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25.3: Physical and Spectroscopic Properties

  • Page ID
    22359
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    The \(\alpha\)-amino acids crystallize as the dipolar forms, \(\ce{H_3} \overset{\oplus}{\ce{N}} \ce{-CHR-C} \overset{\ominus}{\ce{O_2}}\), and the strong intermolecular electrical forces in the crystals lead to higher melting points than those of simple amines or monocarboxylic acids (see Table 25-1). The melting points are so high that decomposition often occurs on melting. The solubility characteristics of amino acids in water are complex because of the acid-dissociation equilibria involved, but they are least soluble at their isoelectric points. The dipolar structures of amino acids greatly reduce their solubility in nonpolar organic solvents compared to simple amines and carboxylic acids.

    The infrared spectra of \(\alpha\)-amino acids in the solid state or in solution do not show a carbonyl absorption band at \(1720 \: \text{cm}^{-1}\) characteristic of a carboxyl group. Rather, they show a strong absorption near \(1600 \: \text{cm}^{-1}\) typical of the carboxylate anion. the \(\ce{N-H}\) stretch appears as a strong, broad band between \(3100\)-\(2600 \: \text{cm}^{-1}\):

    Roberts and Caserio Screenshot 25-3-1.png

    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."


    This page titled 25.3: Physical and Spectroscopic Properties is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.