Alkanes and alkenes are two fundamental families in organic chemistry. Although their formulas are similar, their bonding and reactivity differ significantly. IB Chemistry students must know how to distinguish them using both structural features and experimental tests. This article explains everything clearly and equips you with reliable ways to tell these homologous series apart.
Structural Difference Between Alkanes and Alkenes
The key structural difference is simple:
Alkanes
- Contain only single C–C bonds
- Saturated hydrocarbons
- General formula: CₙH₂ₙ₊₂
Alkenes
- Contain at least one C=C double bond
- Unsaturated hydrocarbons
- General formula: CₙH₂ₙ
This double bond gives alkenes unique reactivity patterns that alkanes do not share.
How the Double Bond Changes Reactivity
Alkenes undergo addition reactions because the C=C bond is electron-rich and can easily be broken to form new bonds.
Alkanes undergo substitution reactions, typically requiring UV light and halogens.
This difference forms the basis of the most famous test used to distinguish them.
The Bromine Water Test (Most Common IB Method)
Alkene + bromine water
- Orange/brown bromine water decolorizes
- The reaction is rapid
- Addition reaction occurs:
C=C + Br₂ → dibromo compound
Alkane + bromine water
- No reaction in the dark
- Bromine water stays orange/brown
- Only reacts in UV light via substitution
This test is the standard IB Chemistry answer because it directly links structure to reactivity.
Why Alkenes React but Alkanes Don’t
The C=C bond has:
- A strong σ-bond
- A weaker π-bond
The π-electrons are exposed above and below the plane of the carbon atoms.
Halogens attack this electron-rich site easily, opening the double bond.
Alkanes have only strong σ-bonds, which do not break easily.
This makes them far less reactive under normal conditions.
Physical Differences Between Alkanes and Alkenes
Although the bromine test is the most reliable method, there are also subtle physical differences:
Boiling Point
- Increases with chain length in both
- No simple universal difference between the two groups
Geometry
- Alkane carbons are tetrahedral (109.5°)
- Alkene carbons are trigonal planar (120° around the double bond)
Combustion Flame
- Alkenes tend to burn with a smokier flame because they are more unsaturated
- Alkanes burn more cleanly
Not a formal test but sometimes mentioned.
Laboratory Methods in IB Chemistry
IB-approved methods for distinguishing the two include:
1. Bromine Water Test (Primary Test)
- Rapid, clear, and directly linked to double bonds
- Most accepted exam answer
2. Reaction with Acidified Potassium Permanganate (KMnO₄)
- Alkenes turn purple KMnO₄ colorless/brown, forming diols
- Alkanes show no reaction
3. Infrared (IR) Spectroscopy
- Alkenes: clear C=C stretch around ~1650 cm⁻¹
- Alkanes: no such peak
Spectroscopy questions sometimes appear in IB Paper 2.
Summary Table
Feature Alkane Alkene Key bond C–C single C=C double Reactivity Low High Main reaction type Substitution Addition Bromine water No change Decolorizes Saturation Saturated Unsaturated
(This is presented textually to avoid table formatting restrictions.)
FAQs
Can an alkene behave like an alkane?
Yes, once the double bond reacts (e.g., through addition), the resulting product may have only single bonds, behaving like an alkane. But initially, the double bond defines its reactivity.
Are all hydrocarbons with double bonds alkenes?
Not necessarily—alkynes have triple bonds. But among the simplest double-bonded hydrocarbons, alkenes are the category IB focuses on.
Will cycloalkanes react with bromine water?
Cycloalkanes generally do not decolorize bromine water in the dark, so they behave like alkanes despite their formula differing slightly.
Conclusion
You can distinguish alkanes and alkenes reliably by using the bromine water test, where alkenes decolorize bromine due to the reactive C=C double bond, while alkanes do not. Structural and spectroscopic differences further support identification. Mastering these distinctions prepares you to analyze organic reactions, identify functional groups, and handle IB Chemistry exam questions with confidence.
