Alkanes and alkenes are both families of hydrocarbons — molecules containing only carbon and hydrogen — but they differ fundamentally at the particle level: alkanes have only single bonds between carbon atoms (saturated), while alkenes contain at least one carbon-carbon double bond (unsaturated). That double bond changes everything about how alkenes react.
What are hydrocarbons and homologous series?
A hydrocarbon is a compound containing only carbon and hydrogen atoms. A homologous series is a family of compounds with the same general formula, similar chemical properties, and properties that change gradually as the chain length increases. Both alkanes and alkenes are homologous series.
Members of a homologous series:
- Have the same functional group (or none, in alkanes)
- Differ by a CH₂ unit from one member to the next
- Show a gradual trend in physical properties (boiling point, viscosity) as chain length increases
What are alkanes?
Alkanes are saturated hydrocarbons — every carbon-carbon bond is a single bond (C–C), so the carbon atoms are bonded to the maximum possible number of hydrogen atoms.
General formula: CₙH₂ₙ₊₂
| Name | Formula | Number of carbons | State at room temperature |
|---|---|---|---|
| Methane | CH₄ | 1 | Gas |
| Ethane | C₂H₆ | 2 | Gas |
| Propane | C₃H₈ | 3 | Gas |
| Butane | C₄H₁₀ | 4 | Gas |
| Pentane | C₅H₁₂ | 5 | Liquid |
| Decane | C₁₀H₂₂ | 10 | Liquid |
Alkanes are relatively unreactive because C–C and C–H bonds are strong and non-polar, and there is no site for attacking reagents to latch onto easily. Their main reaction at GCSE is combustion.
What are alkenes?
Alkenes are unsaturated hydrocarbons — they contain at least one carbon-carbon double bond (C=C).
General formula: CₙH₂ₙ (for simple alkenes with one double bond)
| Name | Formula | Number of carbons |
|---|---|---|
| Ethene | C₂H₄ | 2 |
| Propene | C₃H₆ | 3 |
| Butene | C₄H₈ | 4 |
The C=C double bond makes alkenes far more reactive than alkanes. The double bond is a region of high electron density that attracts electrophiles (reagents that seek electrons).
How do alkanes and alkenes react with bromine water?
The bromine water test (also called the decolorisation test) distinguishes alkanes from alkenes:
| Alkane | Alkene | |
|---|---|---|
| Observation with bromine water | No change — remains orange/brown | Decolourises to colourless |
| Reason | C–C single bonds do not react with Br₂ | C=C double bond undergoes addition |
In the alkene reaction, bromine adds across the double bond in an addition reaction:
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (ethene + bromine → 1,2-dibromoethane)
The product is a colourless dibromo compound, so the orange bromine water becomes colourless. This is a definitive test for unsaturation in an organic molecule.
What combustion reactions do alkanes and alkenes undergo?
Both alkanes and alkenes burn in oxygen. The completeness of combustion depends on the oxygen supply:
Complete combustion (excess oxygen): CH₄ + 2O₂ → CO₂ + 2H₂O (methane — alkane) C₂H₄ + 3O₂ → 2CO₂ + 2H₂O (ethene — alkene)
Incomplete combustion (limited oxygen): Produces carbon monoxide (CO) — a toxic gas — and soot (carbon particles). Alkenes produce more soot than alkanes of similar chain length because they have a higher carbon-to-hydrogen ratio (lower H:C ratio → sootier flame). A burning alkene typically produces a yellow, smoky flame compared with the bluer, cleaner flame of an alkane.
What addition reactions do alkenes undergo?
The C=C double bond undergoes addition reactions in which two molecules react to give a single product — the double bond "opens" and new atoms bond to the two carbon atoms.
| Reagent | Reaction | Product |
|---|---|---|
| Hydrogen (H₂), heat + Ni catalyst | Hydrogenation | Alkane (saturated) |
| Bromine water (Br₂) | Bromination | Dibromo compound (colourless) |
| Steam (H₂O), H₃PO₄ catalyst, heat | Hydration | Alcohol (e.g. ethanol from ethene) |
The hydration of ethene to produce ethanol is an important industrial process: ethene gas and steam at about 300 °C and 60–70 atm over a phosphoric acid catalyst produces ethanol efficiently.
Frequently asked questions
What does "saturated" mean in chemistry?
A saturated organic compound contains only single bonds between carbon atoms, so the carbon atoms are bonded to the maximum possible number of hydrogen atoms — they are "saturated" with hydrogen. Alkanes are saturated hydrocarbons. An unsaturated compound (like an alkene) contains at least one double or triple bond between carbons, meaning fewer hydrogen atoms are bonded to each carbon than the maximum possible.
Why do alkanes not decolourise bromine water but alkenes do?
Alkanes have only strong C–C and C–H single bonds with no regions of particularly high electron density. Bromine (Br₂) cannot easily attack these bonds under normal conditions, so the orange bromine water remains unchanged. Alkenes have a C=C double bond — a region of high electron density that readily attracts and reacts with electrophiles like bromine. The bromine molecule adds across the double bond in a rapid addition reaction, consuming the bromine and producing a colourless product.
How is ethene used to make ethanol industrially?
Ethene (from the cracking of crude oil fractions) reacts with steam in the presence of a phosphoric acid catalyst at around 300 °C and 60–70 atmospheres pressure. This hydration reaction adds a water molecule across the C=C double bond: CH₂=CH₂ + H₂O → CH₃CH₂OH (ethanol). This process is continuous and efficient. An alternative method is the fermentation of sugars by yeast, which is slower but uses renewable feedstocks and is used for beverages and some fuel ethanol.
What is the trend in boiling points across alkane homologues?
As the number of carbon atoms in an alkane increases, the boiling point rises. Longer carbon chains have greater surface area, which means stronger intermolecular forces (van der Waals/London dispersion forces) between molecules. More energy is required to separate the molecules from each other, raising the boiling point. This is why short alkanes (methane, ethane, propane) are gases at room temperature, medium-length ones (pentane to nonane) are liquids, and very long-chain alkanes (waxes) are solids.
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