Resonance illustrating orbital overlap

Olah received the Nobel Prize in 1994 for proving its existence. Note that some textbooks still use erroneous nomenclature. Cations are very reactive, but can be stabilized by inductive or resonance effects. Resonance stabilization is illustrated in benzyl and allyl cations (Figures 1.6 and 1.7). The double bond in the ally lie cation is also in sp hybridization, with the p orbitals overlapping. Sp hybridization in ethylene is shown in Figure 1.8. 

The anomeric effect in terms of a stabilizing effect can be illustrated by the concept of "double-bond - no-bond resonance" (14, 15) shown by the resonance structures 4 and 2 or by the equivalent modern view (16, 17) that this electronic delocalization is due to the overlap of an electron pair orbital of an oxygen atom with the antibonding orbital of a C —OR sigma bond (12). 

It is well known that in many brominations and protonations of cyclohexenols (91) axial entry is favored (Eliel et al., 1965). This is attributed to the parallel alignment of the v orbitals on the three centers. The overlap preference is well illustrated in the oxidation of allyl vs. saturated alcohols. Normally, axial alcohols are oxidized more rapidly by chromic acid than equatorial alcohols. In the absence of large strain factors, equatorial allyl alcohols are oxidized faster than axial alcohols by chromic acid hydrogen is abstracted in the rate-determining step. The contribution of a-j8 ketonic resonance lowers the activation energy,... 

Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms. As a result, the jc electrons are not localized between the carbon atoms of the double bonds, but rather delcx alized over four atoms. This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present. This shortens the bond. Drawing resonance structures illustrates this delcx alization. 

Why is a conjugated diene more stable than an isolated diene Because a conjugated diene has overlapping p orbitals on four adjacent atoms, its 71 electrons are delocalized over four atoms. This delocalization, which cannot occur in an isolated diene, is illustrated by drawing resonance structures. 

However, experiments show that all the carbon-carbon bonds in benzene are the same. In other words, benzene is a molecule with resonance structures. Figure 5 illustrates how the electron orbitals in benzene overlap to form continuous molecular orbitals known as delocalized clouds. The following structural formula is often used to show the ring structure of benzene. 

Figure 2.1 Preferred conformations of butane and 1,2-difluoroethane, illustrating the gauche effect. In the gauche conformation each C-F a orbital can overlap with a C-H a orbital on the vicinal carbon, corresponding to no-bond resonance as shown. Such no-bond resonance would be disfavoured if the C-F bonds were trans, since it would remove electron density from an electronegative element. The effect is strong enough in this case to overcome the electrostatic repulsion between the two C-F dipoles, which favours the trans form.

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