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Class 11 Chemistry Notes

Hydrocarbons Class 11 Notes

Complete, exam-ready notes on hydrocarbons: alkanes, alkenes, alkynes and arenes — their preparations and characteristic reactions, Markovnikov's rule and peroxide effect, addition and elimination mechanisms, and the aromatic chemistry of benzene — written for CBSE, JEE and NEET revision.

Class11SubjectChemistryCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What are hydrocarbons in one line?

Hydrocarbons are the simplest organic compounds, containing only carbon and hydrogen — as open-chain alkanes, alkenes, alkynes or aromatic arenes.

Alkanes — Structure and Preparation

Alkanes

CnH2n+2C_nH_{2n+2}

Saturated hydrocarbons with only C–C and C–H σ bonds, all carbons sp³. They are relatively unreactive (paraffins) — reactions happen mainly by substitution or combustion.

  • From alkenes/alkynes: hydrogenation with a metal catalyst (Ni, Pd, Pt).
  • From alkyl halides: Wurtz reaction 2RX + 2Na → R–R + 2NaX.
  • From carboxylic acids: Kolbe's electrolysis of sodium salts.
  • From Grignard reagents: RMgX with water gives RH.
  • Physical trend: boiling points rise with carbon number; branched isomers boil lower.

Reactions of Alkanes

  • Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O, releasing heat — alkanes are fuel.
  • Halogenation (free-radical substitution): heat or UV light drives CH₄ + Cl₂ → CH₃Cl + HCl, proceeding through initiation, propagation and termination.
  • Chlorination can over-halogenate; reactivity order F₂ > Cl₂ > Br₂ > I₂.
  • Controlled oxidation gives alcohols, aldehydes or acids depending on conditions.

Alkenes — Unsaturation and Preparation

Alkenes

CnH2nC_nH_{2n}

Alkenes carry one C=C double bond (one σ + one π), with trigonal planar sp² carbons. The π bond is weaker, making alkenes far more reactive than alkanes.

  • Cracking of hydrocarbons, and elimination of water from alcohols (dehydration with H₂SO₄).
  • Dehydrohalogenation of alkyl halides with alcoholic KOH (Saytzeff rule: the more substituted alkene forms).
  • Geometric isomerism: cis/trans around the double bond because rotation is blocked by π bonding.

Addition Reactions of Alkenes

Electrophilic addition — Markovnikov's rule

HX adds so that H goes to the carbon with more hydrogens\text{HX adds so that } H \text{ goes to the carbon with } \textbf{more} \text{ hydrogens}

In HX addition the proton attaches to the carbon already holding more hydrogens, giving the more stable (more substituted) carbocation intermediate. Addition of HBr in the presence of peroxides reverses the regiochemistry (anti-Markovnikov, radical mechanism).

  • Catalytic hydrogenation adds H₂ across the double bond.
  • Halogenation: bromine water is decolourised — a simple test for unsaturation.
  • Hydration with acid gives alcohols; hydrohalogenation gives alkyl halides.
  • Ozonolysis cleaves the double bond to carbonyl compounds, revealing the alkene's structure.
  • Polymerisation: many alkenes join to give polyethene, polypropene, PVC.

Alkynes

Alkynes

CnH2n−2,HC≡CHC_nH_{2n-2},\qquad HC\equiv CH

Alkynes have a C≡C triple bond (linear, sp) — two π bonds around a σ bond. They are more reactive to addition and are acidic enough that terminal alkynes form acetylides with strong bases or silver nitrate.

  • Preparation: dehydrohalogenation of vicinal dihalides with alcoholic KOH.
  • Acidity: terminal H (R–C≡C–H) is the most acidic of hydrocarbon hydrogens.
  • Addition reactions mirror alkenes but can add two molar equivalents — e.g. 2H₂, 2Br₂.
  • Calcium carbide + water gives acetylene in the laboratory.

Aromatic Hydrocarbons — Benzene

Aromaticity + Huckel's rule

(4n+2)π  electrons(4n+2)\pi\;\text{electrons}

Benzene C₆H₆ is a planar, cyclic, fully conjugated molecule whose 6 π electrons satisfy Huckel's rule (n = 1). The resonance stabilisation makes it very stable and substitution-prone rather than addition-prone.

  • Electrophilic aromatic substitution (EAS): nitration, halogenation, sulphonation, Friedel–Crafts alkylation and acylation.
  • A substituent on benzene directs and activates: –OH, –CH₃, –NH₂ are ortho/para directors; –NO₂, –CN, –CHO are meta directors.
  • Benzene does not normally undergo addition reactions (unlike alkenes), showing its resonance stability.
  • Toxicity: benzene is carcinogenic; toluene is used as a safer substitute in solvents.

Solved Examples

Example: Predict the addition product of propene (CH₃–CH=CH₂) with HBr.

Solution: By Markovnikov's rule, H goes to the terminal carbon (more hydrogens), and Br joins the middle carbon, giving 2-bromopropane. In the presence of peroxides, the product is 1-bromopropane.

Example: Why does benzene prefer substitution over addition?

Solution: Addition would break the stable 6π aromatic system. Substitution keeps the aromatic ring intact, so benzene undergoes electrophilic aromatic substitution far more readily than addition.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Alkanes

CnH2n+2C_nH_{2n+2}

Alkenes

CnH2nC_nH_{2n}

Alkynes

CnH2n−2C_nH_{2n-2}

Combustion

CH4+2O2→CO2+2H2OCH_4 + 2O_2 \to CO_2 + 2H_2O

Wurtz reaction

2RX+2Na→R−R+2NaX2RX + 2Na \to R{-}R + 2NaX

Markovnikov rule

H→C with more HH \to C \text{ with more } H

Huckel's aromatic rule

4n+2  π  electrons4n+2\;\pi\;\text{electrons}

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

  • Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂.
  • Alkane reactions: combustion and free-radical halogenation.
  • Alkenes add electrophiles; Markovnikov rule with H on the more H-rich carbon.
  • Peroxide effect flips HBr addition to anti-Markovnikov (radical).
  • Alkynes are terminal-acidic; acetylides form with base.
  • Benzene: (4n+2)π aromatic, reacts by electrophilic substitution.
  • Bromine-water decolourisation tests for C=C and C≡C.

FAQ

Common questions

What is the difference between alkanes, alkenes and alkynes?

Alkanes have only C–C single bonds (CₙH₂ₙ₊₂), alkenes contain a C=C double bond (CₙH₂ₙ) and alkynes a C≡C triple bond (CₙH₂ₙ₋₂). Alkenes and alkynes are unsaturated and far more reactive than alkanes.

What is Markovnikov's rule?

In the addition of HX to an alkene, the hydrogen adds to the carbon that already has more hydrogens, and the halogen to the more substituted carbon, giving the more stable carbocation intermediate. Peroxides reverse this for HBr.

Why is benzene extraordinarily stable?

Benzene is planar, cyclic and fully conjugated with 6 π electrons — satisfying Huckel's (4n+2) rule. The delocalised electrons stabilise the ring so much that it undergoes substitution, not addition.

How do you prepare an alkene from an alcohol?

Dehydration with concentrated sulphuric acid removes water — an elimination reaction. Following Saytzeff's rule, the more highly substituted (more stable) alkene is the major product.

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