Chapter 5 - Alkenes: Bonding, Nomenclature, and Properties

  • The configuration about each double bond in these representations is cis.
      * Trans Cyclooctene is the smallest trans cycloalkene produced in pure form that is stable at room temperature.
      * Even in this trans cycloalkene, there is significant angle strain; the 2p orbitals of the double bond create an angle of 448 to each other.
      * Ciscyclooctene is 38 kJ (9.1 kcal)/mol more stable than its trans isomer, as shown in the attached image.
  • Even though it lacks a chiral core, the trans isomer is chiral.
      * A bridgehead carbon is carbon found in both rings of a hydrocarbon.
      * For example, the carbons with arrows indicated below are at bridgeheads.
      * The double bond in norbornene does not have a bridgehead carbon, whereas the double bond in the other picture has.
  • Becausethealkenecannotbeplanarwhiletherestofthecarbonsinthebicyclicsystemspantherings,thisdoublebondconfigurationcausessignificantstrain.Because the alkene cannot be planar while the rest of the carbons in the bicyclic system span the rings, this double bond configuration causes significant strain.

   

  • Terpenes: demonstrate a key feature of biological systems' molecular logic.
      * Terpene research reveals the incredible diversity that nature can create from a simple carbon skeleton.
      * Small subunits are bound together enzymatically via an iterative process in the construction of big molecules, and then changed by following precise enzyme-catalyzed reactions.
  • Inthelaboratory,chemistsapplythesameprinciples,buttheirmethodslacktheaccuracyandselectivityofenzyme−catalyzedreactionsinlivesystems.In the laboratory, chemists apply the same principles, but their methods lack the accuracy and selectivity of enzyme-catalyzed reactions in live systems.
      * The terpenes you are most likely familiar with, at least by odor, are components of so-called essential oils derived by steam distillation or ether extraction of various plant parts.
      * Essential oils include the low-molecular-weight compounds that are responsible for the distinctive plant aromas.
      * In fragrances, several essential oils, particularly those derived from flowers, are employed.
  • Head-to-tail bonds between isoprene units are far more abundant in nature than head-to-head or tail-to-tail connections.
      * The structural formulae of five more terpenes formed from two isoprene units.
      * Geraniol and myrcene share the same carbon backbone.
      * The carbon atoms in myrcene and geraniol are cross-linked to form cyclic structures in the last four terpenes of the image attached.
  • The carbon atoms of the geraniol skeleton are numbered 1 through 8 to assist you to identify the places of cross-linkage and ring formation.
  • This numbered system is intended to indicate crosslinking places in the remaining terpenes.
      * Acarbon−carbonbondexistsbetweencarbons1and6inbothlimoneneandmenthol.A carbon-carbon bond exists between carbons 1 and 6 in both limonene and menthol.
      * Carbon−carbonbondsexistina−pinenebetweencarbons1and6andcarbons4and7.Carbon-carbon bonds exist in a-pinene between carbons 1 and 6 and carbons 4 and 7.
      * Theyarefoundincamphorbetweencarbons1and6andcarbons3and7.They are found in camphor between carbons 1 and 6 and carbons 3 and 7.
      * In the attached image, myrcene is illustrated.

(a) structural formula and

(b) ball-and-stick model

 

  • One of the unifying concepts of organic chemistry is that molecules having electron rich regions, often lone pairs or bonds, exhibit distinct patterns of reactivity.
      * Similarly, molecules with electron-poor regions or weak bonds exhibit distinct reactivity patterns.
      * Three distinct sets of words are used to characterize such electron-rich and electron-poor entities.
  • Chapter 4 introduced the Brnsted-Lowry and Lewis acid and base definitions in the context of acid-base chemistry.
      * Proton transfers are the only ones covered by the Brnsted-Lowry definitions.
      * Chemists might refer to the reactants in other reactions as Lewis acids and bases (as shown in the image attached).
      * RememberthataLewisacidisaspeciesthatcanacceptanelectronpairfromaLewisbasebecausetheLewisacidhasanemptyorbitalwhiletheLewisbasecontainstheelectronpair.Remember that a Lewis acid is a species that can accept an electron pair from a Lewis base because the Lewis acid has an empty orbital while the Lewis base contains the electron pair.
      * Thecoordinationofammoniatoboraneisoneexample(asshownintheimageattached).The coordination of ammonia to borane is one example (as shown in the image attached).
  • A Brnsted-Lowry acid, such as H-X, also contains an empty orbital (the antibonding H-X sigma orbital) that may take a lone pair from a base, thus breaking the bond.
      * Clearly, the Lewis acid-base definition is more expansive!
      * In fact, it is so wide that many chemists consider most reactions (save those involving radicals) to be Lewis acid-base interactions.
      * In practice, however, most chemists refer to Brnsted-Lowry acid-base reactions as proton transfers, and we shall use this nomenclature throughout this book.
      * In reality, beginning in Chapter 6, the Brnsted-Lowry base will most likely be an organic functional group, such as an alkene, alcohol, or ester.
      * To explain proton transfers, we shall use phrases such as "add a proton" or "take a proton away".