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 properties. Chemical procedures will be discussed elsewhere in connection with the appropriate classes of compounds.
- 9.1: Prelude to Separation, Purification, and Identification
- The separation of mixtures of compounds to give the pure components is of great practical importance in chemistry. Many synthetic reactions give mixtures 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 biochemical interest occur naturally as components of very complex mixtures from which they can be separated only with considerable difficulty.
- 9.2: How do we know when an Organic Compounds is Pure?
- The classical criteria for determining the purity of organic compounds are correct elemental compositions 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.
- 9.3: Chromatographic Separation Procedures
- Many separation methods are based on chromatography, that is, separation of the components of a mixture by differences in the way they become distributed (or partitioned) between two different phases. Liquid-solid chromatography originally was developed for the separation of colored substances, hence the name chromatography, which stems from the Greek word chroma meaning color.
- 9.4: Why Can't We See Molecules?
- The most straightforward way to determine the structures of molecules would be to "see" how the nuclei are arranged and how the electrons are distributed. This is not possible with visible light, because the wavelengths of visible light are very much longer than the usual molecular dimensions. A beam of electrons can have the requisite short wavelengths, but small organic molecules are destroyed rapidly by irradiation with electrons of the proper wavelengths.
- 9.5: Atomic Energy States and Line Spectra
- A spectroscopic change related to a change in energy associated with the absorption of a quantum of energy. Spectra are the result of searches for such absorptions over a range of wavelengths. If one determines and plots the degree of absorption by a monoatomic gas, a series of very sharp absorption bands or lines are observed. The lines are sharp because they correspond to specific changes in electronic configuration without complication from other possible energy changes.
- 9.6: Energy States of Molecules
- The energy states and spectra of molecules are much more complex than those of isolated atoms. In addition to the energies associated with molecular electronic states, there is kinetic energy associated with vibrational and rotational motions.
- 9.7: Microwave (Rotational) Spectra
- ecause electronic and vibrational energy levels are spaced much more widely, and because changes between them, are induced only by higher-energy radiation, microwave absorptions by gaseous substances can be characterized as essentially pure “rotational spectra.” It is possible to obtain rotational moments of inertia from microwave spectra, and from these moments to obtain bond angles and bond distances for simple molecules.
- 9.8: Infrared (Rovibrational) Spectroscopy
- Infrared spectroscopy was the province of physicists and physical chemists until about 1940. At that time, the potential of infrared spectroscopy as an analytical tool began to be recognized by organic chemists. The change was due largely to the production of small, quite rugged infrared spectrophotometers and instruments of this kind now are virtually indispensable for chemical analysis. A brief description of the principles and practice of this spectroscopic method is the topic of this section
- 9.9: Raman Spectroscopy
- Raman spectroscopy often is a highly useful adjunct to infrared spectroscopy. The experimental arrangement for Raman spectra is quite simple in principle. Monochromatic light, such as from an argon-gas laser, is passed through a sample, and the light scattered at right angles to the incident beam is analyzed by an optical spectrometer.
- 9.10: Electronic Spectra of Organic Molecules
- Absorption of light in the ultraviolet and visible regions produces changes in the electronic energies of molecules associated with excitation of an electron from a stable to an unstable orbital. Because the energy required to excite the valence-shell electrons of molecules is comparable to the strengths of chemical bonds, absorption may lead to chemical reactions. We discussed this briefly in connection with photochemical halogenation of alkanes.
- 9.11: Nuclear Magnetic Resonance Spectroscopy
- uclear magnetic resonance (NMR) spectroscopy is extremely useful for identification and analysis of organic compounds. The principle on which this form of spectroscopy is based is simple. The nuclei of many kinds of atoms act like tiny magnets and tend to become aligned in a magnetic field. In NMR spectroscopy, we measure the energy required to change the alignment of magnetic nuclei in a magnetic field.
- 9.12: Mass Spectroscopy
- The usual application of mass spectroscopy to organic molecules involves bombardment with a beam of medium-energy electrons in high vacuum, and analysis of the charged particles and fragments so produced. Most mass spectrometers are set up to analyze positively charged fragments, although negative-ion mass spectrometry also is possible.
- 9.E: Separation, Purification, and Identification of Organic Compounds (Exercises)
- These are the homework exercises to accompany Chapter 9 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
Contributors and Attributions
John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."