NCERT Notes Class 11 Chemistry (Part-II) Chapter 9: Hydrocarbons (Free PDF)

Hydrocarbons are considered to be important compounds because they are used as fuels, lubricants, solvents, and as starting materials for the synthesis of many other useful compounds like plastics, fibres, drugs, explosives, dyes, etc. Below, we have provided important notes of NCERT Class 11 Chemistry (Part II), Chapter 9: Hydrocarbons, to help you understand the key concepts easily and prepare effectively for exams.

Explore Notes of Class 11 Chemistry

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Download PDF of NCERT Notes Class 11 Chemistry (Part-II) Chapter-9: Hydrocarbons

Introduction

Hydrocarbons are compounds containing only carbon and hydrogen atoms. It plays a major role in daily life as fuel, raw materials, and in manufacturing.

Common Fuels and Sources

Common fuels and sources are:

• LPG (Liquefied Petroleum Gas):
  • Domestic fuel has the least pollution.
  • Obtained from petroleum.
• CNG (Compressed Natural Gas):
  • Formed by compressing natural gas.
  • Used in automobiles, it causes less pollution.
• LNG (Liquified Natural Gas):
  • Produced by the liquefaction of natural gas.
• Petrol, Diesel, Kerosene Oil:
  • Obtained by fractional distillation of petroleum from the earth’s crust.
  • Petrol and diesel are used in automobiles; petrol causes less pollution.
  • Kerosene oil is used domestically, but it causes some pollution.
• Coal Gas: Obtained by destructive distillation of coal.
• Natural Gas: Found in upper strata during oil drilling.

Uses of Hydrocarbons

The uses of hydrocarbons are

• As fuels: LPG, CNG, LNG, petrol, diesel, kerosene, and coal gas.
• Manufacturing of polymers: Polythene, polypropene, polystyrene, etc.
• Solvents: Higher hydrocarbons are used as solvents for paints.
• Chemical industry: Starting materials for dyes, drugs, and other organic compounds.

Classification of Hydrocarbons

Hydrocarbons are classified into three main types based on the type of carbon–carbon bonds:

1. Saturated Hydrocarbons
   
   • Contain only single bonds (C–C and C–H).
   • Open chain: Alkanes (straight or branched).
   • Closed chain: Cycloalkanes.
2. Unsaturated Hydrocarbons
   • Contain double bonds (alkenes), triple bonds (alkynes), or both.
3. Aromatic Hydrocarbons
   • Special type of cyclic compounds, often with delocalised π-electrons (e.g., benzene).
   • Can be homocyclic or heterocyclic.

Also Read: NCERT Notes Class 11 Psychology Chapter 1: What is Psychology? (Free PDF)

Alkanes

In this section, we have discussed the alkanes, along with their nomenclature, isomerism, preparation, and properties.

• Saturated, open-chain hydrocarbons with C–C single bonds.
• General formula: CₙH₂ₙ₊₂
• Example: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈).
• Bonding: sp³ hybridised carbon atoms; tetrahedral geometry; bond angle = 109.5°; C–C bond length = 154 pm; C–H bond length = 112 pm.

Nomenclature & Isomerism

The nomenclature and isomerism of alkanes are discussed below.

• The first three alkanes (CH₄, C₂H₆, C₃H₈) have only one structure; higher alkanes exhibit structural isomerism.
• Chain isomerism: Same molecular formula, different carbon chain arrangement.
  • Example: C₄H₁₀ → n-butane & isobutane.
• Carbon atom classification:
  • Primary (1°) – attached to 1 carbon.
  • Secondary (2°) – attached to 2 carbons.
  • Tertiary (3°) – attached to 3 carbons.
  • Quaternary (4°) – attached to 4 carbons.

Preparation of Alkanes

In this section, we have provided the key details relevant to the preparation of alkanes.

1. From Unsaturated Hydrocarbons (Hydrogenation)
   • Alkene/alkyne + H₂ → alkane
   • Catalyst: Pt, Pd (room temp) or Ni (high temp/pressure).
2. From Alkyl Halides
   • Reduction with Zn + dil. HCl → alkane.
   • Wurtz reaction: Alkyl halide + Na (dry ether) → higher alkane.
3. From Carboxylic Acids
   • Decarboxylation: Sodium salt of acid + soda lime → alkane (one C less).
   • Kolbe’s electrolysis: Aqueous sodium salt of acid → alkane (even number of C atoms).

Properties of Alkanes

The properties of alkanes are discussed below.

Physical:

• Non-polar, insoluble in water, soluble in non-polar solvents.
• C₁–C₄ → gases, C₅–C₁₇ → liquids, ≥ C₁₈ → solids.
• Boiling point ↑ with molecular mass; branching ↓ boiling point.

Chemical:

1. Substitution Reactions
   • Halogenation: CH₄ + Cl₂ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄
   • Mechanism: Free radical chain (initiation, propagation, termination).
2. Combustion
   • Complete combustion → CO₂ + H₂O (high heat output, used as fuels).
   • Incomplete combustion → carbon black.
3. Controlled Oxidation
   • CH₄ + O₂ (Cu catalyst) → CH₃OH
   • CH₄ + O₂ (Mo₂O₃ catalyst) → HCHO
   • Tertiary alkanes oxidised by KMnO₄ → alcohol.
4. Isomerisation
   • n-Alkane → branched alkane (AlCl₃ + HCl).
5. Aromatization (Reforming)
   • n-Alkane (≥ C₆) → aromatic hydrocarbon (V₂O₅, MoO₃, Cr₂O₃ on Al₂O₃).
6. Reaction with Steam
   • CH₄ + H₂O (Ni, 1273 K) → CO + H₂.
7. Pyrolysis (Cracking)
   • High temp decomposition → lower alkanes + alkenes.

Conformations of Alkanes

Conformations (also called conformers or rotamers) are different spatial arrangements of atoms in a molecule that can be interconverted by rotation around a single C–C sigma bond without breaking any bonds. Due to the symmetrical electron distribution of σ bonds along the C–C axis, alkanes allow such rotation.

• Rotation & Energy Barrier:
  • Rotation is not completely free – hindered by torsional strain (1–20 kJ mol⁻¹) due to repulsion between electron clouds of adjacent bonds.
  • Torsional strain = repulsive interaction between electron clouds in bonds on adjacent carbons.
• Conformations of Ethane (C₂H₆):
  • Contains one C–C single bond, with each carbon bonded to three H atoms.
  • Infinite conformations are possible by rotation, but two extreme cases:
    • Eclipsed Conformation:
      • H atoms on one carbon are as close as possible to H atoms on the adjacent carbon.
      • Maximum torsional strain → Higher energy → Less stable.
    • Staggered Conformation:
      • H atoms are as far apart as possible.
      • Minimum torsional strain → Lower energy → Most stable.
    • Skew Conformation:
      • Any intermediate arrangement between staggered and eclipsed.
• Representations of Conformations:
  • Sawhorse Projection:
    • C–C bond drawn as a slanted line; front carbon shown at lower end, rear carbon at upper end; bonds drawn at 120°.
  • Newman Projection:
    • View along C–C axis; front carbon as a point, rear carbon as a circle; H atoms shown at 120°.
• Relative Stability:
  • Staggered: Minimum electron repulsion, lowest energy, most stable.
  • Eclipsed: Maximum electron repulsion, highest energy, least stable.
  • Energy difference ≈ 12.5 kJ mol⁻¹ (small enough for free rotation at room temperature).

Also Read: NCERT Solutions Class 11 Psychology Chapter 1: What is Psychology? (Free PDF)

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond. General formula: CₙH₂ₙ (two hydrogen atoms less than alkanes). Also called olefins (“oil-forming”) – e.g., ethylene (ethene, C₂H₄) forms oily liquids with Cl₂.

Structure of a Double Bond

The structure of a double bond is described below.

• C=C consists of:
  • One σ-bond: head-on overlap of sp² hybrid orbitals (bond enthalpy ≈ 397 kJ mol⁻¹).
  • One π-bond: lateral overlap of unhybridised p-orbitals (bond enthalpy ≈ 284 kJ mol⁻¹).
• Bond length: C=C (134 pm) < C–C (154 pm).
• π-bond is weaker → electrons are loosely held → site for electrophilic attack.
• Total bond strength: C=C (681 kJ mol⁻¹) > C–C in ethane (348 kJ mol⁻¹).

Nomenclature (IUPAC)

The IUPAC Nomenclature of alkenes is provided below.

• The longest chain containing C=C is chosen.
• Numbering starts from the end nearer to the double bond.
• Replace -ane with -ene.
• Examples:
  • CH₃–CH=CH₂ → Propene
  • CH₃–CH₂–CH=CH₂ → But-1-ene
  • CH₃–CH=CH–CH₃ → But-2-ene
  • CH₂=CH–CH=CH₂ → Buta-1,3-diene
  • CH₂=C(CH₃)₂ → 2-Methylprop-1-ene

Isomerism

In this section, we have discussed the isomerism of alkenes.

1. Structural Isomerism

• Chain & position isomerism is possible from C₄H₈ onwards.
• Examples (C₄H₈): But-1-ene, But-2-ene, 2-Methylprop-1-ene.

2. Geometrical (cis–trans) Isomerism

• Due to restricted rotation around C=C.Ciss: identical groups on the same side of C=C.
• Trans: identical groups on opposite sides.
• Example: cis- and trans-But-2-ene.
• cis form → more polar (e.g., μ = 0.33 D), trans form → nearly non-polar.

Preparation

In this section, we have discussed the preparation of alkenes.

1. From Alkynes
   • Partial hydrogenation with Lindlar’s catalyst → cis-alkene.
   • Reduction with Na / liquid NH₃ → trans-alkene.
2. From Alkyl Halides (Dehydrohalogenation)
   • Heat with alcoholic KOH → β-elimination → alkene.
   • Rate: RI > RBr > RCl; tertiary > secondary > primary.
3. From Vicinal Dihalides (Dehalogenation)
   • Treat with Zn → ZnX₂ eliminated → alkene.
4. From Alcohols (Acidic Dehydration)
   • Heat with conc. H₂SO₄ → β-elimination → alkene.

Properties

In this section, we have discussed the properties of alkenes,

Physical

• First 3 members: gases; next 14: liquids; last: solids.
• Insoluble in water; soluble in non-polar solvents.
• Boiling point ↑ with chain length: straight-chain > branched.

Chemical – Mainly Electrophilic Addition Reactions

1. Addition of H₂ – Ni, Pd, Pt catalysts → alkane.
2. Addition of Halogens – Br₂ / Cl₂ → vicinal dihalides; bromine water test for unsaturation.
3. Addition of HX
   • Symmetrical alkenes → single product.
   • Unsymmetrical alkenes → Markovnikov’s Rule applies (negative part attaches to C with fewer H atoms).
   • Anti-Markovnikov (Peroxide effect) – only with HBr; free radical mechanism → opposite product.
4. Addition of H₂SO₄ – Cold conc. H₂SO₄ → alkyl hydrogen sulfate → hydrolysis → alcohol (Markovnikov).
5. Hydration – H₂O + a few drops of conc. H₂SO₄ → alcohol (Markovnikov).
6. Oxidation
   • Cold, dilute KMnO₄ (Baeyer’s test) → vicinal glycols.
   • Acidic KMnO₄ / K₂Cr₂O₇ → ketones/acids.
7. Ozonolysis – O₃ addition + Zn/H₂O cleavage → carbonyl compounds; locates C=C position.
8. PolYmerisation – High T, high P, catalyst → polythene, polypropene.

Also Read: NCERT Notes Class 11 Psychology Chapter 3: Human Development (Free PDF)

Alkynes

Alkynes are unsaturated hydrocarbons containing at least one C≡C triple bond. General formula: CₙH₂ₙ₋₂ (less hydrogen than alkenes or alkanes). First stable member: Ethyne (acetylene) – used in oxyacetylene welding. Serve as starting materials for various organic compounds.

Nomenclature & Isomerism

The nomenclature and isomerism of alkynes are discussed below.

• Common system: derivatives of acetylene.
• IUPAC system: derivatives of alkanes with -yne suffix; position of triple bond indicated by the first triply bonded carbon.
• Examples:
  • HC≡CH → Ethyne
  • CH₃–C≡CH → Propyne
  • CH≡C–CH₂–CH₃ → But-1-yne
  • CH₃–C≡C–CH₃ → But-2-yne
• Isomerism:
  • Position isomerism: same chain, different location of triple bond (But-1-yne & But-2-yne).
  • Chain isomerism: difference in carbon chain arrangement.

Structure of Triple Bond

The structure of a triple bond is discussed below.

• Example: Ethyne
  • Each carbon: sp hybridised (linear geometry, 180° bond angle).
  • One σ bond (sp–sp overlap) between C atoms, two σ bonds (sp–1s overlap) with H atoms.
  • Two π bonds from lateral overlap of unhybridised p orbitals.
• Bond details:
  • C≡C bond enthalpy: 823 kJ mol⁻¹ (strongest).
  • Bond lengths: C≡C (120 pm) < C=C (133 pm) < C–C (154 pm).
• Molecule is linear; electron cloud cylindrically symmetrical.

Preparation

In this section, we have discussed the ways to prepare alkynes.

1. From Calcium Carbide
   • CaCO₃ → CaO → CaC₂ (with coke) → reacts with water → C₂H₂.
2. From Vicinal Dihalides
   • Alcoholic KOH → β-elimination → alkenyl halide → treated with NaNH₂ → alkyne.

Properties

The physical and chemical properties of alkynes are discussed below.

Physical Properties

• First 3 members: gases; next 8: liquids; last: solids.
  
• Colourless (ethyne has a characteristic odour; others are odourless).
  
• Weakly polar; insoluble in water; soluble in organic solvents.
  
• Boiling/melting points ↑ with molar mass.

Chemical Properties

A. Acidic Character

• Terminal alkynes (HC≡CH, CH₃–C≡CH) have acidic H atoms attached to sp-hybridised carbons (50% s-character → higher electronegativity).
• React with Na or NaNH₂ → sodium alkynides + H₂ gas.
• Acidity trend:
  HC≡CH > H₂C=CH₂ > CH₃–CH₃
  HC≡CH > CH₃–C≡CH >> CH₃–C≡C–CH₃.
• Test: distinguishes alkynes from alkenes & alkanes.

Addition Reactions (Electrophilic Addition)

1. Addition of H₂
   • Catalyst: Ni/Pd/Pt.
   • Stepwise: Alkyne → Alkene → Alkane.
   • Lindlar’s catalyst → cis-alkene; Na/NH₃ → trans-alkene.
2. Addition of Halogens (X₂)
   • Decolourises Br₂ in CCl₄ (test for unsaturation).
   • Yields di- or tetra-halo products.
3. Addition of Hydrogen Halides (HX)
   • 2 molecules add → geminal dihalides.
   • Unsymmetrical alkynes follow Markovnikov’s rule.
4. Addition of Water (Hydration)
   • Catalyst: HgSO₄ + dilute H₂SO₄, 333 K.
   • Yields carbonyl compounds via enol-keto tautomerism.

Polymerisation

1. Linear Polymerisation
   • Forms polyacetylene (polyethyne) – a conductive polymer.
2. Cyclic Polymerisation
   • 3 ethyne molecules → benzene (at 873 K, red-hot Fe tube) – key aliphatic → aromatic conversion.

Aromatic Hydrocarbons

Aromatic hydrocarbons, also known as arenes, are hydrocarbons in which the carbon atoms are arranged in one or more planar cyclic structures with a high degree of unsaturation. Most of these compounds have a pleasant smell (Greek word aroma = pleasant smelling), hence the term “aromatic compounds. The most common example is the benzene ring.

• Classification:
  • Benzenoids: Aromatic compounds containing at least one benzene ring.
    Examples: Benzene, Toluene, Naphthalene, Biphenyl.
  • Non-benzenoids: Aromatic compounds without a benzene ring but containing another highly unsaturated ring with aromatic character.

Nomenclature and Isomerism

All six hydrogen atoms in benzene are equivalent, so replacement of one hydrogen atom gives only one monosubstituted product.

Disubstitution:

• Replacement of two hydrogen atoms can yield three position isomers:
  • Ortho (o): Substituents at 1,2- or 1,6-positions.
  • Meta (m-): Substituents at 1,3- or 1,5-positions.
  • Para (p-): Substituents at 1,4-positions.

Structure of Benzene

Historical background of the Benzene structure

• Discovery: Michael Faraday (1825).
• Formula: C₆H₆ → shows high unsaturation but unusually high stability.
• Observation:
  • Benzene forms a triozonide, indicating the presence of three double bonds.
  • Produces only one ortho-disubstituted derivative, meaning all C–C bonds and all hydrogen atoms are equivalent.

Kekulé’s Structure (1865)

• Proposed a cyclic hexagonal structure with alternating single and double bonds, each carbon bonded to one hydrogen atom.
• Problem: This structure predicts two distinct 1,2-dibromobenzenes (due to alternating single/double bonds), but experimentally, only one is obtained.

Modification – Oscillation of Double Bonds

• Kekulé suggested that double bonds oscillate between positions, making all C–C bonds equivalent.

Resonance and Stability

• Valence Bond Theory: Benzene is a resonance hybrid of two Kekulé structures.
• Resonance structure is shown as a hexagon with a circle representing six delocalised π electrons.
• All six carbon atoms are sp² hybridised:
  • Two sp² orbitals form σ bonds with adjacent carbons.
  • One sp² orbital forms a σ bond with a hydrogen atom.
  • The unhybridised p orbitals on each carbon atom overlap laterally to form a delocalised π electron cloud above and below the ring.

Bond length and planarity

• X-ray diffraction: all C–C bond lengths = 139 pm, intermediate between single bond (154 pm) and double bond (133 pm).
• Benzene is planar, and the delocalisation makes it more stable than a hypothetical cyclohexatriene.
• Delocalisation explains why benzene prefers substitution reactions rather than addition reactions.

Aromaticity

A compound is considered aromatic if it satisfies the following conditions (Hückel’s Rule):

1. The molecule is planar.
2. Complete delocalisation of π electrons over the ring.
3. The ring contains (4n + 2) π electrons, where n = 0, 1, 2…

Examples: Benzene, Pyridine, Furan, Naphthalene.

Preparation of Benzene

The benzene preparation is discussed below.

1. Cyclic Polymerisation of Ethyne
   • At 873 K over red-hot iron:
     3 C₂H₂ → C₆H₆
2. Decarboxylation of Aromatic Acids
   • Sodium benzoate + sodalime (NaOH + CaO, heat) → Benzene + Na₂CO₃.
3. Reduction of Phenol
   • Phenol vapours passed over heated zinc dust:
     C₆H₅OH + Zn → C₆H₆ + ZnO.
     

Properties

Some of the physical and chemical properties are discussed below.

Physical Properties

• Non-polar molecules.
• Colourless liquids or solids with a characteristic aroma.
• Immiscible with water, miscible with organic solvents.
• Burn with sooty flame (due to high carbon content).

Chemical Properties

(A) Electrophilic Substitution Reactions (SE)

1. Nitration:
   • Reagents: Conc. HNO₃ + Conc. H₂SO₄ (nitrating mixture).
   • Example: Benzene → Nitrobenzene.
2. Halogenation:
   • Cl₂/Br₂ in the presence of FeCl₃/FeBr₃ → haloarenes.
3. Sulphonation:
   • Heating benzene with fuming H₂SO₄ → benzene sulphonic acid.
4. Friedel–Crafts Alkylation:
   • Benzene + R–X + AlCl₃ → alkylbenzene.
5. Friedel–Crafts Acylation:
   • Benzene + RCOCl + AlCl₃ → acylbenzene.

Mechanism of Electrophilic Substitution:

1. Generation of Electrophile (E⁺) – e.g., NO₂⁺ in nitration, Cl⁺ in halogenation.
2. Formation of σ-complex (arenium ion) – one C becomes sp³ hybridised.
3. Loss of Proton – aromaticity restored.

Addition Reactions

• Hydrogenation (Ni catalyst, high T/P) → cyclohexane.
• Chlorination under UV → benzene hexachloride (BHC).

Combustion

• Burns with sooty flame:
  C₆H₆ + 15/2 O₂ → 6 CO₂ + 3 H₂O.

Directive Influence of Substituents

Ortho/Para Directing Groups: Activate the ring, increase electron density at o and p-positions.

• Examples: –OH, –NH₂, –OCH₃, –CH₃, –C₂H₅.
• Halogens: o/p directing via resonance but overall deactivating via –I effect.

Meta Directing Groups: Strongly electron-withdrawing, reducing electron density at o and p-positions.

• Examples: –NO₂, –CN, –CHO, –COR, –COOH, –SO₃H.

Carcinogenicity and Toxicity

Benzene and polynuclear aromatic hydrocarbons (PAHs) are toxic and carcinogenic. Examples: Anthracene, Benzo[a]pyrene. Formed during the incomplete combustion of tobacco, coal, and petroleum. It can enter the human body, damage DNA, and cause cancer.

Important Formulas in NCERT Notes Class 11 Chemistry (Part-II) Chapter 9: Hydrocarbons

Here are the important formulas from NCERT Class 11 Chemistry (Part-II) Chapter 9: Hydrocarbons.

• Hydrocarbons – Compounds made up of only carbon and hydrogen atoms.
• Alkanes – Saturated hydrocarbons containing only single covalent (σ) bonds between carbon atoms with the general formula CnH₂n+₂.
• Alkenes – Unsaturated hydrocarbons containing at least one double bond between carbon atoms with the general formula CnH₂n.
• Alkynes – Unsaturated hydrocarbons containing at least one triple bond between carbon atoms with the general formula CnH₂n–₂.
• Aromatic Hydrocarbons (Arenes) – Hydrocarbons containing one or more benzene rings (benzenoids) or other highly unsaturated rings (non-benzenoids).
• Conformations – Different spatial arrangements of atoms in a molecule that can be interconverted by rotation around a single bond.
• Torsional Strain – Repulsive interaction between electron clouds of bonds on adjacent atoms, which hinders free rotation around C–C single bond.
• Isomerism in Hydrocarbons – Existence of two or more compounds with the same molecular formula but different structures (structural or conformational).
• Resonance in Benzene – Delocalisation of π-electrons over the entire benzene ring, making all C–C bonds equal in length and providing extra stability.
• Aromaticity – Property of cyclic, planar, conjugated compounds to be unusually stable due to complete delocalisation of π-electrons following Hückel’s rule (4n+2 π electrons).

Explore Notes of Class 11 Chemistry

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Download the Solutions of Other Chapters of Class 11 Chemistry

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Related Reads

NCERT Notes Class 11 English Woven Words Poem 1: The Peacock (Free PDF)	NCERT Solutions Class 11 English Woven Words Poem 1: The Peacock (Free PDF)
NCERT Notes Class 11 English Woven Words Chapter 8: The Luncheon (Free PDF)	NCERT Solutions Class 11 English Woven Words Chapter 8: The Luncheon (Free PDF)
NCERT Class 11 Sociology Chapter 1 Sociology And Society Notes (Free PDF)	NCERT Class 11 Sociology Chapter 1 Sociology and Society Solutions (Free PDF)

Source- Ignited Minds

Explore notes on other subjects in the NCERT Class 11

English

Sociology

Geography

Political Science

Psychology

FAQs

For more topics, follow LeverageEdu NCERT Study Material today!