Identifying Organic Compounds

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A successful systematic scheme of qualitative analysis for identifying organic compounds was developed early in the twentieth century. This scheme, together with some more recent modifications, is termed classic qualitative organic analysis, and it is the basis for most textbook discussions on the subject (Ref. 1 at the end of Section 25.5, for example). 

Once the purity of the unknown is established, various physical properties are determined. The melting point of a solid or boiling point of a liquid is considered essential. Occasionally, the density and/or refractive index of a liquid may be useful, and for certain compounds, either liquid or solid, the specific rotation can be determined if a substance is optically active. 

Establishing what elements other than carbon and hydrogen are present is critical for identifying the compound, and techniques for elemental analysis are described later. Molar mass, as determined by cryoscopic techniques or mass spectrometry (Sec. 8.5), or percentage by mass composition of the elements present, also provides important data. The solubility of the unknown compound in water, in dilute acids and bases, or in various organic solvents may signal the presence or absence of various functional groups. 

Final assignment of a structure to the unknown compound is achieved by one of several procedures. The classic method involves the chemical conversion of the substance into a solid derivative. The success of this technique depends on the availability of information about the unknown and its various derivatives. Of prime importance is knowledge of the melting or boiling points of possible candidates for the unknown as well as the melting points of solid derivatives. Many tabulations of organic compounds are available for this purpose, and references to two of them are provided at the end of Section 25.5. Abbreviated tables of liquid and solid organic compounds and of their solid derivatives are provided at the website associated with this textbook. 

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It will be seen that the para compound is frequently the only isomeride which is solid at room temperature, a fact which should be borne in mind when identifying organic compounds. 

Students arc strongly advised not to attempt identification of the soluble carbohydrates by taste—quite apart from the fact that other compounds (c.g., saccharin] also have a sweet taste, the tasting of an imperfectly identified organic compound u too dangerous an operation. 

Qualitative identification. The spectrum is of help in identifying organic compounds. If two compounds are identical, the electronic spectra must be identical the converse is not necessarily true and in this... 

As in any organic mass spectrometric technique, ToF-SIMS first aims to identify organic compounds. As stated earlier, m/z ratios do not usually exceed 2000 due to the ionization process. Nevertheless, even for large molecules like polymers, interesting information can be obtained. 

IR spectroscopy, unlike ultraviolet-visible (UV-Vis) spectroscopy, can be used to directly measure both inorganic and organic components in soil, although it is more commonly used to identify organic compounds. It is carried out in two different wavelength ranges, the NIR, which is from 0.8 to 2.5 pm (800-2500 nm), and the MIR, which is from 2.5 to 25 pm (4000-400 cm ). ... 

Mass spectrometer (1) A scientific instrument used to identify organic compounds. (1) A scientific instrument used to measure the relative abundance of stable isotopes in a sample, e.g. isotope ratio mass spectrometer (IRMS). 

This book was originally written to teach the organic chemist how to identify organic compounds from the synergistic information afforded by the combination of mass (MS), infrared (IR), nuclear magnetic resonance (NMR), and ultraviolet (UV) spectra. Essentially, the molecule is perturbed by these energy probes and the molecule s responses are recorded as spectra. 

The use of NMR spectrometry of nuclei other than 3H and 13C to characterize and identify organic compounds is now commonplace. The use of other nuclei in NMR experiments ranges from such diverse areas as simply determining whether an unknown compound contains nitrogen to more complex questions of stereochemistry and reaction mechanisms. Although our discussions will be limited to four other nuclei, we should not limit our outlook with respect to the possibilities of other nuclei or other experiments. In fact, our intention here is to broaden our outlooks to the nearly limitless possibilities with NMR and the periodic table. 

IDENTIFIED ORGANIC COMPOUNDS PRESENT IN COAL-DERIVED OIL FRACTION (74-45) BY MASS SPECTROSCOPY... 

Thurman (1985) provided a comprehensive assessment of the distribution of identifiable organic compounds in natural waters. Total hydrolyzable amino acids accounted for 2-3% of DOC in rivers and 3 -13 % of DOC in eutrophic lakes. Total hydrolyzable sugars accounted for 5-10% of DOC in rivers and 8-12% of DOC in lakes. Thurman (1985) also summarized the... 

Identified organic compounds in Lippe river water samples... 

Conceive syntheses of the chosen, final, and intermediate compounds Select appropriate starting materials, reagents, reactions, and conditions Monitor the progress of reactions, employing suitable analytical methods Isolate, purify, characterize, and identify organic compounds Operate standard Items of laboratory equipment Spectrometers Infrared... 

Historically, Woodward and Fieser s early empirical correlation of various organic substructures with their associated UV spectra (23,24) could be considered one of the first attempts to partially identify organic compounds on the basis of nondestructive physical characterization. Woodward was able to predict the X, ax for number of conjugated systems, such as variously substituted enones and dienes, based on literature precedent. Contemporary structure determination of purified compoimds now relies most heavily on NMR and MS techniques, the UV spectrum normally contributing only a minor component to this process. However, the advent of HPLC and UV diode array detectors has enabled the acquisition of a UV spectrum for every component represented in an HPLC chromatogram. Consequently, the UV spectrum has become one of the most readily accessible pieces of information pertaining to structure, and interest has revived in exploiting its usefulness. 

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. 

Following the spectral examples, a discussion section provides details about the functional groups and other information that may be of use in identifying organic compounds. You may choose to omit this section on your first reading of the material. 

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