Alkenes: EZ system, nomenclature, stereochemistry, and properties

Structure and importance of alkenes

  • Polymers derive from alkenes (double bond): examples include polyethylene, polypropylene, PTFE (Teflon), PVC, polystyrene, PMMA, neoprene, natural rubber.

  • Monomers contain a C=C that reacts to form macromolecules (plastics).

  • Topics covered: structure of alkenes, cis/trans isomerism, naming (IUPAC and EZ system), and structure–property relationships.

Structure of alkenes

  • Unsaturated hydrocarbon: contains C and H and one or more multiple bonds (C=C, C≡C).

  • Acyclic alkenes general formula: C<em>nH</em>2nC<em>nH</em>{2n} (one double bond; subtract 2 H per additional double bond or ring).

  • Degree of unsaturation describes how many rings/double bonds are present.

  • Geometry: C=C carbons are trigonal planar (VSEPR). Bond angles are ~120exto120^ ext{o}. Substituents can cause slight angle strain (e.g., propene ~124exto124^ ext{o} versus ethene ~121exto121^ ext{o}).

  • Planarity is required for π-bond overlap; p orbitals on each carbon overlap to form the π bond, giving the alkene its rigidity and preventing rotation.

  • Hybridization: each C is sp^2; one σ bond to each neighbor plus a π bond from p orbital overlap.

Stereochemistry: cis/trans and EZ

  • Alkenes with two different substituents on each carbon can have stereoisomerism (cis/trans) when rotation around the C=C is restricted.

  • Cis (Z) vs Trans (E):

    • Cis: higher energy due to steric interactions when substituents are on the same side.

    • Trans: lower energy, more stable, less reactive.

  • Rotation about C=C is not allowed under standard conditions because the π bond requires planarity.

  • EZ system used when there are more than one substituent on each carbon:

    • Determine CIP priorities on each carbon side.

    • E (entgegen): higher-priority groups on opposite sides of the double bond.

    • Z (zusammen): higher-priority groups on the same side.

    • If one side has identical substituents, there is no E/Z designation for that double bond.

Nomenclature of acyclic alkenes (IUPAC)

  • Name the parent chain that contains the double bond; number from the side closest to the double bond to give the lowest possible locant to the double bond.

  • The double bond carbon that receives the first number is assigned that locant (e.g., hex-3-ene, 1-heptene as acceptable forms).

  • For substituted alkenes, name substituents as in alkanes and attach to the parent chain containing the double bond (e.g., 2,3-diethyl-4-methylhex-1-ene).

  • Verification: count carbons to ensure the correct chain length.

Common names for alkenes

  • Methylene replaces the IUPAC name for the methylene group: methylenecyclopentane.

  • Vinyl replaces ethenyl: vinylcyclopentane (instead of ethenylcyclopentane).

  • Allyl replaces 2-propenyl: allylcyclopentane.

  • Use common names as shortcuts for substituent fragments attached to a parent hydrocarbon.

Examples of assigning E/Z configurations

  • Example: 3-chloro-4-methylhex-3-ene

    • Determine priorities on each side of the double bond; if high-priority groups are on the same side, it’s Z; if opposite sides, it’s E.

    • In this example, the configuration is Z.

  • If one side of the double bond has identical substituents (e.g., two methyls), there is no E/Z designation.

  • Practice: analyze multiple double bonds by comparing the highest-priority substituents on each carbon and then across the bond.

Cycloalkenes: cis/trans in rings

  • For cyclic alkenes, cis/trans can describe substituent positions around the double bond.

  • Cis: substituents on the same side of the double bond.

  • Trans: substituents on opposite sides.

  • Ring strain: in small rings, trans configurations can be less feasible; transcyclooctane exists but involves considerable strain.

Physical properties of alkenes

  • Intermolecular forces: dispersion (London/Van der Waals) forces dominate.

  • Alkenes are colorless, nonpolar, insoluble in water, and typically less dense than water.

  • Unsaturation affects packing and melting points:

    • Saturated chains (no double bonds) pack well, yielding higher melting points.

    • Each cis double bond disrupts packing, reducing dispersion forces and lowering melting points.

    • Example trends with C18 chains:

    • Stearic acid (C18:0) mp ≈ 70extoC70^ ext{oC} (fully saturated).

    • Oleic acid (C18:1, cis) mp ≈ 13extoC13^ ext{oC} (one cis double bond).

    • Polyunsaturated cis fatty acids mp ≈ −17extoC-17^ ext{oC} (multiple cis double bonds).

  • Overall: more unsaturation, especially cis, lowers melting temperature due to poorer packing.”