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.

• 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.

• 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.

  • A carbon-carbon bond exists between carbons 1 and 6 in both limonene and menthol.
  • Carbon-carbon bonds exist in a-pinene between carbons 1 and 6 and carbons 4 and 7.
  • 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).
  • 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.
  • 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".

\