Class 11 Chemistry Notes Chapter 15 (Chapter 15) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 15, 'Hydrocarbons', from your NCERT Exemplar. This is a crucial chapter, forming the bedrock of organic chemistry, and frequently tested in various government examinations. Pay close attention to the reactions, mechanisms, and specific rules discussed.

Chapter 15: Hydrocarbons - Detailed Notes for Exam Preparation

1. Introduction & Classification

• Hydrocarbons: Organic compounds containing only Carbon (C) and Hydrogen (H). They are the simplest organic compounds and serve as the parent structures for many others.
• Sources: Major sources are petroleum (crude oil) and natural gas. Coal is also a source, especially for aromatic hydrocarbons.
• Classification:
  • Acyclic (Open Chain):
    • Saturated: Contain only C-C single bonds (Alkanes). General formula: CnH2n+2.
    • Unsaturated: Contain C=C double bonds (Alkenes) or C≡C triple bonds (Alkynes).
      • Alkenes: General formula: CnH2n.
      • Alkynes: General formula: CnH2n-2.
  • Cyclic (Closed Chain/Ring):
    • Alicyclic: Properties similar to acyclic compounds (e.g., Cyclohexane, Cyclohexene).
    • Aromatic: Contain at least one benzene ring or exhibit aromatic character (special stability, Huckel's rule). (e.g., Benzene, Toluene).

2. Alkanes (Paraffins)

• Nomenclature: Follows IUPAC rules (Root word + ane).
• Isomerism: Exhibit Chain Isomerism. Example: Butane (C4H10) exists as n-butane and isobutane (2-methylpropane).
• Structure: sp3 hybridised carbon atoms, tetrahedral geometry, C-C bond length ≈ 154 pm, C-H bond length ≈ 112 pm, bond angle ≈ 109.5°.
• Preparation:
  • Hydrogenation of Alkenes/Alkynes (Sabatier-Senderens Reaction): Alkene/Alkyne + H2 (in presence of Ni/Pt/Pd catalyst) → Alkane.
  • From Alkyl Halides:
    • Reduction: RX + Zn + H+ → RH + ZnX+ OR RX + H2 (Ni/Pt/Pd) → RH + HX OR RX + LiAlH4 → RH
    • Wurtz Reaction: 2RX + 2Na (in dry ether) → R-R + 2NaX. (Used for preparing symmetrical alkanes with even number of carbons. Methane cannot be prepared. Tertiary alkyl halides tend to undergo elimination).
  • From Carboxylic Acids:
    • Decarboxylation: RCOONa + NaOH (in presence of CaO, Soda-lime) + Heat → RH + Na2CO3. (Produces alkane with one carbon less).
    • Kolbe's Electrolytic Method: 2RCOONa (aq) --Electrolysis--> R-R (at anode) + 2CO2 + H2 (at cathode) + 2NaOH. (Produces symmetrical alkanes with even number of carbons).
• Physical Properties:
  • Non-polar, insoluble in water, soluble in organic solvents.
  • Boiling point increases with molecular mass (due to increased van der Waals forces). Branching decreases boiling point (less surface area).
  • C1 to C4 are gases, C5 to C17 are liquids, C18 onwards are solids at room temp.
• Chemical Properties: Generally unreactive (paraffins = little affinity). Undergo substitution reactions.
  • Halogenation: Free-radical substitution mechanism (Initiation, Propagation, Termination steps). Occurs in presence of UV light or high temperature. Reactivity order: F2 > Cl2 > Br2 > I2. Selectivity order: 3° H > 2° H > 1° H. (Example: CH4 + Cl2 --UV light--> CH3Cl + HCl, further substitution possible).
  • Combustion: Complete combustion yields CO2 and H2O, releasing large amount of heat. CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O. Incomplete combustion yields CO or Carbon black.
  • Controlled Oxidation: Depends on catalyst and conditions.
    • CH4 + O2 --Cu/523K/100atm--> 2CH3OH (Methanol)
    • CH4 + O2 --Mo2O3/Heat--> HCHO (Methanal) + H2O
    • RCH3 --(CH3COO)2Mn/Heat--> RCOOH (Carboxylic acid)
    • Tertiary alkanes --KMnO4--> Tertiary alcohols
  • Isomerisation: n-Alkanes --Anhydrous AlCl3/HCl, Heat--> Branched alkanes.
  • Aromatization (Reforming): n-Alkanes (≥ 6 carbons) --Cr2O3 or V2O5 or Mo2O3 / Al2O3 support, 773K, 10-20 atm--> Benzene or its derivatives + H2. (Example: n-Hexane → Benzene).
  • Pyrolysis (Cracking): Decomposition of higher alkanes into smaller alkanes, alkenes by heat. Free-radical mechanism. Used in petroleum industry.
• Conformations: Different spatial arrangements arising due to rotation around C-C single bond.
  • Ethane: Infinite conformations. Extreme forms:
    • Eclipsed: H atoms on adjacent carbons are directly behind each other. Maximum repulsion, least stable.
    • Staggered: H atoms on adjacent carbons are maximally far apart. Minimum repulsion, most stable.
  • Representations: Sawhorse and Newman projections.
  • Torsional Strain: Repulsion between electron clouds of bonds in eclipsed conformation.

3. Alkenes (Olefins)

• Nomenclature: Root word + ene. Position of double bond indicated by lowest number.
• Isomerism:
  • Structural: Chain and Position isomerism.
  • Geometrical (cis-trans): Arises due to restricted rotation around C=C bond. Requires each double-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. (Generally more stable due to less steric hindrance).
• Structure of Double Bond: One sigma (σ) bond (sp2-sp2 overlap) and one pi (π) bond (sideways overlap of unhybridized p-orbitals). C=C bond length ≈ 134 pm. Trigonal planar geometry around each carbon, bond angle ≈ 120°. Pi bond is weaker than sigma bond.
• Preparation:
  • From Alkynes: Partial reduction using Lindlar's catalyst (Pd/CaCO3 deactivated with quinoline/sulfur) gives cis-alkene. Reduction with Na/liquid NH3 (Birch reduction) gives trans-alkene.
  • From Alkyl Halides (Dehydrohalogenation): RX + Alcoholic KOH + Heat → Alkene + KX + H2O. Follows Saytzeff's Rule (Zaitsev's Rule): More substituted (more stable) alkene is the major product. Elimination reaction (usually E2).
  • From Vicinal Dihalides: R-CHX-CHX-R' + Zn (dust) + Heat → R-CH=CH-R' + ZnX2. (Dehalogenation).
  • From Alcohols (Dehydration): R-CH2-CH2-OH + Conc. H2SO4 or H3PO4 / Heat → R-CH=CH2 + H2O. Follows Saytzeff's rule. Ease of dehydration: 3° > 2° > 1° alcohol. Carbocation intermediate.
• Physical Properties: Similar trends to alkanes, but slightly more polar due to sp2 carbons. Cis isomers generally have higher boiling points (more polar) and lower melting points (poorer packing) than trans isomers.
• Chemical Properties: More reactive than alkanes due to the presence of the pi bond. Undergo Electrophilic Addition Reactions.
  • Addition of H2: (Hydrogenation - see Alkanes preparation).
  • Addition of Halogens (X2 = Cl2, Br2): R-CH=CH2 + Br2 (in CCl4) → R-CHBr-CH2Br (Vicinal dihalide). Decolourisation of reddish-brown bromine water is a test for unsaturation. Cyclic halonium ion intermediate. Anti-addition.
  • Addition of Hydrogen Halides (HX = HCl, HBr, HI): Follows Markovnikov's Rule: The negative part of the addendum (X-) attaches to the carbon atom having fewer hydrogen atoms (or the more substituted carbon). Mechanism involves formation of a more stable carbocation (3° > 2° > 1°).
    • Peroxide Effect (Anti-Markovnikov's Addition): Addition of HBr only, in the presence of organic peroxide (e.g., Benzoyl peroxide), occurs against Markovnikov's rule. Free-radical mechanism. Br• adds first to form a more stable free radical.
  • Addition of H2SO4: Follows Markovnikov's rule. Alkyl hydrogen sulfates formed.
  • Addition of Water (Hydration): Acid-catalysed addition (follows Markovnikov's rule) → Alcohol.
  • Oxidation:
    • Ozonolysis (O3 followed by Zn/H2O): Cleaves the double bond to form aldehydes and/or ketones. Used to locate the position of the double bond.
      RCH=CHR' --(i) O3 (ii) Zn/H2O--> RCHO + R'CHO
      R2C=CHR' --(i) O3 (ii) Zn/H2O--> R2C=O + R'CHO
    • With Cold, Dilute, Alkaline KMnO4 (Baeyer's Reagent): Hydroxylation occurs. Alkene → Vicinal diol (glycol). Purple colour of KMnO4 is discharged. Test for unsaturation. Syn-addition.
    • With Hot/Acidic KMnO4: Strong oxidation. Cleaves the double bond, forms ketones, carboxylic acids, or CO2 depending on the alkene structure.
  • Polymerisation: Alkenes undergo addition polymerisation at high temp/pressure with catalysts. n(CH2=CH2) → [-CH2-CH2-]n (Polythene).

4. Alkynes

• Nomenclature: Root word + yne. Position of triple bond indicated by lowest number.
• Isomerism: Chain and Position isomerism. (No geometrical isomerism around C≡C).
• Structure of Triple Bond: One sigma (σ) bond (sp-sp overlap) and two pi (π) bonds (sideways overlap of unhybridized p-orbitals). C≡C bond length ≈ 120 pm. Linear geometry, bond angle = 180°.
• Acidity of Terminal Alkynes: Hydrogen attached to sp-hybridised carbon is acidic (due to 50% s-character, high electronegativity). Terminal alkynes react with strong bases like NaNH2 or metals like Na, Ag+, Cu+ to form acetylides. This property distinguishes terminal alkynes from non-terminal alkynes and alkenes/alkanes. Example: HC≡CH + NaNH2 → HC≡C-Na+ + NH3.
• Preparation:
  • From Calcium Carbide: CaC2 + 2H2O → Ca(OH)2 + C2H2 (Ethyne). Industrial method.
  • From Vicinal Dihalides: R-CHX-CHX-R' + 2 Alcoholic KOH + Heat → R-C≡C-R' + 2KX + 2H2O. (Requires stronger conditions than alkene prep). NaNH2 can also be used.
  • From Geminal Dihalides: R-CX2-CH2-R' + 2 Alcoholic KOH + Heat → R-C≡C-R' + 2KX + 2H2O.
• Physical Properties: Similar trends to alkanes and alkenes. First three are gases, next eight liquids, higher ones solids. Slightly more polar than alkenes.
• Chemical Properties: Undergo electrophilic addition reactions, similar to alkenes, but can add two molecules of reagent. Also show acidity of terminal H.
  • Addition of H2: C2H2 --H2/Ni--> C2H4 --H2/Ni--> C2H6. Partial reduction possible (see Alkenes preparation).
  • Addition of Halogens (X2): C2H2 --Br2--> CHBr=CHBr --Br2--> CHBr2-CHBr2 (Tetrahaloalkane).
  • Addition of HX: Follows Markovnikov's rule. C2H2 --HCl--> CH2=CHCl (Vinyl chloride) --HCl--> CH3-CHCl2 (Geminal dihalide).
  • Addition of Water (Hydration - Kucherov's Reaction): Alkyne + H2O (in presence of HgSO4/dil. H2SO4, 333K) → Intermediate enol → Ketone (or Ethanal from Ethyne).
    CH≡CH + H2O --Hg2+/H+--> [CH2=CHOH] (Unstable enol) → CH3CHO (Ethanal)
    CH3C≡CH + H2O --Hg2+/H+--> [CH3C(OH)=CH2] (Unstable enol) → CH3COCH3 (Propanone)
  • Polymerisation:
    • Linear: Ethyne --Catalyst--> Polyacetylene (Polyethyne).
    • Cyclic: 3 C2H2 --Red hot iron tube, 873K--> C6H6 (Benzene).

5. Aromatic Hydrocarbons (Arenes)

• Introduction: Benzene (C6H6) is the parent aromatic hydrocarbon. Characterised by pleasant odour (aroma - historical term).
• Nomenclature: Substituted benzenes named as derivatives (e.g., Methylbenzene = Toluene, Hydroxybenzene = Phenol) or using prefixes (o-, m-, p- for disubstituted).
• Structure of Benzene: Cyclic, planar structure. All C-C bond lengths intermediate (139 pm) between single (154 pm) and double (134 pm) bonds. Each C is sp2 hybridised. Delocalised pi electron cloud above and below the ring plane. Resonance hybrid structure. Exceptionally stable.
• Aromaticity (Huckel's Rule): Conditions for aromaticity:
  1. Cyclic
  2. Planar
  3. Complete delocalization of pi electrons in the ring.
  4. Presence of (4n + 2) pi electrons, where n is an integer (0, 1, 2, ...). Benzene has 6 pi electrons (n=1).
• Preparation of Benzene:
  • Cyclic Polymerisation of Ethyne: (See Alkynes).
  • Decarboxylation of Aromatic Acids: Sodium benzoate + Soda-lime + Heat → Benzene + Na2CO3.
  • Reduction of Phenol: Phenol + Zn (dust) + Heat → Benzene + ZnO.
• Physical Properties: Non-polar, immiscible with water, soluble in organic solvents. Characteristic smell. Burn with sooty flame (high carbon content).
• Chemical Properties: Despite unsaturation, Benzene primarily undergoes Electrophilic Substitution Reactions (ESR), preserving the stable aromatic ring.
  • Mechanism of ESR:
    1. Generation of electrophile (E+).
    2. Attack of electrophile on pi electron cloud to form carbocation intermediate (arenium ion/sigma complex), which is resonance stabilised. (Slow, rate-determining step).
    3. Loss of proton (H+) from the carbocation to restore aromaticity. (Fast step).
  • Nitration: Benzene + Conc. HNO3 + Conc. H2SO4 (Nitrating mixture) + Heat (323-333K) → Nitrobenzene + H2O. (Electrophile: NO2+ nitronium ion).
  • Halogenation: Benzene + X2 (Cl2 or Br2) + Anhydrous AlX3 or FeX3 (Lewis acid catalyst) → Halobenzene + HX. (Electrophile: X+ halonium ion generated by catalyst). Direct iodination is reversible, done in presence of oxidising agent (HNO3, HIO3). Fluorination is too violent.
  • Sulphonation: Benzene + Conc. H2SO4 (or Fuming H2SO4/Oleum) + Heat → Benzenesulphonic acid + H2O. (Electrophile: SO3). Reversible reaction.
  • Friedel-Crafts Alkylation: Benzene + R-X (Alkyl halide) + Anhydrous AlCl3 → Alkylbenzene (Toluene) + HX. (Electrophile: R+ carbocation). Limitations: Polyalkylation common, rearrangements of carbocation possible, deactivated rings (e.g., nitrobenzene) and aryl halides do not react.
  • Friedel-Crafts Acylation: Benzene + R-CO-Cl (Acyl chloride) or (RCO)2O (Acid anhydride) + Anhydrous AlCl3 → Acylbenzene (Ketone) + HCl. (Electrophile: R-C≡O+ acylium ion). Advantage: No rearrangement, product is deactivated, preventing polyacylation. Product ketone can be reduced to alkyl group (Clemmensen or Wolff-Kishner reduction).
  • Addition Reactions: Occur only under drastic conditions (high temp/pressure, UV light), destroying aromaticity.
    • Benzene + 3H2 --Ni/Pt/Pd, High T/P--> Cyclohexane.
    • Benzene + 3Cl2 --UV light, 500K--> Benzene hexachloride (BHC, Gammexane - insecticide).
  • Combustion: Burns with sooty flame: 2C6H6 + 15O2 → 12CO2 + 6H2O.
• Directive Influence of Functional Groups in Monosubstituted Benzene:
  • Substituents already present on the ring direct the incoming electrophile to specific positions (ortho, meta, para).
  • Activating Groups (Electron Donating Groups - EDG): Increase electron density in the ring, make ESR faster than benzene. They are ortho- and para- directing. Examples: -OH, -NH2, -NHR, -NR2, -OCH3, -CH3, -C2H5 etc. (Increase stability of arenium ion intermediate when attack is at o/p positions via resonance or inductive effect).
  • Deactivating Groups (Electron Withdrawing Groups - EWG): Decrease electron density in the ring, make ESR slower than benzene. They are meta- directing. Examples: -NO2, -CN, -SO3H, -CHO, -COR, -COOH, -COOR, -NR3+. (Decrease stability of arenium ion intermediate, especially when attack is at o/p positions).
  • Halogens (-F, -Cl, -Br, -I): Exception! They are deactivating (due to strong -I effect) but are ortho- and para- directing (due to +R effect involving lone pairs, which stabilises o/p attack intermediates more than m- attack).
• Carcinogenicity and Toxicity: Polycyclic aromatic hydrocarbons (PAHs) containing more than one fused benzene ring (e.g., benz[a]pyrene, benz[a]anthracene) found in coal tar, tobacco smoke, incomplete combustion products are potent carcinogens (cancer-causing agents). Benzene itself is toxic and carcinogenic.

Multiple Choice Questions (MCQs)

1. Which of the following reactions is used to prepare methane?
   (a) Wurtz Reaction
   (b) Kolbe's Electrolytic Method
   (c) Decarboxylation of Sodium Acetate with Soda-lime
   (d) Hydrogenation of Ethene

2. Addition of HBr to propene in the presence of benzoyl peroxide yields:
   (a) 1-Bromopropane
   (b) 2-Bromopropane
   (c) 1,2-Dibromopropane
   (d) Allyl bromide

3. Which of the following compounds exhibits geometrical isomerism?
   (a) But-1-ene
   (b) 2-Methylbut-2-ene
   (c) But-2-ene
   (d) 2,3-Dimethylbut-2-ene

4. Ozonolysis 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 reagent is used to distinguish between ethyne and ethene?
   (a) Bromine water
   (b) Baeyer's reagent (alkaline KMnO4)
   (c) Ammoniacal silver nitrate solution (Tollen's reagent)
   (d) Chlorine water

6. Identify the compound that follows Huckel's rule for aromaticity:
   (a) Cyclooctatetraene
   (b) Cyclobutadiene
   (c) Benzene
   (d) Cyclopentadiene

7. The major product formed when toluene undergoes nitration with nitrating mixture is:
   (a) m-Nitrotoluene
   (b) o-Nitrotoluene and p-Nitrotoluene
   (c) 2,4-Dinitrotoluene
   (d) Benzene

8. Which conformation of ethane is the most stable?
   (a) Eclipsed
   (b) Staggered
   (c) Skew
   (d) Gauche (for butane, not ethane extremes)

9. The reaction of benzene with acetyl chloride in the presence of anhydrous AlCl3 is an example of:
   (a) Friedel-Crafts Alkylation
   (b) Nucleophilic Substitution
   (c) Friedel-Crafts Acylation
   (d) Electrophilic Addition

10. Saytzeff's rule is associated with which type of reaction?
    (a) Electrophilic addition to alkenes
    (b) Dehydration of alcohols
    (c) Free radical halogenation of alkanes
    (d) Ozonolysis of alkenes

Answer Key for MCQs:

1. (c)
2. (a) - Peroxide effect leads to Anti-Markovnikov addition of HBr.
3. (c) - But-2-ene (CH3-CH=CH-CH3) has two different groups (H, CH3) on each double-bonded carbon.
4. (d) - Ozonolysis of 2,3-Dimethylbut-2-ene [(CH3)2C=C(CH3)2] yields two molecules of propanone.
5. (c) - Only terminal alkynes (like ethyne) react with Tollen's reagent due to acidic hydrogen. Both react with (a) and (b).
6. (c) - Benzene has 6 pi electrons (4n+2, where n=1), is cyclic, planar, and has delocalized pi electrons.
7. (b) - The methyl group (-CH3) in toluene is an activating, ortho/para directing group.
8. (b) - Staggered conformation has minimum torsional strain.
9. (c) - Introduction of an acyl group (CH3CO-) onto the benzene ring.
10. (b) - Saytzeff's rule predicts the major product in elimination reactions like dehydration of alcohols and dehydrohalogenation of alkyl halides, favoring the more substituted alkene.

Study these notes thoroughly, focusing on understanding the reaction mechanisms and the application of rules like Markovnikov's, Saytzeff's, and Huckel's. Practice predicting products of various reactions. Good luck with your preparation!