Saturated & Unsaturated Compounds
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Saturated & Unsaturated Compounds Saturated Compound A hydrocarbon compound in which all carbon-carbon bonds are single bonds (C-C). The molecule contains the maximum possible number of hydrogen atoms for the number of carbon atoms present.
Characteristics of Saturated Compounds Only single C-C bonds: All carbon atoms are joined by single covalent bonds Maximum hydrogen atoms: Cannot add any more hydrogen without breaking the carbon chain General formula: Alkanes follow C n H 2n+2 Less reactive: Do not undergo addition reactions easily "Saturated" with hydrogen: No room for additional atoms without removing existing ones Examples of Saturated Compounds (Alkanes) [Diagram: Displayed formulae of methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) showing all single C-C and C-H bonds] Name Molecular Formula Structural Formula Type of Bonds Methane CH₄ CH₄ Only C-H single bonds Ethane C₂H₆ CH₃-CH₃ C-C and C-H single bonds Propane C₃H₈ CH₃-CH₂-CH₃ C-C and C-H single bonds Butane C₄H₁₀ CH₃-CH₂-CH₂-CH₃ C-C and C-H single bonds Unsaturated Compound A hydrocarbon compound that contains at least one carbon-carbon double bond (C=C) or triple bond (C≡C).
The molecule can potentially add more hydrogen atoms. Characteristics of Unsaturated Compounds Contains C=C or C≡C bonds: At least one multiple bond between carbon atoms Fewer hydrogen atoms: Has less than the maximum possible number of hydrogen atoms Can undergo addition reactions: The double/triple bond can break to add new atoms More reactive than saturated compounds: Multiple bonds are reactive sites "Unsaturated" with hydrogen: Can add more hydrogen through addition reactions Examples of Unsaturated Compounds (Alkenes) [Diagram: Displayed formulae of ethene (C₂H₄), propene (C₃H₆), and butene (C₄H₈) showing C=C double bonds] Name Molecular Formula Structural Formula Type of Bonds Ethene C₂H₄ CH₂=CH₂ One C=C double bond Propene C₃H₆ CH₃-CH=CH₂ One C=C double bond, C-C single bonds Butene C₄H₈ CH₃-CH₂-CH=CH₂ One C=C double bond, C-C single bonds Pentene C₅H₁₀ CH₃-CH₂-CH₂-CH=CH₂ One C=C double bond, C-C single bonds Comparing Saturated and Unsaturated Compounds [Diagram: Side-by-side comparison of ethane (C₂H₆) with single C-C bond and ethene (C₂H₄) with C=C double bond, highlighting the difference in hydrogen content] Property Saturated Compounds (Alkanes) Unsaturated Compounds (Alkenes) Carbon-carbon bonds Only single bonds (C-C) At least one double bond (C=C) General formula C n H 2n+2 C n H 2n (for one C=C) Hydrogen content Maximum possible hydrogen atoms Less hydrogen atoms than maximum Reactivity Less reactive (mainly combustion and substitution) More reactive (addition reactions) Test with bromine water No visible change (remains orange/brown) Decolorizes bromine water (turns colorless) Examples Methane, ethane, propane, butane Ethene, propene, butene Testing for Unsaturation - Bromine Water Test The bromine water test is used to distinguish between saturated and unsaturated compounds: Saturated compound (alkane): Bromine water remains orange/brown - no reaction in the dark Unsaturated compound (alkene): Bromine water turns from orange/brown to colorless - addition reaction occurs [Diagram: Two test tubes showing bromine water test - one with alkane (remains orange), one with alkene (becomes colorless)] Addition Reaction with Bromine C₂H₄ + Br₂ → C₂H₄Br₂ The C=C double bond in ethene opens up to add two bromine atoms, forming 1,2-dibromoethane Why Unsaturated Compounds Are More Reactive The C=C double bond consists of: One sigma (σ) bond: Strong bond formed by direct overlap One pi (π) bond: Weaker bond formed by sideways overlap of p orbitals The π bond is weaker and more easily broken, making the C=C double bond a reactive site that can undergo addition reactions.
Converting Between Saturated and Unsaturated Compounds Unsaturated → Saturated (Hydrogenation): Adding hydrogen (H₂) to an alkene in the presence of a nickel catalyst converts it to an alkane [Diagram: Reaction showing ethene + H₂ → ethane, with Ni catalyst, illustrating addition of H atoms across C=C bond] Hydrogenation Example C₂H₄ + H₂ → C₂H₆ Ethene (unsaturated) + Hydrogen → Ethane (saturated) Saturated: Only single C-C bonds, maximum hydrogen atoms (C n H 2n+2 ) Unsaturated: Contains C=C or C≡C bonds, fewer hydrogen atoms (C n H 2n ) Reactivity: Unsaturated compounds are more reactive than saturated ones Bromine water test: Distinguishes saturated (no color change) from unsaturated (decolorizes) Addition reactions: Unsaturated compounds can add atoms across the double bond Hydrogenation: Converts unsaturated to saturated by adding H₂
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