZnX2.
    • From Alcohols (Acid-catalyzed Dehydration): Alcohol + Conc. H2SO4 or H3PO4 (Δ) → Alkene + H2O. Follows Saytzeff's rule. Ease of dehydration: 3° > 2° > 1° alcohol. Carbocation intermediate mechanism.
  • Physical Properties: Similar to alkanes, but slightly more polar. First few members are gases, then liquids, then solids. Insoluble in water. BP increases with mass, decreases with branching.
  • Chemical Properties: More reactive than alkanes due to the presence of the accessible π-electron cloud. Undergo Electrophilic Addition Reactions.
    • Addition of Dihydrogen (H2): Alkene + H2 (Ni/Pt/Pd) → Alkane.
    • Addition of Halogens (X2): Alkene + X2 (in CCl4) → Vicinal dihalide. Decolourisation of bromine water (reddish-brown) is a test for unsaturation. Cyclic halonium ion intermediate mechanism. Anti-addition observed.
    • Addition of Hydrogen Halides (HX): Alkene + HX → Alkyl halide.
      • Markovnikov's Rule: (For unsymmetrical alkenes and unsymmetrical reagents) The negative part of the addendum (adding molecule) gets attached to the carbon atom possessing fewer hydrogen atoms. Mechanism involves formation of the more stable carbocation (3° > 2° > 1°).
      • Anti-Markovnikov's Rule (Peroxide Effect/Kharasch Effect): In the presence of peroxide (e.g., Benzoyl peroxide), addition of HBr (only HBr) to unsymmetrical alkenes occurs contrary to Markovnikov's rule. Free radical mechanism.
    • Addition of Sulphuric Acid: Follows Markovnikov's rule. Alkyl hydrogen sulphates formed.
    • Addition of Water (Hydration): Alkene + H2O (H+, Δ) → Alcohol. Follows Markovnikov's rule. Acid-catalyzed mechanism via carbocation.
    • Oxidation:
      • With Cold, Dilute, Alkaline KMnO4 (Baeyer's Reagent): Alkene + H2O + [O] (from KMnO4) → Vicinal diol (glycol). Purple colour of KMnO4 is discharged. Test for unsaturation. Syn-addition.
      • With Hot/Acidic KMnO4: Cleavage of C=C bond occurs, forming ketones, acids, or CO2 depending on the alkene structure.
      • Ozonolysis (O3 followed by Zn/H2O): Cleavage of C=C bond to form aldehydes and/or ketones. Used to determine the position of double bonds.
        Alkene + O3 → Ozonide --(Zn/H2O)--> Carbonyl compounds. Zn prevents further oxidation of aldehydes to acids by H2O2 formed.
    • Polymerisation: Alkenes undergo addition polymerisation at high temperature/pressure and/or catalysts. n(CH2=CH2) → -(CH2-CH2)n- (Polythene).

IV. Alkynes (Acetylenes)

  • Structure: At least one C≡C triple bond. Carbon atoms in triple bond are sp hybridized. Linear geometry (bond angle 180°). C≡C bond consists of one strong σ bond and two weak π bonds. C≡C bond length ~120 pm.
  • Nomenclature: Root word + suffix '-yne'. Position of triple bond indicated by lowest number.
  • Isomerism: Chain and Position isomerism. No geometrical isomerism due to linear structure.
  • Preparation:
    • From Calcium Carbide: CaC2 + 2H2O → Ca(OH)2 + C2H2 (Ethyne).
    • From Vicinal Dihalides: CHX2-CHX2 + 2 Alc. KOH (Δ) → Alkyne + 2KX + 2H2O. Stronger base like NaNH2 may be needed for terminal alkyne formation.
    • From Geminal Dihalides: R-CX2-CH3 + 2 Alc. KOH (Δ) → R-C≡CH + 2KX + 2H2O.
  • Physical Properties: Similar to alkanes and alkenes. First few members are gases. Insoluble in water. BP/MP increase with mass.
  • Chemical Properties: More reactive than alkenes towards electrophilic addition (though initial step may be slower due to higher electron density being held more tightly by sp carbons). Also show acidic character for terminal alkynes.
    • Acidic Character of Terminal Alkynes: H atom attached to sp hybridized carbon is acidic (due to 50% s-character, high electronegativity of sp carbon). React with strong bases like NaNH2 or metals like Na, Ag+, Cu+.
      • HC≡CH + NaNH2 → HC≡C-Na+ + NH3 (Sodium acetylide)
      • HC≡CH + 2AgNO3 + 2NH4OH → Ag-C≡C-Ag (Silver acetylide, white ppt) + 2NH4NO3 + 2H2O (Tollen's test)
      • HC≡CH + 2CuCl + 2NH4OH → Cu-C≡C-Cu (Copper acetylide, red ppt) + 2NH4Cl + 2H2O
      • This property distinguishes terminal alkynes from non-terminal alkynes and alkenes. Acidity order: H2O > ROH > HC≡CH > NH3 > RH.
    • Electrophilic Addition Reactions:
      • Addition of H2: Alkyne + H2 (Ni/Pt/Pd) → Alkane. Alkyne + H2 (Lindlar's catalyst) → cis-Alkene. Alkyne + Na/Liq. NH3 → trans-Alkene.
      • Addition of X2: Alkyne + X2 → Dihaloalkene + X2 → Tetrahaloalkane.
      • Addition of HX: Alkyne + HX → Haloalkene + HX → Geminal dihalide. Follows Markovnikov's rule.
      • Addition of Water (Hydration - Kucherov's reaction): Alkyne + H2O (HgSO4/dil. H2SO4, 333K) → Unstable Enol → Carbonyl compound (Ketone, except ethyne which gives ethanal).
    • Polymerisation:
      • Linear Polymerisation: n(HC≡CH) → -(CH=CH-CH=CH)n- (Polyacetylene)
      • Cyclic Polymerisation: 3(HC≡CH) (Red hot Fe tube, 873K) → C6H6 (Benzene).

V. Aromatic Hydrocarbons (Arenes)

  • Structure of Benzene (C6H6): Cyclic, planar structure. All C-C bond lengths equal (139 pm, intermediate between single and double bond). Each C is sp2 hybridized. Delocalized π-electron cloud (6 π electrons) above and below the planar ring. Resonance hybrid structure.
  • Aromaticity (Huckel's Rule): For a compound to be aromatic, it must be:
    1. Cyclic
    2. Planar
    3. Have complete delocalization of π electrons in the ring.
    4. Possess (4n + 2) π electrons, where n is an integer (0, 1, 2, ...). (e.g., Benzene n=1, Naphthalene n=2, Anthracene n=3, Cyclopentadienyl anion n=1).
  • Nomenclature: Substituted benzenes named as derivatives (e.g., Methylbenzene = Toluene, Hydroxybenzene = Phenol, Aminobenzene = Aniline). For disubstituted benzenes, use prefixes ortho- (1,2), meta- (1,3), para- (1,4).
  • Isomerism: Position isomerism in substituted benzenes (o, m, p).
  • Preparation of Benzene:
    • Cyclic Polymerisation of Ethyne: (See Alkynes)
    • Decarboxylation of Aromatic Acids: Sodium benzoate + Soda-lime (Δ) → Benzene + Na2CO3.
    • Reduction of Phenol: Phenol + Zn dust (Δ) → Benzene + ZnO.
  • Physical Properties: Non-polar, immiscible with water. Colourless liquids or solids. Characteristic odour. Burn with sooty flame (high carbon content).
  • Chemical Properties: Undergo Electrophilic Substitution Reactions (SER), preserving the stable aromatic ring. Addition reactions occur under drastic conditions.
    • Electrophilic Substitution Mechanism:
      1. Generation of electrophile (E+).
      2. Attack of electrophile on π-electron cloud forming arenium ion intermediate (sigma complex), which is resonance stabilized. This step is slow (rate-determining).
      3. Loss of proton (H+) from the arenium ion to restore aromaticity. This step is fast.
    • Specific SERs:
      • Nitration: Benzene + Conc. HNO3 + Conc. H2SO4 (Nitrating mixture, <333K) → Nitrobenzene + H2O. Electrophile: NO2+ (nitronium ion).
      • Halogenation: Benzene + X2 (Anhydrous AlX3 or FeX3 - Lewis acid catalyst) → Halobenzene + HX. Electrophile: X+ (generated by Lewis acid). Direct iodination requires an oxidizing agent. Fluorination is too vigorous.
      • Sulphonation: Benzene + Conc. H2SO4 (or Oleum, H2S2O7) (Δ) → Benzenesulphonic acid + H2O. Electrophile: SO3. Reaction is reversible.
      • Friedel-Crafts Alkylation: Benzene + R-X (Anhydrous AlCl3) → Alkylbenzene + HX. Electrophile: R+ (carbocation). Limitations: Polyalkylation common, rearrangements of carbocation possible, aryl halides cannot be used, strongly deactivated rings do not react.
      • Friedel-Crafts Acylation: Benzene + R-COCl or (RCO)2O (Anhydrous AlCl3) → Acylbenzene (Ketone) + HCl. Electrophile: RCO+ (acylium ion). Advantage: No polyacylation (product is deactivated), no rearrangement. Ketone can be reduced to alkyl group (Clemmensen or Wolff-Kishner reduction).
    • Directive Influence of Substituents: Substituents already present on the benzene ring determine the position (orientation) and rate (reactivity) of further substitution.
      • Activating Groups (Electron Donating Groups - EDG): Increase electron density in the ring, make it more reactive than benzene. They are ortho, para-directing. Examples: -OH, -NH2, -NHR, -NR2, -OCH3, -CH3, -C2H5 etc. (Reactivity order: -NH2 > -OH > -OCH3 > -CH3). They stabilize the arenium ion intermediate when attack occurs at o- or p-positions.
      • Deactivating Groups (Electron Withdrawing Groups - EWG): Decrease electron density in the ring, make it less reactive than benzene. They are meta-directing. Examples: -NO2, -CN, -SO3H, -CHO, -COR, -COOH, -COOR, -NR3+. Exception: Halogens (-F, -Cl, -Br, -I) are deactivating but are ortho, para-directing due to dominance of +R effect over -I effect in stabilizing the intermediate during o/p attack (though overall ring is deactivated by strong -I effect).
    • Addition Reactions (Under Drastic Conditions):
      • Hydrogenation: Benzene + 3H2 (Ni/Pt, High T/P) → Cyclohexane.
      • Halogenation: Benzene + 3Cl2 (UV light, 500K) → Benzene hexachloride (BHC or Gammexane - an insecticide). Free radical mechanism.
    • Combustion: Benzene + O2 → CO2 + H2O. Burns with a sooty flame.

VI. Carcinogenicity and Toxicity

  • Polycyclic aromatic hydrocarbons (PAHs) containing more than one fused benzene ring (e.g., benz[a]pyrene, benz[a]anthracene) found in tobacco smoke, coal tar, incomplete combustion products are potent carcinogens (cancer-causing agents).
  • Benzene itself is toxic and carcinogenic.

Multiple Choice Questions (MCQs)

  1. Which of the following reagents will yield cis-alkene upon reaction with an alkyne?
    A) H2, Ni/Pt/Pd
    B) Na / Liquid NH3
    C) H2, Pd/C, quinoline
    D) Zn / H+

  2. The reaction of HBr with propene in the presence of benzoyl peroxide primarily yields:
    A) 1-Bromopropane
    B) 2-Bromopropane
    C) 1,2-Dibromopropane
    D) Allyl bromide

  3. Identify the compound that exhibits geometrical isomerism:
    A) But-1-ene
    B) 2-Methylbut-2-ene
    C) But-2-ene
    D) 2,3-Dimethylbut-2-ene

  4. Ozonolysis (O3 followed by Zn/H2O) of an alkene produces only propanone (acetone). The alkene is:
    A) Propene
    B) But-2-ene
    C) 2-Methylpropene
    D) 2,3-Dimethylbut-2-ene

  5. Which of the following hydrocarbons is most acidic?
    A) Ethane
    B) Ethene
    C) Ethyne
    D) Benzene

  6. The electrophile generated during the nitration of benzene using nitrating mixture (Conc. HNO3 + Conc. H2SO4) is:
    A) NO3-
    B) NO2+
    C) NO+
    D) HSO4-

  7. Which group among the following is meta-directing for electrophilic aromatic substitution?
    A) -OH
    B) -CH3
    C) -Cl
    D) -CHO

  8. Kolbe's electrolytic method using sodium acetate solution produces which hydrocarbon at the anode?
    A) Methane
    B) Ethane
    C) Ethene
    D) Ethyne

  9. The most stable conformation of n-butane is:
    A) Eclipsed
    B) Gauche
    C) Fully Eclipsed
    D) Anti

  10. Aromatization of n-heptane using Cr2O3/Al2O3 at 773 K and 10-20 atm pressure gives:
    A) Benzene
    B) Toluene
    C) Hept-1-ene
    D) Cycloheptane

Make sure you understand the mechanisms, rules like Markovnikov's and Saytzeff's, the concept of aromaticity, and the directive influence of substituents, as these are frequently tested areas. Good luck with your preparation!

Read more