Organic Chemistry I - Structure and Properties of Alkanes - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Structure and Properties ofAlkanes1.Alkanes: Natural SourcesAlkanes occur naturallyand are mainly obtained fromnatural gasandpetroleum.1.1Natural GasNatural gas is made upmostly of methane (CH₄).It also containssmall amounts of other low-molecular-weight alkanes, such as ethane andpropane.Because these alkanes are small and light, they exist mainly asgasesunder normal conditions.1.2Petroleum (Crude Oil)Petroleumis themain source of most alkanes.It is a complex mixture containing many different hydrocarbons.When petroleum is processed bydistillation:•Lighter fractionsare separated first and formliquid alkanes•Theheavier residueleft behind containssolid alkanesThis separation allows alkanes of different sizes and properties to be collected and used.Key Takeaways•Alkanes come primarily fromnatural gas and petroleum•Natural gas contains mostlymethane•Petroleum suppliesliquid and solid alkanes•Distillation separates alkanes based onmolecular weight

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Study Guide2.Alkanes: PreparationsIn practice, alkanes arerarely prepared from other compoundsbecause it is usuallynoteconomical. Most alkanes are obtained directly fromnatural gas and petroleum.However, from atheoretical and laboratory point of view, alkanescanbe prepared using severalchemical methods. Ignoring cost and practicality, the main preparation methods are described below.1. From Unsaturated Compounds (Catalytic Reduction)Alkenes and alkynescan be converted into alkanes byadding hydrogenin the presence of acatalyst.•Hydrogen gas (H₂) is added across the multiple bond•A metal catalyst such asplatinum (Pt), palladium (Pd), or nickel (Ni)is used•The double or triple bond is converted into asingle bondThis process is calledcatalytic hydrogenation(or catalytic reduction).Example idea:Alkene + H₂→ Alkane2. From Alkyl Halides (Wurtz Reaction)Alkanes can be prepared bycoupling alkyl halidesusing sodium metal.•The reaction is carried out indry ether•Two alkyl halide molecules join together•Ahigher alkaneis formed

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Study GuideThis reaction is called theWurtz reaction.Key point:•Best suited for preparingsymmetrical alkanes•Less useful for unsymmetrical products due to mixtures3. From Alkyl Halides via Grignard ReagentsAlkyl halides can also be converted into alkanes throughGrignard reagents.This happens in two steps:Step 1: Formation of Grignard Reagent•Alkyl halide reacts withmagnesium (Mg)in dry ether•This forms analkyl magnesium halide(Grignard reagent)Step 2: Hydrolysis•The Grignard reagent reacts withwater (H₂O)•The final product is analkaneImportant idea:Grignard reagents act as strong bases and react readily with water.

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Study Guide4. From Alkyl Halides by ReductionAlkyl halides can be converted directly into alkanes byreduction.•The halogen atom is replaced by hydrogen•Common reducing agents includezinc in acidor other metal–acid systemsThis method removes the halogen and forms aC–H bond.Reaction Schemes vs Chemical EquationsAlthough organic chemists often call these drawings“equations”, they are usuallynot fullybalanced.•Not all reactants or by-products are shown•These diagrams mainly showhow atoms are rearrangedBecause of this, they are more accurately calledreaction schemes, not true chemical equations.

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Study GuideKey Takeaways•Alkanes are rarely synthesized due to cost•They can be prepared from:oUnsaturated compounds (hydrogenation)oAlkyl halides (Wurtz reaction)oGrignard reagentsoReduction of alkyl halides•Most textbook reactions arereaction schemes, not balanced equations3.Alkanes: Molecular and Structural FormulasAlkanesare organic compounds made only ofcarbon and hydrogen atoms, joined together bysingle covalent bonds. Because they follow a consistent pattern, alkanes form ahomologousseries.3.1General Formula of AlkanesAll alkanes have the samegeneral molecular formula:•nrepresents the number of carbon atoms•n can beany whole numberAs the number of carbon atoms increases, the number of hydrogen atoms increases in a predictableway.3.2The Simplest Alkane: MethaneThe simplest alkane ismethane.•Molecular formula:CH₄•It containsone carbon atom•All bonds aresingle covalent bonds

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