Distinguishing Alkanes vs Alkenes in Chemistry

• 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.

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