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9 SEPARATION AND PURIFICATION. IDENTIFICATION OF ORGANIC COMPOUNDS BY SPECTROSCOPIC TECHNIQUES he separation of mixtures of compounds to give the pure components is of T great practical importance in chemistry. ~2n~ synthetic reactions give mix- tures of products and it is necessary for you to have a reasonably clear idea of how mixtures of compounds can be separated. Almost all compounds of bio- chemical interest occur naturally as components of very complex mixtures from which they can be separated only with considerable difficulty. Separations can be achieved by differences in physical properties, such as differences in boiling point, or by chemical means, wherein differences in physical properties are enhanced by chemical reactions. In this chapter we will consider some separations of compounds based on differences in physical
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258 9 Separat~on and Pur~ficat~on ldent~f~cat~on of Organ~c Compounds by Spectroscop~c Techn~ques properties. Chemical procedures will be discussed elsewhere in connection with the appropriate classes of compounds. Identification and structure determination are often closely allied to the problem of separation. Once a compound is separated, how do we determine whether it is identical to some previously known compound (identification) or, if that can't be done, how do we determine its chemical structure? The spectro- scopic properties of molecules have proven to be extremely informative for both identification and structure determination and this chapter is mainly con- cerned with the application of spectroscopy for such purposes. We will give you now an overview of the spectroscopic properties of the major classes of organic compounds. In subsequent chapters, spectroscopic properties will be discussed in the context of the class of compounds under consideration. 9-1 HOW DO WE KNOW WHEN AN ORGANIC COMPOUND IS PURE? The classical criteria for determining the purity of organic compounds are correct elemental compositions (Section 1- 1A) and sharpness of melting point or constancy of boiling point. Important though these analytical and physical criteria are, they can be misleading or even useless. For instance, the analytical criterion is of no help with possible mixtures of isomers because these mixtures have the same elemental composition. The simple physical criteria are not applicable to substances that decompose when one attempts to determine the melting point or boiling point. Furthermore, boiling points are not very helpful for liquids that are mixtures of substances with nearly the same boiling point or are a~eotropes.~ Similar difficulties may be encountered with mixtures of solid substances that form mixed crystals or are eutectic~.~ Much sharper criteria for the purity of organic compounds now are provided through use of "super-separation" methods to see if any contaminants can be separated, or by spectroscopic techniques, as will be discussed later in this chapter. We begin here with a brief description of chromatographic methods of separation.
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This note was uploaded on 07/17/2011 for the course CHEM 101 taught by Professor Karim during the Spring '11 term at Cairo University.

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