pyramidalization

Examples of pyramidalization in the following topics:

• The Configuration of Free Radicals

  • Since the difference in energy between a planar radical and a rapidly inverting pyramidal radical is small, radicals generated at chiral centers generally lead to racemic products.
  • However, unlike carbocation intermediates, which prefer to be planar, radicals tolerate being restricted to a pyramidal configuration.
  • Initial formation of a carboxyl radical is followed by loss of carbon dioxide to give a pyramidal bridgehead radical.
  • Although this is a 3º-alkyl halide, it does not undergo SN1 solvolysis reactions because of the strain imposed on the carbocation intermediate by its pyramidal confinement.

• Stereogenic Nitrogen

  • As noted earlier, single-bonded nitrogen is pyramidal in shape, with the non-bonding electron pair pointing to the unoccupied corner of a tetrahedral region.
  • However, pyramidal nitrogen is normally not configurationally stable.

• The Shape of Molecules

  • Of course, it is the configuration of atoms (not electrons) that defines the the shape of a molecule, and in this sense ammonia is said to be pyramidal (not tetrahedral).
  • For purposes of discussion we shall consider three other configurations for CH4, square-planar, square-pyramidal and triangular-pyramidal.
  • Since the tetrahedral, square-planar and square-pyramidal configurations have structurally equivalent hydrogen atoms, they would each give a single substitution product.
  • However, in the trigonal-pyramidal configuration one hydrogen (the apex) is structurally different from the other three (the pyramid base).

• Reactive Intermediates

  • Carbanions are pyramidal in shape (tetrahedral if the electron pair is viewed as a substituent), but these species invert rapidly at room temperature, passing through a higher energy planar form in which the electron pair occupies a p-orbital.
  • Radicals are intermediate in configuration, the energy difference between pyramidal and planar forms being very small.

• Nomenclature and Structure of Amines

  • In contrast, atropine, coniine, morphine, nicotine and quinine have stereogenic pyramidal nitrogen atoms in their structural formulas (think of the non-bonding electron pair as a fourth substituent on a sp3 hybridized nitrogen).
  • The other stereogenic nitrogens are free to assume two pyramidal configurations, but these are in rapid equilibrium so that distinct stereoisomers reflecting these sites cannot be easily isolated.
  • It should be noted that structural factors may serve to permit the resolution of pyramidal chiral amines.
  • An increase in angle strain in the sp2-hybridized planar transition state is responsible for the greater stability of the pyramidal configuration.

• Lone Electron Pairs

  • The lone pair orbital will point toward the fourth corner of the tetrahedron, but since that position will be vacant, the NH3 molecule itself cannot be tetrahedral; instead, it assumes a pyramidal shape, more specifically, that of a trigonal pyramid (a pyramid with a triangular base).

• Applying the VSEPR Model

  • Molecules with a coordination number of 5 are in the shape of a trigonal bipyramid; this consists of two triangular-based pyramids joined base-to-base.
  • In an AX6 molecule, six electron pairs will try to point toward the corners of an octahedron (two square-based pyramids joined base-to-base).
  • Here are the shapes that we will talk about: tetrahedral, trigonal pyramidal, bent, trigonal planar, linear.

• Four-Membered Rings

  • Such electron pair delocalization is diminished in the penicillins, leaving the nitrogen with a pyramidal configuration and the carbonyl function more reactive toward nucleophiles.

• Trihalides: Boron-Halogen Compounds

  • This trend is commonly attributed to the degree of π-bonding in the planar boron trihalide that would be lost upon pyramidalization (the conversion of the trigonal planar geometry to a tetrahedral one) of the BX3 molecule, which follows this trend: BF3 > BCl3 > BBr3 (that is, BBr3 is the most easily pyramidalized).

• Oxidation States of Sulfur Compounds

  • Sulfoxides have a fixed pyramidal shape (the sulfur non-bonding electron pair occupies one corner of a tetrahedron with sulfur at the center).