Curriculum
Login

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

0/28

Chapter 2 - Chemical Analysis

0/22

Chapter 3 - Chemical Equilibrium

0/22

Chapter 4 - Electrochemistry

0/33

Chapter 5 - Organic Chemistry

0/72

More lessons

0/23
Text lesson

Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons

Chemistry

Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons

Overview

Overview

🌳 Concept tree

  1. Alkynes (Acetylene) and Cyclic Hydrocarbons
  2. ├── Structure and nomenclature
  3. ├── Preparation of ethyne
  4. ├── Properties of ethyne
  5. └── Cycloalkanes, CₙH₂ₙ
Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 1 / 6
Structure and nomenclature

Structure and nomenclature

  • Alkynes are unsaturated open-chain hydrocarbons with at least one triple bond C≡C: one strong σ bond and two weak π bonds, so alkynes are very reactive.
  • General formula CₙH₂ₙ₋₂ two H fewer than the alkene and four fewer than the alkane with the same number of carbons.
  • The first member is ethyne, C₂H₂, with the common name acetylene.
  • Nomenclature longest chain containing the triple bond; -ane → -yne; number from the end nearer the triple bond; give the number of the first carbon of the triple bond. Examples: 3-bromo-1-butyne is CH≡C–CHBr–CH₃; 5-chloro-2-pentyne is CH₃–C≡C–CH₂–CH₂Cl; CH≡C–CH₂CH₃ 1-butyne.
Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 2 / 6
Preparation of ethyne

Preparation of ethyne

Laboratory→drop water on calcium carbide (CaC₂):

CaC₂ + 2H₂O → HC≡CH + Ca(OH)₂

  • Commercial calcium carbide contains impurities (calcium phosphide and sulphide), so the gas contains phosphine (PH₃) and hydrogen sulphide (H₂S); it is passed through copper sulphate solution acidified with dilute sulphuric acid (or bubbled through acidified CuSO₄) before collecting, to remove them.
  • The gas is collected over water.
  • Industrial from methane of natural gas heated to above 1400 °C and then cooled very rapidly (quenching):

CH₄ —(>1400 °C, rapid cooling)→ HC≡CH + 3H₂

Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 3 / 6
Properties of ethyne

Properties of ethyne

Comparison aspectMain ideaExampleKeywords
1. Combustion.
  • In limited air it burns with a smoky luminous flame (incomplete combustion
  • soot)
2C₂H₂ + O₂ → 4C + 2H₂O (carbon is released as soot). In excess oxygen: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + heat. The oxy-acetylene flame reaches about 3000 °C and is used to weld and cut metalslimitedburnssmokyluminous
2. Addition reactionsin two stages (triple → double → single bond)—stagestripledoublesingle
Comparison aspectMain ideaKeywords
Hydrogenation (Ni, heat)HC≡CH + H₂ → CH₂=CH₂ → (+H₂) CH₃–CH₃
Halogenation
  • HC≡CH + Br₂ → CHBr=CHBr (1
  • 2-dibromoethene)
  • then + Br₂ → CHBr₂–CHBr₂ (1
  • 2-tetrabromoethane). Decolourisation of bromine in CCl₄ is a test for unsaturation. With chlorine the reaction is violent
CHBrdibromoethenethenCHBr₂
Hydrogen halides
  • HC≡CH + HBr → CH₂=CHBr (bromoethene)
  • + HBr (Markovnikov) → CH₃–CHBr₂ (1
  • 1-dibromoethane). 1
  • 2-Dibromoethane is not formed because the second H adds to the carbon that already carries more hydrogens
CHBrbromoetheneMarkovnikovCHBr₂
Catalytic hydration
  • with dilute H₂SO₄ and HgSO₄ catalyst at about 60 °C
  • water adds to give an unstable enol that rearranges to ethanal (acetaldehyde): HC≡CH + H₂O → CH₃CHO. Oxidation of ethanal with acidified KMnO₄ gives ethanoic acid
  • reduction with H₂ (Ni) gives ethanol: CH₃CHO + [O] → CH₃COOH
  • CH₃CHO + H₂ → CH₃CH₂OH
diluteH₂SO₄HgSO₄catalyst
3. Polymerisation to benzene→3C₂H₂ → C₆H₆ when passed through a red-hot tube (~500 °C).
Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 4 / 6
Cycloalkanes, CₙH₂ₙ

Cycloalkanes, CₙH₂ₙ

  • Hydrocarbons with three or more carbons can form rings.
  • Saturated rings are cycloalkanes, general formula CₙH₂ₙ — the same as alkenes, so cycloalkanes are isomeric with alkenes of the same carbon number (C₃H₆ is propene or cyclopropane), and one must look at the structure.
  • Names put "cyclo" before the alkane name: cyclopropane C₃H₆, cyclobutane C₄H₈, cyclopentane C₅H₁₀, cyclohexane C₆H₁₂.
  • Stability and ring strain. Normal tetrahedral carbon has bond angles of 109.5°. The C–C–C angle in the cyclopropane ring is 60° and in cyclobutane about 90°; this forces the orbitals to overlap poorly ("bent bonds"), so the C–C bonds are weak and these small rings are very reactive (cyclopropane forms explosive mixtures with air and can open its ring). Cyclopentane (angle ≈ 108°) and cyclohexane (≈ 109.5° in the non-planar chair form) have nearly normal angles, good overlap and strong σ bonds, so they are stable.
Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 5 / 6
Part 6

Part 6

  • 📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

  • Lesson 3 – Alkynes (Acetylene) and Cyclic Hydrocarbons
  • Detailed Academic Lesson Overview:
  • Alkynes contain a carbon-carbon triple bond (one σ and two weak π bonds, general formula CnH2n-2).
  • Acetylene (ethyne) is synthesized in the lab and burns with a high-temperature oxy-acetylene flame (3000°C) used in welding.

🌳 Hierarchy Classification & Conceptual Roadmap

Alkynes and Cycloalkanes

├── 1. Acetylene (Ethyne – C2H2)

Comparison aspectMain ideaKeywords
│ ├── Laboratory PreparationDropping water on Calcium Carbide (CaC2 + 2 H2O → C2H2 + Ca(OH)2)DroppingwaterCalciumCarbide
│ ├── IndustrialThermal quenching of Methane (2 CH4 at 1500°C rapid cool → C2H2 + 3 H2)ThermalquenchingMethane1500
•│ └── CombustionCombustion
│ ├── Limited O2Smoky flame due to unburned carbonSmokyflameunburnedcarbon
│ └── Abundant O2Oxy-acetylene flame (3000°C) for cutting and welding metalsacetyleneflame3000cutting
•├── 2. Addition Reactions of Ethyne (Occurs in two sequential steps due to 2 π bonds)AdditionReactionsEthyneOccurs
│ ├── Halogenation
  • C2H2 + 2 Br2 → 1
  • 2-tetrabromoethane
C2H2tetrabromoethane
│ ├── Hydrohalogenation
  • C2H2 + HBr → Vinyl bromide → 1
  • 1-dibromoethane (Markovnikov)
C2H2Vinylbromidedibromoethane
│ └── Catalytic HydrationC2H2 + H2O (40% H2SO4 / 1% HgSO4 at 60°C) → Acetaldehyde (CH3CHO)C2H2H2SO4HgSO4Acetaldehyde
•
  • └── 3. Cycloalkanes (General formula: CnH2n
  • isomers of alkenes)
CycloalkanesGeneralformulaCnH2n
├── Cyclopropane (bond angle 60°) and Cyclobutane (90°)Unstable and highly active due to poor orbital overlapUnstablehighlyactivepoor
•└── Cyclopentane (108°) and Cyclohexane (109.5°): Completely stable due to tetrahedral anglesCyclopentaneCyclohexaneCompletelystable
•📋 Master Teacher's Blackboard Rules & Exam SecretsMasterTeacherBlackboardRules
RULE 1
  • Addition of HBr to acetylene in step 2 obeys Markovnikov's rule
  • yielding 1
  • 1-dibromoethane and NEVER 1
  • 2-dibromoethane
Additionacetylenestepobeys
RULE 2
  • Catalytic hydration of ethyne initially forms unstable vinyl alcohol (CH2=CH-OH)
  • immediately tautomerizes to acetaldehyde (CH3-CHO)
Catalytichydrationethyneinitially
RULE 3
  • Cyclopropane is an isomer of propene (both C3H6)
  • but cyclopropane is a saturated cyclic ring whereas propene is an open-chain unsaturated alkene
Cyclopropaneisomerpropeneboth
📊 Detailed Comparison MatrixRing Strain and Stability Comparison in CycloalkanesRingStrainStabilityCycloalkanes
Compound Ring Bond Angle Orbital Overlap Quality Chemical Activity / Stability
Cyclopropane (C3H6) 60° Extremely weak overlap Extremely active / flammable gas
Cyclobutane (C4H8) 90° Weak overlap Active / burns easily
Cyclopentane (C5H10) 108° Strong orbital overlap Stable non-reactive ring
Cyclohexane (C6H12) 109.5° (Chair/Boat) Ideal tetrahedral overlap Highly stable / mimics straight alkanes
Lesson 3: Alkynes (Acetylene) and Cyclic Hydrocarbons · 6 / 6