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Chemistry LibreTexts

7.S: Alkenes- Structure and Reactivity (Summary)

  • Page ID
    203920
  • Concepts & Vocabulary

    7.1 Industrial Preparation and Use of Alkenes

    • Breaking up of large hydrocarbon molecules into smaller, useful molecules is called cracking.

    7.2 Calculating Degree of Unsaturation

    • Saturated molecules contain only single bonds and no rings.
    • Saturated hydrocarbons have the formula CnH2n+2, where n can be any integer.
    • Degrees of unsaturation account for the total number of rings and pi bonds in a molecule.
    • Each degree of unsaturation reduces the number of hydrogens in the molecule by 2.

    7.3 Naming Alkenes

    • When the two largest groups are on the same side of the double bond (top or bottom) they are called cis or Z.
    • When the two largest groups are on opposite sides of the double bond (top or bottom) they are called trans or E.
    • Endocyclic double bonds occur when there is a pi bond within a ring.

    7.4 Cis-Trans Isomerism in Alkenes

    7.5 Alkene Stereochemistry and the E, Z Designation

    • E and Z are less limited than cis and trans in naming.
    • E and Z configurations use the same priority rules as R and S (CIP rules).

    7.6 Stability of Alkenes

    • Relative stability of alkenes can be measured by using heats of hydrogenation upon reduction to the related alkane.
    • More substituted alkenes are more stable than less substituted.
    • Alkenes with the largest groups trans are more stable than cis.

    7.7 Electrophilic Addition Reactions of Alkenes

    • In electrophilic addition reactions, the pi bond of the alkene acts as the nucleophile.
    • Electrophilic addition reactions occur faster with larger hydrogen halides as well as more substituted alkenes.

    7.8 Orientation of Electrophilic Additions: Markovnikov's Rule

    • The more substituted carbocation intermediate forms during electrophilic addition reactions, since more substituted carbocations are more stable. This is known as Markovnikov's rule.

    7.9 Carbocation Structure and Stability

    • Molecules or ions that can disperse (delocalize) charge are more stable than structures with charge localized on a single atom.
    • Due to inductive stabilization, carbocation stability follows the order:

    tertiary > secondary > primary > methyl

    • Electron donating groups stabilize carbocations.
    • Electron withdrawing groups destabilize carbocations.
    • Resonance effects can stabilize a carbocation (some examples include benzylic and allylic carbocations).
    • Vinylic carbocations are unstable and are unlikely to form.

    7.10 The Hammond Postulate

    • The Hammond Postulate states that transition state structure most resembles the nearest stable species.
    • Based on the Hammond Postulate, transition states for exothermic reaction steps resemble reactants, while endergonic step transition states resemble products.

    7.11 Evidence for the Mechanism of ELectrophilic Additions: Carbocation Rearrangements

    • Carbocations will rearrange from less stable to more stable isomers through hydride shifts or alkyl shifts.

    Skills to Master

    • Skill 7.1 Calculate degree of unsaturation for organic molecular formulae.
    • Skill 7.2 Draw isomers from a molecular formula.
    • Skill 7.3 Name alkenes following IUPAC rules, including configuration (E, Z).
    • Skill 7.4 Draw structures from IUPAC name.
    • Skill 7.5 Describe bonding in alkenes including bond length, strength, angle and restricted rotation.
    • Skill 7.6 Explain stability of alkenes.
    • Skill 7.7 Rank alkenes in order of stability.
    • Skill 7.8 Draw mechanism for electrophilic addition of HX to alkenes, including regiochemistry.
    • Skill 7.9 Explain stability of carbocations.
    • Skill 7.10 Explain transition states related to the Hammond Postulate.
    • Skill 7.11 Explain products formed by carbocation rearrangements.

    Memorization Tasks

    MT 7.1 Memorize formula for saturated hydrocarbons CnH2n+2.

    MT 7.2 Memorize basic IUPAC naming rules.

    MT 7.3 Memorize relative stability of alkenes.

    MT 7.4 Memorize relative stability of carbocations.

    Contributors

    • Layne Morsch (University of Illinois Springfield)
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