Resonance structures conjugated double bonds

Follow the four-step procedure for the composite model of bonding. Use localized bonds and hybrid orbitals to describe the bonding framework and the inner atom lone pairs. Next, analyze the system, paying particular attention to resonance structures or conjugated double bonds. Finally, make sure the bonding inventory accounts for all the valence electrons and all the valence orbitals. 

Resonance or electromeric effects. Certain molecular structures are characterized by the possibility of having two or more compatible electronic structures and the molecules exist in a resonance state intermediate between the several extremes. These effects are particularly characteristic of aromatic structures and other molecules containing conjugate double bonds. 

For compounds that require metabolic activation, avoiding structural features that may provide resonance stabilization of electrophilic metabolites (e.g., conjugated double bonds, or conjugated system/aryl moiety) will decrease the lifetime of the reactive intermediates. 

HYPERCONJUGATION. The description of the properties of a molecule m terms of resonance structures in which an atom or group is not joined by any sort of bond to the atom to which it is ordinarily considered linked. Also called no-hond resonance. The hypothesis of hyperconjugaiion has been advanced lo interpret some properties of substances containing but I double bond by analogy wilh those of substances containing conjugated double bonds. Consider a substance with a terminal... 

Methine dyes have an extended system of conjugated double bonds. A large number of resonance structures may therefore be formulated, of which (1) and (2) are the most important, i.e., the electron density is lowest on the nitrogen atoms. 

At this point, we can define an aromatic compound to be a cyclic compound containing some number of conjugated double bonds and having an unusually large resonance energy. Using benzene as the example, we will consider how aromatic compounds differ from aliphatic compounds. Then we will discuss why an aromatic structure confers extra stability and how we can predict aromaticity in some interesting and unusual compounds. 

Fig. 7.13 Resonance structures in some food colorants (a) conjugated double bonds, e.g. carotenoids (b) due to lone pair electrons from nitrogen, e.g., betainins.

Diverse spectroscopic methods have been employed to characterise triterpenes. Ultraviolet (UV) and infrared (IR) spectroscopy are not very useful techniques in elucidating the structure of triterpenes, but the former gives information about compounds with conjugated double bonds and the latter may provide some information about substituents like the hydroxyl group, ester carbonyl group or a,p-unsaturate carbonyl. Other physical data may be of interest to characterise new compounds, but the use of modem spectroscopic methods of nuclear magnetic resonance (NMR) and mass spectroscopy (MS) are essential for the structural determination. 

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

Such delocalization occurs for systems that are referred to as conjugated. Conjugation means a lack of an intervening atom between tt bonds or between TT bonds and lone-pair electrons. Whenever you encounter a structure that has conjugated double bonds or lone-pair electrons conjugated with double bonds, you should consider that resonance is likefy. 

Because of its conjugated double-bond system, vitamin A and other retinoids can be characterized and quantitated by their UV absorption spectra and often by their fluorescence. The light absorbances of selected retinoids are given in Table 1 (3). Proton and nuclear magnetic resonance (NMR) spectra are also useful in characterizing cis-trans isomers, and mass spectrometry (MS) is valuable in assessing molecular weights and structures (1). 

Undoubtedly, the possible addition of other potential independent parameters to equation 3e would increase the correlation coefficient and decrease the standard error of the estimate. Specifically, the following variables were also investigated but were found to result only in negligible contributions to the quality of the regressions modified Swain and Lupton s field (F) and resonance (R) parameters (Hansch and Leo 1979), (log P), (AMR), the number of conjugated double bonds, ionization potential (in methanol), and several indicator variables for structural fragments, such as N, NH2, CO, and so forth. However, for several of these parameters only incomplete data sets are available and no significant increase of the correlations over that of equation 3d with up to five independent variables was observed. 

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