Cross-conjugation
As a cross-conjugation (English cross - conjugation ) a special kind of conjugation of double bonds is known in organic chemistry. Run at least three single bonds linked to each other double bonds must be present. One of them has a bifurcation exhibit ( see below for examples ). This is the " classic " definition. Another is based on a specific molecular orbital theory (PMO method according to Dewar ).
Cross-conjugated double bond systems
Once in organic chemistry at the end of the 19th century clarity about the constitution of the main " unsaturated " compounds was achieved, it remained puzzling how the constitution and color of organic substances were related. For example, 1,4- benzoquinone or phorone was yellow, whereas other ketones, such as acetone or acetophenone showed no color.
Johannes Thiele reported in 1900 on reactions of cyclopentadiene with various ketones, in which unsaturated hydrocarbons emerged that he "because of the bright color " fulvenes called (Latin fulvus, brown yellow, red-yellow ). Thiele saw this as proof that "the coloring of organic compounds mainly by the type of arrangement of double bonds is conditional. "
For this particular arrangement of double bonds has the term " crossed - conjugated " naturalized. The term was and Thiele is attributed. In its press release, however, the word " crossed " not before. In contemporary literature there can be found in Hermann Staudinger and Auer.
Since then, numerous molecules have been discovered and synthesized with the structure element " cross-conjugated double bond ".
An explanation of the relationship between constitution and color was made possible by quantum chemistry until the 20th century. It has been recognized that it is confusing to compare the color of the fulvenes and unsaturated carbonyl compounds; because this results from different electronic transitions ( electronically excited states ), namely an n- π * transition ( quinone ) and π - π * transition ( fulvene ).
Crossed - effects of conjugated double bonds
Results in the presence of crossed -conjugated double bonds in a molecule observable effects, and if so, which ones?
To explain the color, or more precisely the absorption of visible or ultraviolet light (UV / VIS spectrum, electronic spectrum ), the term cross-conjugation is likely to be unnecessary now.
Has cross-conjugation significant influence on the structure of organic compounds, that is, heats of formation, bond lengths, bond angles, or dipole moments?
Phelan and Orchin loaded 1968 field work in which they 3 -methylene-1 ,4- pentadiene in terms of VB theory and the then available Hückel approximation ( HMO model ) investigated the hydrocarbon in the MO theory. This unsaturated hydrocarbon can be considered as a prototype of the crossed -conjugated systems, so to speak, an "open fulvene " (Staudinger ). Here, however, different conformations are possible between those in the HMO approximation, no distinction is possible.
Phelan and Orchin found that the Hückel resonance energy is 0,900 ft, and the bond order of the C-1/C-2-Bindung is higher than the reference molecule 1,3-butadiene. For binding C-3/C-6 ( branching ) of the " double-bond character " should be smaller than the butadiene. The authors concluded: " ... in the ground state there is some net bonding or conjugation Between The conjugated centers. "
Experimental data to verify this prediction does not appear to be present. The molecule is very unstable.
Phelan and Orchin extended the concept of cross-conjugation of other connections even urea. In this case, which are "free " (non- binding ) considers pairs of electrons on the nitrogen atoms as equivalents of a C = C π bond. How useful is this extension remains to be seen.
As for the " crossed -conjugated " carbonyl compounds, gave the HMO calculations reveal that, on the whole, the extent of electronic interaction with the second C = C double bond is slim at best.
Today we know the limits of HMO considerations, and it begs the question whether the results of this theory are consistent with experiment. Until this is clear, the term " cross-conjugation " should be used with great caution. He comes from a time before the development of quantum- chemical theories of the chemical bond.