Alkyne and Alkene Reactivities

Although ethylene has similar 0 orbitals, these are not only appreciably different in energy from the n orbital of ethylene, but the overlap between the hydrogen orbitals and the ethylene n orbitals is smaller upon bending both effects cause much less mixing of these orbitals upon deplanarization of ethylene. 

The fact that the w orbital drops drastically on bending is, of course, related to the fact that the vinyl anion (sp2 lone pair) is more stable than the ethyl anion (higher energy sp3 lone pair). That is, upon bending, more s character is mixed into the LUMO of the alkyne, which ultimately becomes the sp2 anion lone pair upon addition of a nucleophile. However, our treatment also explains why nucleophilic attack on alkynes is preferred even for early transition states or for those reactions, such as cycloadditions and thermal rearrangements, in which anions are not formed. 

The calculations also suggest why tram addition of nucleophiles to alkynes is invariably observed, and why the apparently unusual trajectories proposed by Baldwin48 for such attacks occur. Table 3 shows the energy changes which occur upon various distortions of acetylene and ethylene, as well as energies of interaction of the various geometries of these species with hydride ion, a model nucleophile. 

Each bend was 25° for acetylene, or 11.9° for the HCH bisector of ethylene, each corresponding to a 42% geometry change from acetylene to ethylene, or ethylene to ethane. The H CC angles were optimized and are shown in parentheses. 

The greater bending of acetylene is more than compensated for by the increased stabilization upon interaction, a result which we attribute to increased hydride HOMO-acetylene LUMO interactions. 7rans-bending is strongly favored over one-end or as-bending, even for this very early transition state, since a constrained c/s-bent transition state collapses to the frans-bent without activation upon attempted optimization. 

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