Organic compounds resonance structures

Determining the structure of an organic compound was a difficult and time-consuming process in the 19th and early 20th centuries, but powerful techniques are now available that greatly simplify the problem. In this and the next chapter, we ll look at four such techniques—mass spectrometry (MS), infrared (IR) spectroscopy, ultraviolet spectroscopy (UV), and nuclear magnetic resonance spectroscopy (NMR)—and we U see the kind of information that can be obtained from each. 

A common biologically active radical is the pentadienyl radical, RCHCHCHCHCHR, where the carbons form a long chain, with R and R, which can be a number of different organic groups, at each end. Draw three resonance structures for this compound that maintain carbon s valence of four. 

Nuclear magnetic resonance (NMR) is the principal technique for the identification of organic compounds and is among the leading techniques for the determination of their structures. The technique has also been developed, as magnetic resonance imaging (MRI), as a diagnostic procedure in medicine. 

On the other hand, nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools for the structure elucidation of organic compounds. However, to solve the molecnlar strnctnre of a novel substance by NMR spectroscopy alone is often time-consnming (when compared to MS). Besides, the identification of components in a complex mixture usually requires the separation and/or isolation of the components of interest prior to NMR analysis. Therefore mnltiple preparatory chromatographic... 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

There are many organic compounds stabilized by resonance. As a first example we may cite benzene, in which there is resonance between the two Kekule structures. In this case, the resonance energy could be calculated to give good agreement with the experimental data on the heat of formation of the molecule. Perhaps even more remarkable is the compound KC5H5, which is formed when potassium reacts with ryr/opentadiene. 

For tabulations of NMR chemical shifts and coupling constants, see F. A. Bovey, NMR Data Tables for Organic Compounds, vol. 1, Interscience, New York, 1967 W. Briigel, Nuclear Magnetic Resonance Spectra and Chemical Structure, vol. 1, Academic Press, New York, 1967. 

Fortunately for organic chemists, hydrogen and carbon are the most common nuclei found in organic compounds, and the ability to probe these nuclei by NMR is invaluable for organic structure determination. Since proton magnetic resonance (PMR) is tire most common type, tire behavior of nuclei in magnetic fields will serve as a model for other nuclei which have spin quantum numbers I = and thus behave similarly (13C, 19F, etc.). 

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