Axial and equatorial lone pairs

Heteroatoms in rings have axial and equatorial lone pairs... 

Table 5.1 The NBO s-character, hybridization, and energy of all lone pairs (X=N, O, S) in selected saturated heterocycles at the B3LYP/6-31C level, the NBO plots of the lone pairs and s-character in C-X bonds. The axial and equatorial lone pairs are drawn as dissected by H -C3-Xi or H -C3-XI planes, respectively. If two lone pairs are present at an atom, data for equatorial lone pair are given in parenthesis.

Figure 5.13 NBO energies (in a.u., 1 a.u. = 627.5kcai/moi) of axial and equatorial lone pairs in oxa-, thia-,...

For oxathiane 1, lone pair selectivity is controlled by steric interactions of the gem-dimethyl group and an anomeric effect, which renders the equatorial lone pair less nucleophilic than the axial lone pair. Of the resulting ylide conformations, 25a will be strongly preferred and will react on the more open Re face, since the Si face is blocked by the gem-dimethyl group (Scheme 1.9) [3, 15]. 

Second-row heteroatoms are known to show a substantial anomeric effect. There appears to be evidence for a reverse anomeric effect in 2-aminotetrahydro-pyrans. ° It has been called into question whether a reverse anomeric effect exists at all. ° In 94, the lone-pair electrons assume an axial conformation and there is an anomeric effect. In 95, however, the lone-pair electron orbitals are oriented gauche to both the axial and equatorial oc-CH bond and there is no anomeric effect. ... 

The difference between equatorial and axial positions determines the arrangement of bonding pairs and lone pairs around an atom with a steric number of 5. An example is provided by sulfur tetrafluoride, a colorless gas that has industrial uses as a potent fluorinating agent. The Lewis structure of SFq shows four S—F bonds and one lone pair of electrons on the sulfur atom. These five pairs of electrons are distributed in a trigonal bipyramid around the sulfur atom. 

When the steric number is 5, there are two distinct positions, equatorial and axial. Placing lone pairs in equatorial positions always leads to the greatest stability. Thus, CIF3 is T-shaped with two equatorial lone pairs. 

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

The length of the coordinative bond (2.86 A) corresponds to a normal Sb-Sb single bond. The coordination geometries of the donor or acceptor antimony atoms are distorted tetrahedral for the former and pseudo trigonal bipyramidal for the latter with the iodine atoms in axial (I-Sb-I 169.71°) and the lone pair, the Me3Sb and the methyl group in equatorial positions. 

Conformational analysis in connection with determinations of ffee-energy differences (AG°) between axial and equatorial conformers is still attracting interest. Schneider and Hoppen (114) discussed A values ( —AG°) and preferred orientations of axial substituents with lone pairs at heteroatoms directly attached to C (e.g., -OR, -NR2, and -N3), as well as of some other nonspherical substituents (X = -NC, -NCS, -CN, -C CH). Phenyl and vinyl groups were investigated by Eliel and Manoharan (277), who found A values of 2.87 0.09 kcal/mol for phenyl and 1.68 0.06 kcal/mol for vinyl. The latter value was essentially confirmed by Buchanan (196) the formyl group A = 0.84 0.08 kcal/mol) in axial position adopts a predominant (93%) conformation (305) with the plane of the axial CHO group nearly perpendicular to the plane of symmetry of the cyclohexyl residue (Scheme 71) (196). 

Fig. 7. Relative orientations of nitrogen and phosphorus lone-pairs in cyclotriphosph(lll)azanes with (59) (axial) and (60) (equatorial) phosphorus substituents, X...

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