Unknown compounds

NMR spectroscopy is probably the singly most powerful technique for the confirmation of structural identity and for stmcture elucidation of unknown compounds. Additionally, the relatively low measurement times and the facility for automation contribute to its usefulness and industrial interest. 

In this way the student s knowledge of the organic reactions is consolidated as he proceeds through the sections, his experience of the general method of identification steadily increases, and the investigation of the unknown compounds forms a welcome break from the systematic pursuit of the sectional work. 

It must be emphasised that the following substances are merely representative of widely different classes of compounds, and that the test when applied to an unknown compound often provides only a general indication of the probable class to which the compound belongs. Moreoi er, the behaviour of a compound when heated is often determined more by the nature of its substituent groups than by its general character. 

If an unknown compound gives a positive test with the 2 4-dinitrophenylhydrazine reagent, it then becomes necessary to decide whether it is an aldehyde or a ketone. Although the dimedone reagent (Section 111,70,2) reacts only with aldehydes, it is hardly satisfactory for routine use in class reactions. It is much simpler to make use of three other reagents given below, the preparation and properties of which have already been described (Section 111,70). 

If the tests for an aldehyde are negative, the unknown compound is a ketone. When once the compound has been established as an aldehyde or ketone, it is permissible to refer to tables of physical constants information thus obtained may be of value in indicating other possible groups to be tested for, with due regard to possible comphcations caused by the presence of these groups. 

Determination of melting points (a-naphthylamine, a-naphthol, benzoic acid, succinic acid and p-nitrobenzoic acid). Use the apparatus shown in Fig. II, 10, 2, a. Construction of calibration curve for thermometer. Determination of m.p. of unknown compound. 

Molecular similarity is also useful in predicting molecular properties. Programs that predict properties from a database usually hrst search for compounds in the database that are similar to the unknown compound. The property of the unknown is probably close in value to the property for the known... 

Run test calculations with the new parameters. Then adjust the parameters as necessary to reproduce experimental results before using them to describe an unknown compound. 

Analyzing an NMR spectrum m terms of a unique molecular structure begins with the mfor matron contained m Table 13 1 By knowing the chemical shifts characteristic of various proton environments the presence of a particular structural unit m an unknown compound may be inferred An NMR spectrum also provides other useful information including... 

Understanding how molecules fragment upon electron impact permits a mass spec trum to be analyzed m sufficient detail to deduce the structure of an unknown compound Thousands of compounds of known structure have been examined by mass spectrome try and the fragmentation patterns that characterize different classes are well docu mented As various groups are covered m subsequent chapters aspects of their fragmentation behavior under conditions of electron impact will be descnbed... 

With the availability of computerized data acquisition and storage it is possible to build database libraries of standard reference spectra. When a spectrum of an unknown compound is obtained, its identity can often be determined by searching through a library of reference spectra. This process is known as spectral searching. Comparisons are made by an algorithm that calculates the cumulative difference between the absorbances of the sample and reference spectra. For example, one simple algorithm uses the following equation... 

The matching of a spectrum for an unknown compound to a reference spectrum stored in a computer database. 

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

Interestingly, if the original comparison of the spectrum from the reference compound with stored m/z data for that compound reveals discrepancies, the stored reference data are updated before the computer goes on to acquire data from the unknown compound. In this mode, the computer is not used simply to acquire and manipulate data but is also used to make decisions... 

Spectra. The abiHty to consult collections of standard spectra is cmcial in the analysis of unknown compounds. A long history of data collection efforts has been aimed at these appHcations. Among the best known of the pubHshed handbooks are the Sadtkr Spectral Data Sheets which include ir,... 

Analysis. Many analytical procedures calling for determination of molecular stmcture are aided by crystallization or requite that the unknown compound be crystalline. Methodologies coupling crystalliza tion and analytical procedures will not be covered here (see X-RAY TECHNOLOGY)... 

Identification of unknown compounds in solutions, liquids, and crystalline materials characterization of structural order, and phase transitions... 

Before the advent of NMR spectroscopy, infrared (IR) spectroscopy was the instrumental method most often applied to determine the striaeture of organic compounds. Although NMR spectroscopy, in general, tells us more about the structure of an unknown compound, IR still retains an important place in the chemist s inventory of spectroscopic methods because of its usefulness in identifying the presence of certain functional groups within a molecule. 

If the unknown compound is made up of x atoms of chlorine and y atoms of oxygen, its molecular formula will be Cl O. Because the molecular weights of Cl and 0 are 70.91 and 32.00 g/g-mole, respectively, we can express the weight composition of the compound as... 

Let s assume that we want to find the structure of an unknown hydrocarbon. A molecular weight determination on the unknown yields a value of 82, which corresponds to a molecular formula of CfcHjo Since the saturated Q alkane (hexane) has the formula C61-114, the unknown compound has two fewer pairs of hydrogens (H]4 - H l() = H4 = 2 H2), and its degree of unsaturation is two. The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. 

The IR spectrum of an unknown compound is shown in Figure 12.16. What functional groups does the compound contain ... 

The determination of how the atoms of a molecule are connected is the most important problem in identifying an unknown compound. It can be as exciting as a detective story, with the chemical and physical properties furnishing the clues. With the right collection of clues, the chemist can ascertain the identity of the molecule. 

Maximum benefit from Gas Chromatography and Mass Spectrometry will be obtained if the user is aware of the information contained in the book. That is, Part I should be read to gain a practical understanding of GC/MS technology. In Part II, the reader will discover the nature of the material contained in each chapter. GC conditions for separating specific compounds are found under the appropriate chapter headings. The compounds for each GC separation are listed in order of elution, but more important, conditions that are likely to separate similar compound types are shown. Part II also contains information on derivatization, as well as on mass spectral interpretation for derivatized and underivatized compounds. Part III, combined with information from a library search, provides a list of ion masses and neutral losses for interpreting unknown compounds. The appendices in Part IV contain a wealth of information of value to the practice of GC and MS. 

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