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Alkanes: Structure and Conformation. Compounds Contain Only C, H General Formula: C n H 2n+2 (Saturated) Common Source of Alkanes: Petroleum Separation Technique: Fractional Distillation Boiling Point (Size) Method of Separating Basic Building Block of More Complex Organics.

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Alkanes structure and conformation l.jpg
Alkanes: Structure and Conformation

  • Compounds Contain Only C, H

    • General Formula: CnH2n+2 (Saturated)

  • Common Source of Alkanes: Petroleum

  • Separation Technique: Fractional Distillation

  • Boiling Point (Size) Method of Separating

  • Basic Building Block of More Complex Organics

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Alkane Shape: Straight Chains

  • “Straight Chain” Better Termed “Unbranched”

  • Actually Zig-Zag Structures (Tetrahedral Carbon Atoms)

  • All Carbons sp3 Hybridized

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Branched Alkanes: Simple

  • Constitutional Isomers: Same Formula; Different Connectivity

    • Butane and Isobutane: C4H10

    • Pentane, Isopentane, Neopentane: C5H12

    • Different Properties: BP, MP, Density, Refractive Index etc.

    • Number of Constitutional Isomers Increases w/ # of Carbons

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Alkane Nomenclature: The Rules

Unbranched Alkanes

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Alkyl Group Nomenclature

Unbranched Alkyl Groups

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Branched Alkanes

  • Locate Longest Continuous Chain (Parent Name)

  • Number Carbons in Chain; Begin @ End Nearest Substituent

  • Use Number of C on Parent Chain; Locate Substituents

  • Assign C Number to Each Substituent

  • Use Same C Number If Multiple Substituents

  • If Substituents Identical, Use Di-, Tri-, Tetra- Designations

  • Equal Length Chains Compete, Parent Chain Most Substituted

  • First Branches Equivalent; Choose Lowest Possible # Set

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Branched Alkanes: Examples

  • Once Long Chain Found; Simplify w/ Alkyl Abbreviations

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Branched Alkyl Groups

  • Can Name as Simple Alkane w/ “yl” Ending Replacing “ane”

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Alkyl Halides

  • Alkyl Halides Named as “Halo”-Alkane

    • “Halo” = Fluoro, Chloro, Bromo, Iodo

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  • Note the “-ol” Ending

  • Use Same Di-, Tri-, Tetra- to Indicate Multiple

  • We’ll Designate Alcohols as Priority Groups (Give Low #)

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Monocyclic Alkanes/Alcohols

Lowest Number


  • “Cyclo” Added  Indicates Cyclic Structure

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Terminal Alkenes

Feature an “ene” Ending

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Properties of Alkanes

  • Boiling Point Increases Regularly for Unbranched

  • Branching Lowers Boiling Points (Van der Waals, SA)

  • Melting Point in Unbranched Increases Regularly Within

  • ODD or EVEN Numbered Series (Not Both)

  • Density Less Than 1.0 g/mL (Less Than H2O)

  • Solubility: Quite Insoluble in H2O; Less Dense  Float

    • Non Polar (Like Dissolves Like)

    • No Hydrogen Bonding

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Sigma Bonds in Hydrocarbons

  • Alkane Sigma (s) Bonds Formed From sp3 Hybridized C

  • These Bonds Can Freely Rotate

  • Temporary Shapes Adapted via Rotation: CONFORMERS

  • Conformers can have Different Energies

  • Need to Have a System for Depicting Various Conformations

    • Newman Projections

    • Sawhorse Formulas

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Newman Projections/Sawhorse Formula

  • R Groups 180° Apart (Anti Conformation)

  • 4 Atom Angle (3 Bond Angle) is a Dihedral (Torsional) Angle

  • Having Large Groups Anti is Low in Energy

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Newman Projections/Sawhorse Formula

  • When R Groups 0° Apart (Ecclipsed Conformation)

  • Having Large Groups Anti is High in Energy

  • Can Adapt any Range of Conformations in Between

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Conformational Analysis: Butane


  • Can Plot these Points and Connect w/ Curve: PES

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Cyclohexane Conformation

  • Cyclohexanes Adapt Chair Conformations (Boats and Others)

  • Ax: Axial (Straight up and Down on Chair)

  • Eq: Equatorial (Parallel to Next Bonds over in Chair)

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Cyclohexane Conformation: Cis

  • Methyls are Cis (Same); Two Energy Equivalent Chairs

  • Ring Flips Interchange Chair Conformations

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Cyclohexane Conformation: Trans

  • Methyls are Cis (Same); Two Energy Inequivalent Chairs

  • Diequatorial Much More Stable than Diaxial Conformation

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Cyclohexane Conformation: Notes

  • With Multiple Same Substituents; More Equatorial = Better

  • Larger Groups Tend to Adopt Equatorial Positions

  • tert-butyl Groups Nearly Always Equatorial

    • Conformation Setters (Lock Ring)

    • Set Other Groups Relative to tert-butyl

  • Fused Rings (Decalin, for Example) Can be Drawn as Chairs

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Reactions: Hydrogenation

  • Addition of Hydrogen (H2) Across a Multiple (p) Bond

  • Ethanol (CH3CH2OH) is a Common Solvent

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Reactions: Alkyl Halide Reducation

  • Zn Transfers Electrons to C of Alkyl Halide

  • Alkyl Halide is Reduced (Reduction = Gains Electrons)

  • Zn: Good Two Electron Donor (Reductant, Reducing Agent)

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Reactions: Alkylation of Terminal Alkynes

  • NaNH2 (-NH2) to Deprotonate Alkyne (Acid/Base Reaction)

  • Anion Reacts with Alkyl Halide (Bromide); Displaces Halide

  • Alkyl Group Added to Alkyne

  • Alkyl Halide Must be 1° or Me; No Branching at 2nd (b) Carbon