IDENTIFYING A COMPOUND

Thus far we have seen how to plan and perform a reaction, to separate and isolate the various products, and check for purity. Sometimes we know what the product or products of a reaction are likely to be, for example, if we are repeating a procedure which is already published in a book or scientific paper. On other occasions, as is the case in research, we know what we hope the product will be, but we cannot be certain. So what do we do in these cases There is no advantage in obtaining a pure, but unknown, product from a reaction, we have to be able to characterize it. The identification techniques available are mostly applicable to both organic and inorganic chemistry, but the priority that each branch of chemistry gives to each technique tends to vary. 

If the preparation has been previously published in a book or scientific paper, then it is usually possible to identify it by comparison with various data listed in the literature. However, the compound may be completely unknown, or maybe it has never been made before so there is no data for comparison, or we may have an unexpected product. We shall see that the methods that are of most use to us in identification through comparison, also allow us to identify molecules when no comparison is possible that is, they allow us to infer the molecular identity. 

Many identification procedures that we use for previously recorded compounds are based on the measurement of some physical property of the compound. We will already have noted some physical properties when the separation was carried out. For example, if we used distillation, we would have noted the boiling temperature if we used chromatography, we would have noted the time taken for the component to travel a certain distance (or the Rf value) or in the case of solvent extraction, we would have noted the solubility characteristics in various solvents. There are many other measurements that we can make. We could determine the mass of the molecule (relative molecular mass), the density of the substance, the acidity, or the amount and frequency of electromagnetic radiation absorbed. 

There is therefore a range of analytical techniques we can employ, but we need to decide which would help most towards the identification of a particular compound. We will start by looking at the techniques which identify the different elements, and which measure how much of each element is present. This allows the formula of the compound to be determined. These techniques are expensive and destructive (you cannot recover your sample) so they tend to be used only as a final check before the publication of results. 

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The NFPA reactivity code, labour regulations, enthalpy and temperature of decomposition of the compound are all listed in the following table. They help to identify a compound s instability. There are a few compounds used in the explosives industry added to the list. These codes are described in Part I (see p. 119-123). 

To identify a compound, five data points per peak may be sufficient. Quantitation may require at least 10 data points across a peak. Many of today s laboratories still house standard detectors (UV, ELSD, fluorescence, etc.) with maximum data acquisition rates at or below 20 Hz. Many conventional LC/MS methods acquire data at rates of 5 Hz or less. As shown in Figure 3.8, this is not sufficient for modem speed optimized chromatography. Obviously, selecting the wrong data acquisition rate will nullify all attempts to optimize chromatography. 

Unknown 1. Try to identify a compound with the spectrum represented in Fig. 5.1. The exact molecular mass of the compound is 60.0211 Da, which defines its elemental composition as C2H4O2. At this stage pay attention only to the most abundant peaks in the spectrum m/z 60 (molecular ion) and primary fragment ions of m/z 45, m/z 43, m/z 28, and m/z 15. Use the masses of elements from the periodic table of chemical elements. 

Because MS identifies components on the basis of their atomic or molecular mass and, in the case of organic compounds, fragmentation pattern, it is a particularly useful and powerful detector. The molecular mass, which is commonly referred to as the molecular weight, is a particularly useful piece of information when trying to identify a compound. For elements, atomic mass determination can also determine the isotope present, which in turn can be used to identify the source and movement of an element through the environment. 

This expresses tR as a function of the fundamental column parameters t0 and k tR can vary between t0 (for k = 0) and any larger value (for k > 0). Since to varies inversely with solvent velocity u, so does tR. For a given column, mobile phase, temperature, and sample component X, k is normally constant for sufficiently small samples. Thus, tR is defined for a given compound X by the chromatographic system, and tR can be used to identify a compound tentatively by comparison with a tR value of a known compound. 

In actual practice, the resulting Rf value of the original compound together with the chromatographic results of the reaction are usually good enough to identify a compound accurately and precisely. 

TLC can be used to identify a compound by comparing the / , vaiues with those ot known compounds. TLC can aiso be used to assess the purity ot a compound as a pure compound wiii give oniy one spot on the deveioped chromatogram. 

Every organic chemical has a mass spectrum, which is a combination of ions with different masses and different intensities (abundances). To identify a compound, its mass spectrum is compared to the mass spectra of standards, analyzed under the same instrument settings, and to the EPA/National Institute for Standards and Technology (NIST) mass spectra library. The EPA/NIST library is stored in the database of the computer that operates the instrument. A comparison to the library spectra is possible only if there is consistency in the compound spectra generated by different GC/MS systems at hundreds of environmental laboratories. To achieve such consistency, the EPA methods for GC/MS analysis include the mass... 

With two heteroatoms in the six-membered ring, it is again useful to identify a compound containing both. The nucleic acid base uracil 87 contains a molecule of urea 88 in its structure but removing that leaves an awkward synthon 89. The question is how are we to add to the enone and get the double bond back The solution is to use the acetylenic acid 90. Heating 90 and 88 together in acid gives uracil in 65% yield.13... 

As mentioned in Section 8.1, it was recognized that it would be beneficial to further tailor the overall profile of propranolol, 6, for use as an anti-anginal agent within the setting of obstructive airway disease. Toward this end, ICI immediately embarked on a program that sought to identify a compound which would have the following profile [60] ... 

In the previous section, you learned how to calculate the percentage composition of a compound from its chemical formula. Now you will do the reverse. You will use the percentage composition of a compound, along with the concept of the mole, to calculate the empirical formula of the compound. Since the percentage composition can often be determined by experiment, chemists use this calculation when they want to identify a compound. 

Just as a symbol identifies an element, a formula is a combination of symbols that identifies a compound, an ion, or a molecule of an element. However, chemical formulas do much more. A formula also indicates the relative quantities of the elements contained in the compound or ion and implies some kind of chemical bonding between the atoms. 

To that end, synthetic analog studies have identified a compound (G5) with comparable or better anticancer activity and certain structural features that are critical to the anticancer activity of this compound type (81). 

How to set a threshold level for the match factor to positively identify a compound spectrum is strongly dependent on a variety of factors. Below are mentioned those that play a major role in the success of the spectral matching procedure and are important tools in optimizing spectral comparison and threshold setting. 

Do the problem-solving LAB below to learn how functional groups give organic compounds characteristic properties that may be used to identify a compound s general type. 

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