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23: Organonitrogen Compounds I- Amines

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  • A wide variety of organic compounds contain nitrogen. In fact, the types of nitrogen compounds are so numerous and diverse that we shall be unable to consider them all. We shall give most attention to the chemistry of amines and amides in this and the following chapter, because these represent the two largest classes of nitrogen compounds.

    • 23.1: Amines Compared with Alcohols
      As you read the chapter you will realize a similarity between the chemistry of amines and the chemistry of alcohols. Primary amines (RNH2) and secondary amines (R2NH) are much weaker acids than alcohols (ROH) and form strongly basic anions.
    • 23.2: Naturally Occurring Amines
      A large and widespread class of naturally occurring amines is known as alkaloids. These are basic organic nitrogen compounds, mostly of plant origin. The structures of the plant alkaloids are extraordinarily complex, yet they are related to the simple amines in being weak nitrogen bases. The first investigator to isolate an alkaloid in pure form was F. W. A. Sertürner who described morphine as basic, salt-forming, and ammonia-like. The term "organic alkali" is derived the name alkaloid.
    • 23.3: Types and Nomenclature of Amines
      Amine bases are classified according to the number of alkyl or aryl groups attached to nitrogen. A further classification exists if the nitrogen is multiply bonded to carbon, as in imines and aromatic nitrogen compounds.
    • 23.4: Physical Properties of Amines
      The physical properties of amines depend in an important way on the extent of substitution at nitrogen. Thus primary amines, \(\ce{RNH_2}\), and secondary amines, \(\ce{R_2NH}\), are less volatile than hydrocarbons of similar size, weight, and shape, as the following examples show.
    • 23.5: Spectroscopic Properties of Amines
      A characteristic feature of the infrared spectra of primary and secondary amines is the moderately weak absorption at 3500cm−1 to 3300cm−1 , which corresponds to  N−H stretching vibrations. Primary amines have two such bands in this region, whereas secondary amines generally show only one band. Absorption is shifted to lower frequencies by hydrogen bonding.
    • 23.6: Stereochemistry of Amines
      In ammonia and amines, the bonds to nitrogen are pyramidal with bond angles closer to the tetrahedral value of 109.5° than to the 90° value expected for the use of pure  p orbitals of nitrogen in bond formation. We consider that the nitrogen in amines is formulated best with hybrid sp3-type orbitals; three of these orbitals are used in σ -bond formation while the fourth contains the nonbonding electron pair.
    • 23.7: Amines as Bases
      Perhaps the most characteristic property of amines is their ability to act as bases by accepting protons from a variety of acids.
    • 23.8: Amines as Acids
      Primary and secondary amines are very weak acids. The lithium salts of such amines can be prepared in ether solution by treatment of the amine with phenyllithium.
    • 23.9: Amines as Nucleophiles
      The unshared electrons on nitrogen play a key role in the reactions of amines. In fact, almost all reactions of amines at the nitrogen atom have, as a first step, the formation of a bond involving the unshared electron pair on nitrogen. A typical example is acylation, which is amide formation through the reaction of an acyl chloride, an anhydride, or an ester with an amine.
    • 23.E: Organonitrogen Compounds I- Amines (Exercises)
      These are the homework exercises to accompany Chapter 23 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
    • 23.10: Amines with Nitrous Acid
      Some of the most important reactions of amines are brought about by nitrous acid (HONO). The character of the products depends very much on whether the amine is primary, secondary, or tertiary. In fact, nitrous acid is a useful reagent to determine whether a particular amine is primary, secondary, or tertiary. With primary amines nitrous acid results in evolution of nitrogen gas; with secondary amines insoluble yellow liquids or solid N-nitroso compounds, R2N−N=O, separate; tertiary alkanamines
    • 23.11: Oxidation of Amines
      Nitrogen has a wide range of oxidation states in organic compounds.
    • 23.12: Synthesis of Amines
      There three main groups of synthesis routes for amines. The first group starts with a simple amine, or with ammonia, and builds up the carbon framework by alkylation or arylation reactions on nitrogen. The second group starts with compounds of the same carbon-nitrogen framework as in the desired amine, but with nitrogen in a higher oxidation state.  The third group of reactions relies on the fact that amides usually can be converted to amines, either by reduction, hydrolysis, or rearrangement.
    • 23.13: Protection of Amino Groups in Synthesis
      We have mentioned previously that it may be difficult to ensure selective chemical reaction at one functional group when other functional groups are present in the same molecule. Amino groups are particularly susceptible to reactions with a wide variety of reagents, especially oxidizing reagents, alkylating reagents, and many carbonyl compounds. Therefore, if we wish to prevent the amino group from undergoing undesired reaction while chemical change occurs elsewhere in the molecule.
    • 23.14: Carcinogenic Nitrogen Compounds
      A number of arenamines are carcinogens. and the most dangerous examples are known to induce human bladder cancer. These chemicals were used widely in the chemical industry (mostly in azo dye manufacture) long before they were recognized as hazardous carcinogens.

    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."

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