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Unsaturated Hydrocarbon

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms.

In one line: Double or triple bond means unsaturated.

Understand it

Unsaturated hydrocarbons are more reactive than saturated hydrocarbons because double and triple bonds can open up for addition reactions. Ethene and ethyne are common examples. In school tests, unsaturation is often checked using bromine water or alkaline potassium permanganate, which get decolourised by such compounds.

Example

Ethene, C₂H₄, is an unsaturated hydrocarbon because it contains a carbon-carbon double bond.

Rule to remember

Open-chain alkenes generally follow CnH2n, and alkynes generally follow CnH2n-2.

The important condition is the presence of double or triple carbon-carbon bonds.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Ethene has the formula C₂H₄.
2Because it contains a double bond between the two carbon atoms, it is unsaturated and can undergo addition reactions easily.

Method you can reuse

1Check the bond between carbon atoms.
2Identify double or triple bonds.
3Match the compound with the unsaturated family.
4Use the bond type to explain higher reactivity.

Sketch it in your answer

Draw ethene with a double bond or ethyne with a triple bond to highlight the unsaturated part clearly.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What bond pattern tells you that a hydrocarbon is unsaturated?

Think first, then show answer

A hydrocarbon is unsaturated if it contains at least one double bond or triple bond between carbon atoms. That bond pattern is the main sign of unsaturation.

Common wrong answer

A common wrong answer is to call propane unsaturated because it burns well. Burning is not the test for unsaturation; the bond pattern is.

Don't mix this up

Students often think any hydrocarbon with many hydrogens is saturated. The actual bond pattern must be checked first.

Exam tip

Look for C=C or C≡C. If either is present, the hydrocarbon is unsaturated.