An unsaturated compound is one that contains at least one carbon-carbon double bond, as in the alkenes.
An unsaturated compound is one that contains at least one carbon–carbon double bond, C=C, so it does not hold the maximum number of hydrogen atoms. Malay: tak tepu · Chinese: 不饱和.
The double bond is a region of high electron density that can open up, which makes unsaturated compounds reactive: they undergo addition reactions in which atoms add across the C=C and turn it into a single bond. The alkenes are the unsaturated hydrocarbons of this chapter, and their reactivity is the direct opposite of the calm alkanes. This is also why unsaturated compounds can be detected with a simple test.
Example. Ethene (C₂H₄) is unsaturated: its two carbon atoms are joined by a double bond. When ethene is bubbled through bromine water, the brown colour disappears at room temperature as bromine adds across the double bond, the standard test for unsaturation. Acidified potassium manganate(VII) is decolourised in the same way.
Confusion to avoid. Do not confuse unsaturated with saturated. An unsaturated compound decolourises bromine water quickly at room temperature because of its C=C; a saturated compound leaves bromine water unchanged, since it has single bonds only. Remember that it is the double bond, not merely being a hydrocarbon, that gives the positive test.
The reactivity that comes with unsaturation is what makes these compounds so useful in industry. Because the double bond can be opened, ethene is the starting point for making many everyday materials: it can be turned into ethanol by adding steam, hardened with hydrogen to change liquid oils into solid fats, or joined to itself many times over to build long polymer chains. In each case the same feature is doing the work, a double bond that is ready to open and accept new atoms. Seeing unsaturation as an opportunity for the molecule to react, rather than just a structural detail, is what makes the alkene reactions feel like one connected story.
Unsaturation is the key to the whole chemistry of the alkenes, addition, hydrogenation, polymerisation and the bromine-water test all trace back to it, so it is one of the highest-value terms in the Carbon Compounds work that SPM Chemistry examines.
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