Conjugated double bonds, definition

A complete definition of aromatic compounds must wait until Chapter 16. For the present we will define them as benzene and its substituted derivatives. They are also called arenes. These aromatic compounds have a six-membered ring with three conjugated doable bonds. It is this cycle of conjugated double bonds that makes arenes special. Examples include the following ... 

Our working definition of aromatic compounds has included cyclic compounds containing conjugated double bonds with unusually large resonance energies. At this point we can be more specific about the properties that are required for a compound (or an ion) to be aromatic. 

Attempts to bring about the reaction of the simplest diene (allene) with dialkyl aluminum hydrides led to no definite organoaluminum compounds 123). Little is known so far about the reaction of alkenes containing conjugated double bonds (butadiene 290), isopropene, etc.) with dialkyl alanes. Diisobutylalane and butadiene gave products whose hydrolysis... 

The above definition of the enthalpic effects of conjugation is not unique. A second definition decouples the two double bonds by an alternative hydrogenation process (equation 19). 

The description of conjugated dienes as shown by equation 17 and the associated comparison with butadiene in equation 18 corresponds most closely to the conventional definition. The results are plausible in that groups on one double bond that are c/5-situated relative to the other encourage nonplanarity, cause destabilization and result in lessened conjugation energy. Or so we say. The biggest debit of this approach is that the thermochemistry of the monoenes related by single addition of H2 is often absent. An example... 

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

It is clear from this definition that bond (orbital) conjugation is far more common than bond (electron) delocalization. Conjugation does not always lead to bond delocalization and, accordingly, it is not correct to use the two terms indiscriminately13. It is well known that polyenes are typical examples of double bond conjugation but, as has been demonstrated by Dewar and coworkers, their heats of formation as well as other properties can be reproduced by appropriate bond increments. Thus polyenes are not examples of bond or electron delocalization. 

Jerusalem Symposia on Quantum Chemistry and Biochemistry. Aromaticity, Pseudoaromaticity, Antiaromaticity, Israel Academy of Sciences and Humanities 1971 Vol. 3 E. Heilbronner On Mr Binsch s slide the following definition is proposed A conjugated electron system is called aromatic if it shows neither strong first-order nor second-order double bond fixation . Now, could you point out a molecule, except benzene, which classifies as aromatic G. Binsch Benzene is a perfect example E. Heilbronner Name a second one. 

Another aspect of the geometry of the bound olefin that has been barely studied is the orientation of substituents capable of x interactions with the olefin. The most studied x substituent is the cyano group, but its linearity precludes discussion of the nature of the interaction. The structures of two complexes of diphenylethylenes (IV and XVIII, Table I) have been determined. On the basis of electronic effects one would expect a phenyl ring either to be coplanar with the olefin double bond for better conjugation or to be perpendicular to the metal-olefin plane for greatest x overlap (in the cyclopropane model). The limited evidence favors the second orientation. However, structural studies of olefins with substituent groups such as -COH, -COOR, or -N02 would be useful for the further definition of the orientation of x substituents. 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

The non-conjugated BNI double bond is less reactive. However, the polymerization mechanism has not up to now been definitely established although a radical process is postulated for the propagation. Recent approaches towards the structure determinations of crosslinked networks and, by the way, towards the best knowledge of polymerization mechanisms, were undertaken in order to obtain BMI and BNI polymers presenting improved properties. In this section particular attention will be paid to this point. 

Definition a, p-Unsaturated aldehydes and ketones are aldehydes and ketones that are conjugated with a double bond. The a-position is defined as the carbon atom next to the carbonyl group, while the P-position is the carbon atom two bonds removed ... 

D. A conjugated molecule is a molecule with double bonds on adjacent atoms such as the molecule shown in A. Choice B and C give the definition of sigma and pi molecular orbitals. D is false because a resonance form is one of multiple equivalent Lewis structures, but these structures do not describe the actual state of the molecule. The anion will exist in a state between the two forms. 

The FDA s regulatory chemical definition for trans fatty acids is all unsaturated fatty acids that contain one or more isolated (i.e., nonconjugated) double bonds in a trans configuration. Under this wording, conjugated linoleic acid (CLA) would be excluded from the definition of trans fat, but trans vaccenic acid would be included. This definition was reconfirmed as of June 24, 2004.162... 

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