Organic Chemistry - Hydrocarbons
1. Homologous Series
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πΉ Definition:
πΉ A family of organic compounds with the same general formula, similar chemical properties, and a gradual change in physical properties.
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πΉ Key Points:
- πΉ Members differ by a βCHββ group.
- πΉ Examples: Alkanes, Alkenes, Alcohols.
2. Alkanes
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πΉ Definition:
πΉ Saturated hydrocarbons containing only single covalent bonds between carbon atoms.
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πΉ General Formula:
πΉ CβHββββ
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πΉ Properties:
- πΉ Non-polar molecules β insoluble in water, soluble in organic solvents.
- πΉ Generally unreactive due to strong CβC and CβH bonds.
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πΉ Structures & Naming (CββCβ):
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πΉ Unbranched:
- πΉ Methane (CHβ)
- πΉ Ethane (CβHβ)
- πΉ Propane (CβHβ)
- πΉ Butane (CβHββ)
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πΉ Branched Example:
πΉ 2-methylpropane (CβHββ)
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πΉ Unbranched:
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πΉ Reactivity:
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πΉ Combustion:
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πΉ Complete:
πΉ Alkane + Oβ β COβ + HβO
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πΉ Incomplete:
πΉ Produces CO or carbon (soot).
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πΉ Complete:
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πΉ Substitution with Chlorine:
πΉ Occurs in UV light, e.g., CHβ + Clβ β CHβCl + HCl.
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πΉ Combustion:
3. Isomerism
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πΉ Definition:
πΉ Compounds with the same molecular formula but different structural arrangements.
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πΉ Example:
πΉ Butane and 2-methylpropane (CβHββ).
4. Alkenes
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πΉ Definition:
πΉ Unsaturated hydrocarbons containing at least one C=C double bond.
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πΉ General Formula:
πΉ CβHββ
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πΉ Properties:
- πΉ More reactive than alkanes due to double bond.
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πΉ Structures & Naming (CββCβ):
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πΉ Unbranched:
- πΉ Ethene (CHβ=CHβ)
- πΉ Propene (CHβ=CHβCHβ)
- πΉ But-1-ene (CHβ=CHβCHββCHβ)
- πΉ But-2-ene (CHββCH=CHβCHβ)
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πΉ Branched Example:
πΉ 2-methylpropene
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πΉ Unbranched:
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πΉ Manufacture:
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πΉ Cracking:
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πΉ Definition:
πΉ Breaking down large alkanes into smaller alkanes and alkenes.
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πΉ Conditions:
πΉ High temperature + catalyst.
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πΉ Example:
πΉ CββHββ β CβHββ + CβHβ
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πΉ By-product:
πΉ Hydrogen gas.
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πΉ Definition:
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πΉ Cracking:
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πΉ Differences from Alkanes:
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πΉ Bond Type:
πΉ Single bonds only (alkanes) vs at least one double bond (alkenes).
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πΉ Reactivity:
πΉ Less reactive (alkanes) vs more reactive (alkenes).
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πΉ Test:
πΉ No reaction with bromine water (alkanes) vs decolourises bromine water (alkenes).
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πΉ Bond Type:
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πΉ Reactions:
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πΉ Combustion:
πΉ Burns with a smokier flame than alkanes.
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πΉ Polymerisation:
πΉ Forms addition polymers.
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πΉ Addition Reactions:
- πΉ With hydrogen (hydrogenation).
- πΉ With halogens (bromine water decolourises).
- πΉ With steam (forms alcohols).
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πΉ Combustion:
5. Polyunsaturated in Food
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πΉ Definition:
πΉ Contain multiple C=C double bonds in their hydrocarbon chains.
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πΉ Source:
πΉ Oils from plants and fish β healthier than saturated fats.
6. Hydrogenation in Margarine Production
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πΉ Definition:
πΉ Adding hydrogen to unsaturated vegetable oils in the presence of a nickel catalyst.
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πΉ Effect:
πΉ Converts C=C bonds to single bonds, turning oil into semi-solid margarine.
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πΉ Benefits:
πΉ Improves shelf life and texture.
- πΉ Complete combustion of methane: CHβ + 2Oβ β COβ + 2HβO
- πΉ Complete combustion of ethane: 2CβHβ + 7Oβ β 4COβ + 6HβO
- πΉ Incomplete combustion of propane: CβHβ + 3Oβ β 3CO + 4HβO
- πΉ Substitution of methane with chlorine: CHβ + Clβ β CHβCl + HCl (UV light)
- πΉ Cracking of decane: CββHββ β CβHββ + CβHβ
- πΉ Hydrogenation of ethene: CβHβ + Hβ β CβHβ (Ni catalyst)
- πΉ Addition of bromine to ethene: CβHβ + Brβ β CβHβBrβ
- πΉ Hydration of ethene to ethanol: CβHβ + HβO β CβHβ OH (acid catalyst)
- β οΈ Alkanes are completely unreactive (they can react under specific conditions like UV light).
- β οΈ All hydrocarbons react the same way with bromine water (only alkenes decolourise it without heating).
- β οΈ Cracking produces only alkenes (it produces both alkanes and alkenes, sometimes hydrogen).
- β οΈ Polyunsaturated fats are always solid at room temperature (they are usually oils).
- π Memorise general formulas for alkanes (CβHββββ) and alkenes (CβHββ).
- π Be able to identify isomers by drawing different structures for the same molecular formula.
- π Know the bromine water test for unsaturation and how to describe it in words.
- π Remember conditions for cracking: high temperature + catalyst.
- π Draw and name branched hydrocarbons accurately according to IUPAC rules.
- π Explain why alkenes are more reactive using the double bond as the reason.