Hydrocarbons is Chapter 12 of CBSE Class 11 Chemistry — the chapter where organic chemistry stops being abstract and starts making sense. Everything from cooking gas (LPG) to petrol, from polythene bags to naphthalene balls, is a hydrocarbon. Master classification, the named reactions, and the substitution mechanisms here, and a huge slice of NEET, JEE, and your board organic paper becomes routine scoring.
By the end of these notes you will be able to name any alkane, alkene, or alkyne by IUPAC rules, predict the product of Markovnikov, anti-Markovnikov, ozonolysis, and Friedel-Crafts reactions, explain benzene’s aromaticity, and apply the directive influence of groups in electrophilic substitution. This is a high-weightage chapter carrying roughly 6–8 marks in boards, and the launchpad for the entire Class 12 organic syllabus.
Table of Contents
- Key Concepts — Classification, alkanes, alkenes, alkynes, aromatic hydrocarbons, mechanisms, toxicity
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. Classification of Hydrocarbons
Hydrocarbons are compounds made of only carbon and hydrogen. The way the carbon atoms are joined decides everything about their behaviour, so we classify them first.
- Saturated (alkanes): only C–C single bonds. General formula CₙH₂ₙ₊₂. Example: methane CH₄, ethane C₂H₆.
- Unsaturated (alkenes & alkynes): contain C=C or C≡C. Alkenes CₙH₂ₙ (ethene C₂H₄); alkynes CₙH₂ₙ₋₂ (ethyne C₂H₂).
- Aromatic: contain a benzene ring (or fused rings) with delocalised π electrons. Example: benzene C₆H₆.
[DIAGRAM: A tree — Hydrocarbons branching into Aliphatic (acyclic + alicyclic) and Aromatic; aliphatic further splitting into saturated alkanes and unsaturated alkenes/alkynes.]
2. Alkanes — Nomenclature and Isomerism
Alkanes are saturated open-chain hydrocarbons (CₙH₂ₙ₊₂) in which carbon is sp³ hybridised with bond angles of 109.5°. They are also called paraffins because of their low reactivity.
IUPAC naming: pick the longest chain (root word), number it so substituents get the lowest locants, name substituents alphabetically as prefixes, and end with “-ane”.
For example, (CH₃)₂CHCH₂CH₃ is 2-methylbutane — a four-carbon main chain with a methyl branch at C-2.
Chain (Structural) Isomerism
Alkanes from butane onward show chain isomerism — same molecular formula, different skeletons. C₄H₁₀ has two isomers (n-butane and isobutane); C₅H₁₂ has three; C₆H₁₄ has five.
3. Conformations of Alkanes
Conformations are the different spatial arrangements obtained by rotation about a C–C single bond. They are not isomers because they interconvert freely and cannot be isolated.
- Staggered: hydrogen atoms of the two carbons are as far apart as possible — minimum repulsion, most stable.
- Eclipsed: hydrogen atoms directly overlap — maximum torsional strain, least stable.
For ethane the energy difference between staggered and eclipsed forms is only about 12.5 kJ/mol, so rotation is essentially free at room temperature.
[DIAGRAM: Newman projections of ethane — staggered (back H’s at 60° to front H’s) versus eclipsed (back H’s directly behind front H’s).]
4. Preparation of Alkanes
Alkanes are made by adding hydrogen to unsaturated compounds or by removing functional groups.
- Hydrogenation (Sabatier–Senderens): alkene + H₂ → alkane, using Ni/Pt/Pd catalyst.
- Wurtz reaction: 2R–X + 2Na → R–R + 2NaX (gives symmetrical alkanes with an even number of carbons).
- Decarboxylation: sodium salt of a carboxylic acid + soda lime → alkane with one less carbon. CH₃COONa + NaOH → CH₄ + Na₂CO₃.
- Kolbe’s electrolysis: electrolysis of aqueous sodium carboxylate gives an alkane at the anode.
5. Properties of Alkanes & Mechanism of Halogenation
Alkanes are non-polar, insoluble in water, and generally unreactive. Their most important reaction is free-radical substitution with halogens in sunlight (UV light).
CH₄ + Cl₂ → CH₃Cl + HCl (continues to CH₂Cl₂, CHCl₃, CCl₄)
Free-Radical Mechanism (Chlorination of Methane)
- Initiation: Cl₂ → 2Cl• (homolytic cleavage by UV light).
- Propagation: Cl• + CH₄ → •CH₃ + HCl; then •CH₃ + Cl₂ → CH₃Cl + Cl•.
- Termination: radicals combine — Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, •CH₃ + •CH₃ → C₂H₆.
Reactivity order of halogens: F₂ > Cl₂ > Br₂ > I₂. Fluorination is explosive; iodination is reversible and slow.
6. Alkenes — Structure and Nomenclature
Alkenes (CₙH₂ₙ) contain at least one C=C double bond. The doubly bonded carbons are sp² hybridised with bond angles near 120°; the double bond is one strong σ bond plus one weaker π bond.
IUPAC names end in “-ene” and the chain is numbered to give the double bond the lowest locant. For example CH₃–CH=CH–CH₃ is but-2-ene.
Geometrical (cis–trans) Isomerism
Because rotation about C=C is restricted, alkenes can show geometrical isomerism when each doubly bonded carbon carries two different groups.
- cis: identical groups on the same side.
- trans: identical groups on opposite sides (usually more stable).
But-2-ene exists as cis- and trans- forms; but-1-ene does not (one carbon carries two H atoms).
7. Preparation and Properties of Alkenes
Alkenes are made by elimination reactions that introduce a double bond.
- Dehydrohalogenation: alkyl halide + alc. KOH → alkene + KX + H₂O.
- Dehydration of alcohols: alcohol + conc. H₂SO₄ (heat) → alkene + H₂O.
- Dehalogenation: vicinal dihalide + Zn → alkene.
Their characteristic chemistry is electrophilic addition across the electron-rich double bond: addition of H₂, X₂, HX, H₂O, etc.
Baeyer’s test: alkenes decolourise cold dilute alkaline KMnO₄ (purple → colourless), giving a vicinal glycol — a test for unsaturation.
8. Markovnikov & Anti-Markovnikov (Peroxide Effect)
When an unsymmetrical reagent like HBr adds to an unsymmetrical alkene, the orientation matters.
Markovnikov’s rule: the negative part of the reagent adds to the carbon bearing the fewer hydrogen atoms (the H goes to the carbon already having more H’s). This is because the more stable carbocation forms preferentially.
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃ (2-bromopropane).
Anti-Markovnikov Addition (Kharasch / Peroxide Effect)
In the presence of organic peroxides, HBr (only HBr) adds against Markovnikov’s rule, via a free-radical mechanism.
CH₃–CH=CH₂ + HBr (peroxide) → CH₃–CH₂–CH₂Br (1-bromopropane).
Note: HCl and HI do not show the peroxide effect.
9. Ozonolysis of Alkenes
Ozonolysis is the cleavage of a C=C double bond by ozone to locate its position. The alkene reacts with O₃ to form an ozonide, which on reductive cleavage (Zn/H₂O) gives two carbonyl compounds.
CH₃–CH=CH₂ → CH₃CHO + HCHO (ethanal + methanal)
By identifying the aldehydes/ketones formed, we can pinpoint where the double bond was in the original alkene — a favourite reasoning question.
10. Alkynes — Preparation and Acidic Character
Alkynes (CₙH₂ₙ₋₂) contain a C≡C triple bond; the carbons are sp hybridised, linear, with 180° bond angles. The triple bond is one σ and two π bonds.
Preparation
- Ethyne from calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂.
- Double dehydrohalogenation of vicinal/geminal dihalides with alc. KOH.
Acidic Character of Terminal Alkynes
The H attached to a triply bonded (sp) carbon is acidic because the sp carbon, with 50% s-character, holds the bonding electrons tightly. Terminal alkynes therefore react with sodium and ammoniacal AgNO₃/Cu₂Cl₂.
HC≡CH + Na → HC≡C⁻Na⁺ + ½H₂; HC≡CH + 2[Ag(NH₃)₂]⁺ → Ag–C≡C–Ag (white ppt) — a test for terminal alkynes.
11. Addition Reactions of Alkynes
Like alkenes, alkynes undergo electrophilic addition, but in two stages (across each π bond).
- Hydrogenation: with Pd/BaSO₄ (Lindlar’s catalyst) gives cis-alkene; with Ni gives the alkane.
- Addition of water: C₂H₂ + H₂O (dil. H₂SO₄, HgSO₄) → CH₃CHO (acetaldehyde), via an unstable enol.
- Polymerisation: 3 ethyne molecules cyclise (red-hot tube) to give benzene.
12. Aromatic Hydrocarbons — Benzene Structure & Aromaticity
Aromatic hydrocarbons (arenes) contain the benzene ring. Benzene (C₆H₆) is a flat, regular hexagon with all C–C bond lengths equal (139 pm), intermediate between single and double bonds.
Each carbon is sp² hybridised; the six unhybridised p-orbitals overlap to form a delocalised π electron cloud above and below the ring — this delocalisation gives benzene its unusual stability (resonance energy ≈ 152 kJ/mol).
Hückel’s Rule (Aromaticity)
A compound is aromatic if it is planar, cyclic, fully conjugated, and contains (4n + 2) π electrons (n = 0, 1, 2…). Benzene has 6 π electrons (n = 1), so it is aromatic.
13. Electrophilic Substitution in Benzene
Because the π cloud is electron-rich but stable, benzene prefers substitution over addition — it keeps the ring intact. The general mechanism has three steps.
- Generation of electrophile (E⁺).
- Attack of E⁺ on the ring → arenium ion (carbocation, σ-complex), resonance stabilised.
- Loss of H⁺ restoring aromaticity → substituted benzene.
| Reaction | Reagents | Electrophile | Product |
|---|---|---|---|
| Nitration | conc. HNO₃ + conc. H₂SO₄ | NO₂⁺ (nitronium ion) | Nitrobenzene |
| Sulphonation | fuming H₂SO₄ (SO₃) | SO₃ / ⁺SO₃H | Benzenesulphonic acid |
| Halogenation | X₂ + anhyd. FeX₃ / AlX₃ | X⁺ | Halobenzene |
| Friedel–Crafts alkylation | R–X + anhyd. AlCl₃ | R⁺ (carbocation) | Alkylbenzene |
| Friedel–Crafts acylation | RCOCl + anhyd. AlCl₃ | RCO⁺ (acylium ion) | Aryl ketone |
14. Directive Influence of Substituents
A group already present on the ring decides where the next group goes and whether reaction is faster or slower.
- Ortho/para-directing, activating: –OH, –NH₂, –OR, –CH₃, –alkyl. They release electrons, speed up substitution, and direct the new group to the 2- and 4- positions.
- Meta-directing, deactivating: –NO₂, –COOH, –CHO, –SO₃H, –CN. They withdraw electrons, slow substitution, and direct to the 3- position.
- Exception: halogens (–Cl, –Br) are deactivating but still ortho/para-directing.
15. Carcinogenicity and Toxicity
Benzene and polynuclear aromatic hydrocarbons are not just exam topics — they are health hazards. Polynuclear hydrocarbons such as benz[a]pyrene, formed by incomplete combustion of tobacco, coal, and petrol, are carcinogenic (cancer-causing).
These compounds enter the body, get metabolised, and damage DNA, which can trigger cancer. This is why prolonged exposure to vehicle exhaust and cigarette smoke is dangerous.
Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 6–8 marks | IUPAC naming, Markovnikov, mechanisms, directive influence |
| JEE Main / Advanced | 2–3 questions | Ozonolysis, peroxide effect, aromaticity, reaction sequences |
| NEET | 2–3 questions | Named reactions, acidic character of alkynes, aromatic substitution |
[TABLE: Question-type split — VSA (1 mark): definitions & reagents; SA (2–3 marks): mechanisms, Markovnikov products, conformations; LA (5 marks): full electrophilic substitution mechanism, directive influence reasoning.]
Important Definitions
| Term | Definition |
|---|---|
| Hydrocarbon | A compound containing only carbon and hydrogen |
| Alkane | Saturated hydrocarbon with only C–C single bonds: CₙH₂ₙ₊₂ |
| Alkene | Unsaturated hydrocarbon with a C=C double bond: CₙH₂ₙ |
| Alkyne | Unsaturated hydrocarbon with a C≡C triple bond: CₙH₂ₙ₋₂ |
| Conformation | Spatial arrangement from rotation about a C–C single bond (e.g. staggered, eclipsed) |
| Markovnikov’s rule | Negative part of HX adds to the carbon with fewer H atoms (more stable carbocation) |
| Peroxide effect | Anti-Markovnikov addition of HBr to alkenes in presence of peroxides |
| Ozonolysis | Cleavage of C=C by ozone to give carbonyl compounds, locating the double bond |
| Aromaticity | Extra stability of planar, cyclic, conjugated rings with (4n+2) π electrons (Hückel’s rule) |
| Electrophilic substitution | Replacement of a ring H by an electrophile, keeping aromaticity intact |
Solved Examples
Example 1
Write the IUPAC name of (CH₃)₃C–CH₂–CH₃.
Answer: Longest chain = pentane (5 C); two methyl groups on C-2. Name: 2,2-dimethylbutane (main chain is butane, with two methyls at C-2).
Example 2
Predict the product when propene reacts with HBr (a) without peroxide and (b) with peroxide.
Answer: (a) Markovnikov → 2-bromopropane (CH₃CHBrCH₃). (b) Anti-Markovnikov (peroxide effect) → 1-bromopropane (CH₃CH₂CH₂Br).
Example 3
An alkene on ozonolysis gives only acetone (propanone). Identify the alkene.
Answer: Two acetone units join at the carbonyl carbons → (CH₃)₂C=C(CH₃)₂, i.e. 2,3-dimethylbut-2-ene.
Example 4
Why is ethyne more acidic than ethene and ethane?
Answer: The acidic H is on an sp carbon (50% s-character) in ethyne, which holds the electron pair closer to the nucleus, stabilising the carbanion. Order of acidity: ethyne (sp) > ethene (sp²) > ethane (sp³).
Example 5
Name the electrophile in the nitration of benzene and write the reagents used.
Answer: Electrophile = nitronium ion, NO₂⁺; reagents = conc. HNO₃ + conc. H₂SO₄ (nitrating mixture). Product = nitrobenzene.
Example 6
Toluene (methylbenzene) is nitrated. Predict the major products and explain.
Answer: –CH₃ is an ortho/para-directing, activating group, so nitration gives mainly o-nitrotoluene and p-nitrotoluene, and the reaction is faster than for benzene.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Write the general formula of alkanes, alkenes, and alkynes.
- Why does benzene undergo substitution rather than addition?
- Name the catalyst used to convert an alkyne to a cis-alkene.
- What is the electrophile in Friedel–Crafts acylation?
- Which alkene shows geometrical isomerism — but-1-ene or but-2-ene? Why?
2–3-Mark Questions (SA)
- State Markovnikov’s rule and illustrate with the addition of HBr to propene.
- Explain the peroxide effect with a suitable example and mechanism outline.
- Draw and compare the staggered and eclipsed conformations of ethane; state which is more stable and why.
- Explain why terminal alkynes are acidic but alkenes and alkanes are not.
5-Mark Questions (LA)
- Describe the free-radical mechanism of chlorination of methane (initiation, propagation, termination).
- Explain the mechanism of electrophilic substitution in benzene, taking nitration as an example.
- Discuss the directive influence of substituents in benzene with examples of ortho/para- and meta-directing groups.
Quick Revision Points
- Alkanes CₙH₂ₙ₊₂ (sp³, 109.5°); alkenes CₙH₂ₙ (sp², 120°); alkynes CₙH₂ₙ₋₂ (sp, 180°)
- Conformations: staggered (stable) vs eclipsed (least stable); not isomers
- Alkane prep: hydrogenation, Wurtz, decarboxylation, Kolbe’s electrolysis
- Halogenation of alkanes = free-radical substitution (UV); F₂ > Cl₂ > Br₂ > I₂
- Markovnikov: negative part to C with fewer H; peroxide effect = anti-Markovnikov (HBr only)
- Ozonolysis locates C=C → gives two carbonyl compounds
- Terminal alkyne H is acidic (sp carbon); gives white ppt with ammoniacal AgNO₃
- 3 C₂H₂ → benzene; benzene is aromatic by Hückel’s (4n+2) rule, 6 π electrons
- Electrophiles: nitration NO₂⁺, sulphonation SO₃, halogenation X⁺, F–C R⁺/RCO⁺
- –OH, –NH₂, –CH₃ are o/p-directing activators; –NO₂, –COOH are m-directing deactivators; halogens o/p but deactivating
- Polynuclear aromatics (e.g. benz[a]pyrene) are carcinogenic
Next Chapter: Chapter 14 — Environmental Chemistry
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- Organic Chemistry: Some Basic Principles and Techniques Class 11 Notes
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- Some Basic Concepts of Chemistry Class 11 Notes
Practice What You Learned
Carry these named reactions forward with our Class 12 Chemistry notes once you are board-ready.