Alkanes and Alkenes: Properties and Reactions
Crude Oil and Hydrocarbons
Crude oil is a finite resource formed from the remains of ancient marine organisms over millions of years. It is a mixture of many different hydrocarbons — molecules made of only hydrogen and carbon atoms.
Crude oil is separated into useful fractions by fractional distillation. The fractionating column is hot at the bottom and cool at the top. Hydrocarbons with different boiling points condense at different heights.
Trends in Hydrocarbon Properties
As the chain length increases (more carbon atoms):
- Boiling point increases (stronger intermolecular forces between larger molecules)
- Viscosity increases (thicker, less runny)
- Flammability decreases (harder to ignite)
Short-chain hydrocarbons are more useful as fuels because they ignite easily and burn with a clean flame.
Alkanes
Alkanes are a homologous series of hydrocarbons with the general formula:
CₙH₂ₙ₊₂
They are saturated — all carbon-carbon bonds are single bonds (C−C).
| Name | Formula | Structure |
|---|---|---|
| Methane | CH₄ | 1 carbon |
| Ethane | C₂H₆ | 2 carbons |
| Propane | C₃H₈ | 3 carbons |
| Butane | C₄H₁₀ | 4 carbons |
Properties of Alkanes
- Saturated — contain only single C−C bonds
- Relatively unreactive — single bonds are strong and hard to break
- Main reactions: combustion (burning) and reaction with halogens (in UV light)
Combustion of Alkanes
Complete combustion (plenty of oxygen):
hydrocarbon + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion (limited oxygen):
Produces carbon monoxide (CO) — toxic, and/or carbon/soot (C), plus water.
CH₄ + O₂ → C + 2H₂O (very limited oxygen)
2CH₄ + 3O₂ → 2CO + 4H₂O (limited oxygen)
Incomplete combustion is dangerous because CO is a colourless, odourless, poisonous gas.
Cracking
Cracking breaks down longer-chain hydrocarbons into shorter, more useful ones. It produces alkanes (for fuels) and alkenes (for making polymers).
There are two methods:
Catalytic cracking:
- Hydrocarbon vapour passed over a hot zeolite catalyst at about 600–700°C
- Produces mostly alkenes for polymer production
Steam cracking:
- Hydrocarbon mixed with steam and heated to very high temperatures (~800–850°C)
- Produces a mix of alkenes
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)
The products always include at least one alkene (with a C=C double bond).
Alkenes
Alkenes are a homologous series with the general formula:
CₙH₂ₙ
They are unsaturated — they contain at least one carbon-carbon double bond (C=C).
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| Butene | C₄H₈ |
Properties of Alkenes
- Unsaturated — contain a C=C double bond
- More reactive than alkanes — the double bond can open up and react
- Used as feedstocks for making polymers and other chemicals
Test for Alkenes: Bromine Water
Add bromine water (orange/brown) to the substance:
- Alkene → bromine water turns colourless (decolourised) — the alkene has reacted
- Alkane → bromine water stays orange — no reaction
The alkene reacts by addition across the double bond:
C₂H₄ + Br₂ → C₂H₄Br₂ (dibromoethane)
This is an addition reaction — two molecules combine to make one product, with no other product formed.
Other Addition Reactions of Alkenes
Hydrogenation (addition of hydrogen):
C₂H₄ + H₂ → C₂H₆ (ethane)
Catalyst: nickel, temperature ~150°C. Used to harden vegetable oils for margarine.
Hydration (addition of water/steam):
C₂H₄ + H₂O → C₂H₅OH (ethanol)
Catalyst: phosphoric acid, high temperature and pressure. Industrial method for making ethanol.
Exam Tips
- Know the general formulae: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ
- Alkanes are saturated (single bonds only), alkenes are unsaturated (contain C=C)
- Bromine water test: decolourised = alkene present
- Cracking always produces at least one alkene
- Complete combustion produces CO₂ + H₂O; incomplete produces CO and/or C
- Be able to write balanced equations for combustion and addition reactions