Alkanes and Alkenes perplex

🧪 ALKANES AND ALKENES

Cambridge IGCSE Chemistry

Hydrocarbons

Compounds containing only Hydrogen (H) and Carbon (C) atoms. The building blocks of organic chemistry.

📖 17.1 ALKANES - Saturated Hydrocarbons

Alkanes are hydrocarbons with only single covalent bonds (C-C). They are saturated because each carbon bonds to the maximum of 4 atoms.

General Formula

CnH2n+2

where n = number of carbon atoms

First Four Alkanes

NameFormulaC AtomsState (r.t.p.)
MethaneCH41Gas
EthaneC2H62Gas
PropaneC3H83Gas
ButaneC4H104Gas

Methane Structure

H | H - C - H | H

Ethane Structure

H H | | H - C - C - H | | H H
Properties: Boiling points increase with carbon number. First 4 are gases, pentane+ are liquids/solids.
Uses: Natural gas (CH4), LPG (C3H8, C4H10), petrol, candle wax.

📖 17.2 CHEMICAL PROPERTIES - Alkanes

Alkanes are generally unreactive (strong C-C and C-H bonds). Also called "paraffins" (lacking affinity).

Two main reactions: Combustion and Substitution

1. Combustion

A. Complete Combustion (plenty of O2)

Alkane + Oxygen → Carbon Dioxide + Water
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

✓ Clean blue flame, heat released

B. Incomplete Combustion (limited O2)

Alkane + Oxygen → Carbon Monoxide + Water
2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)
⚠️ Danger: Yellow/orange smoky flame, produces poisonous CO and soot. Never use gas heaters in closed rooms!

2. Substitution Reactions

Substitution: One atom is replaced by another. Alkanes react with chlorine in UV light.

CH₄(g) + Cl₂(g) --UV light--> CH₃Cl(g) + HCl(g)
Methane + Chlorine → Chloromethane + Hydrogen chloride
H H | | H - C - H + Cl-Cl → H - C - Cl + H-Cl | | H H

Multiple substitutions: With excess Cl2, forms CH2Cl2, CHCl3, CCl4 (chloroalkanes).

📖 17.3 SUBSTITUTION REACTIONS

Photochemical Reaction: Chemical reaction caused by UV light. UV provides activation energy (Ea) to break Cl-Cl bonds.

Cl-Cl --UV--> Cl• + Cl• (free radicals)

Note: ANY hydrogen in alkane can be substituted → multiple products possible.

📖 17.4 ALKENES - Unsaturated Hydrocarbons

Alkenes are hydrocarbons with at least one carbon-carbon double bond (C=C). They are unsaturated (can add more atoms).

General Formula

CnH2n

⚠️ No "methene" - minimum 2 carbons needed for C=C bond

First Three Alkenes

NameFormulaC AtomsState (r.t.p.)
EtheneC2H42Gas
PropeneC3H63Gas
ButeneC4H84Gas

Ethene Structure

H H \ / C=C / \ H H

Alkanes vs Alkenes

PropertyAlkanesAlkenes
FormulaCnH2n+2CnH2n
BondsC-C (single)C=C (double)
SaturationSaturatedUnsaturated
ReactivityUnreactiveMore reactive
Bromine TestNo changeDecolorizes
Main ReactionSubstitutionAddition

📖 17.5 CRACKING - Producing Alkenes

Cracking: Breaking down large alkane molecules into smaller molecules using high temperature and catalyst.

Process

Long alkane --heat + catalyst--> Shorter alkane + Alkene + (H₂)
C₆H₁₄ → C₄H₁₀ + C₂H₄
Hexane → Butane + Ethene
Industrial Cracking: Heat to ~500°C, pass over catalyst (Al₂O₃/SiO₂).

Why Crack Alkanes?

  1. Produce alkenes: For plastics (polyethene), ethanol, chemicals
  2. Produce hydrogen: For Haber process (ammonia/fertilizers)
  3. Produce petrol: Convert heavy fractions to useful short-chain alkanes

Bromine Water Test

Test: Add hydrocarbon to red-brown bromine water

Alkane + Br₂(aq) → NO CHANGE (stays brown)
Alkene + Br₂(aq) → DECOLORIZES (turns colorless)

Alkenes react with Br₂ via addition, removing color. Alkanes don't react.

📖 17.6 CHEMICAL PROPERTIES - Alkenes

Alkenes are more reactive than alkanes due to C=C double bond. Main reaction type: ADDITION

Addition Reaction: Unsaturated compound + substance → ONE product only

Three Addition Reactions

1. Bromination (+ Bromine)

C₂H₄(g) + Br₂(aq) → C₂H₄Br₂(l)
Ethene + Bromine → 1,2-dibromoethane

✓ Room temp, no catalyst, brown→colorless

H H H H \ / | | C=C + Br-Br → H - C - C - H / \ | | H H Br Br

2. Hydrogenation (+ Hydrogen)

C₂H₄(g) + H₂(g) --200°C, Ni--> C₂H₆(g)
Ethene + Hydrogen → Ethane (alkene → alkane)

✓ 200°C, Ni catalyst, high pressure

Used to make margarine from vegetable oils!

3. Hydration (+ Steam)

C₂H₄(g) + H₂O(g) --300°C,60atm,H₃PO₄--> C₂H₅OH(l)
Ethene + Steam → Ethanol (alcohol)

✓ 300°C, 60 atm, H₃PO₄ catalyst

Produces ethanol for fuel, solvents, beverages

Addition vs Substitution

TypeMechanismProductsExample
SubstitutionReplace atomTWOCH₄+Cl₂→CH₃Cl+HCl
AdditionAdd to C=CONEC₂H₄+Br₂→C₂H₄Br₂

📝 KEY EQUATIONS

Alkanes

CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion)
2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete)
CH₄ + Cl₂ --UV--> CH₃Cl + HCl (substitution)

Alkenes

C₆H₁₄ → C₄H₁₀ + C₂H₄ (cracking)
C₂H₄ + Br₂ → C₂H₄Br₂ (bromination)
C₂H₄ + H₂ --200°C,Ni--> C₂H₆ (hydrogenation)
C₂H₄ + H₂O --300°C,60atm,H₃PO₄--> C₂H₅OH (hydration)

📚 KEY DEFINITIONS

Alkane: Saturated hydrocarbon, CnH2n+2, single bonds only

Alkene: Unsaturated hydrocarbon, CnH2n, contains C=C

Saturated: Maximum atoms bonded, only single bonds

Unsaturated: Contains double/triple bonds, can add atoms

Cracking: Breaking large molecules with heat + catalyst

Substitution: One atom replaced by another (2 products)

Addition: Molecules combine to form 1 product

Photochemical: Reaction caused by UV light

Hydrogenation: Adding H₂ to unsaturated compound

🎯 SUMMARY

ALKANES: Saturated, unreactive, CnH2n+2. Undergo combustion & substitution. Used as fuels.

ALKENES: Unsaturated, reactive, CnH2n. Undergo addition reactions. Made by cracking. Used for plastics, alcohols, chemicals.

Double bond = Key difference!

⚠️ Safety: Never do combustion without supervision. UV light harmful. Organics flammable!

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