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13: Polyfunctional Compounds, Alkadienes, and Approaches to Organic Synthesis

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    21941
  • Organic compounds of natural origin rarely have simple structures. Most have more than one functional group in each molecule. Usually the chemical behavior of a functional group is influenced significantly by the presence of another functional group, especially when the groups are in close proximity. Indeed, the complexities that are associated with polyfunctionality are of central importance in biochemical reactions and in the design of organic syntheses. For this reason, you will need to gain experience in judging how and when functional groups in the same molecule interact with one another. We will start by considering the chemistry of alkadienes, which are hydrocarbons with two carbon-carbon double bonds.

    • 13.1: General Comments on Alkadienes
      The molecular properties of alkadienes depend on the relationship between the double bonds, that is, whether they are cumulated, conjugated, or isolated.  The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions: electrophilic, radical, cycloaddition, and polymerization.
    • 13.2: 1,3- or Conjugated Dienes. Electrophilic and Radical Addition
      The reactions of 1,3-butadiene are reasonably typical of conjugated dienes. The compound undergoes the usual reactions of alkenes, such as catalytic hydrogenation or radical and polar additions, but it does so more readily than most alkenes or dienes that have isolated double bonds. Furthermore, the products frequently are those of 1,2 and 1,4 addition.
    • 13.3: Cycloaddition Reactions
      There are a variety of reactions whereby rings are formed through addition to double or triple bonds. Diels-Alder reaction, which has proved so valuable in synthesis, is the most famous and can be called a [4 + 2] cycloaddition and results in the formation of a six-membered ring.
    • 13.4: Polymerization Reactions of Conjugated Dienes
      The general character of alkene polymerization by radical and ionic mechanisms was discussed previosuly. The same principles apply to the polymerization of alkadienes, with the added feature that there are additional ways of linking the monomer units. The polymer chain may grow by either 1,2 or 1,4 addition to the monomer.
    • 13.5: Cumulated Alkadienes
      The 1,2-dienes, which have cumulated double bonds, commonly are called allenes. Allenes of the type may be chiral molecules and can exist in two stereoisomeric forms, i.e., enantiomers. Verification of the chirality of such allenes (originally proposed by van’t Hoff in 1875) was slow in coming and was preceded by many unsuccessful attempts to resolve suitably substituted allenes into their enantiomers.
    • 13.6: Approaches to Planning Practical Organic Syntheses
      Chemical synthesis is not a science that can be taught or learned by any well-defined set of rules. Some classify it as more art than science because, as with all really creative endeavors, to be very successful requires great imagination conditioned by a wealth of background knowledge and experience. The problems of synthesis basically are problems in design and planning. There always is a variety of ways that the objective can be achieved either from the same or different starting materials.
    • 13.7: Building the Carbon Skeleton
      According to the suggested approach to planning a synthesis, the primary consideration is how to construct the target carbon skeleton starting with smaller molecules (or, alternatively, to reconstruct an existing skeleton). Construction of a skeleton from smaller molecules almost always will involve formation of carbon-carbon bonds. Up to this point we have discussed only a few reactions in which carbon-carbon bonds are formed.
    • 13.8: Introducing Functionality
      It may be easy to construct the carbon skeleton of the target compound of a synthesis, but with a reactive functional group at the wrong carbon. Therefore it is important also to have practice at shifting reactive entry points around to achieve the final desired product. We shall illustrate this form of molecular chess with reactions from previous post.
    • 13.9: Construction of Ring Systems by Cycloaddition
      Another example of a synthesis problem makes use of the cycloaddition reactions discussed here. Whenever a ring has to be constructed, you should consider the possibility of cycloaddition reactions, especially [4 + 2] cycloaddition by the Diels-Alder reaction.
    • 13.E: Polyfunctional Compounds, Alkadienes, and Approaches to Organic Synthesis (Exercises)
      These are the homework exercises to accompany Chapter 13 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
    • 13.10: Protecting Groups in Organic Synthesis
      Functional groups usually are the most reactive sites in the molecule, and it may be difficult or even impossible to insulate one functional group from a reaction occurring at another. Therefore any proposed synthesis must be evaluated at each step for possible side reactions that may degrade or otherwise modify the structure in an undesired way. To do this will require an understanding of how variations in structure affect chemical reactivity.
    • 13.11: Building the Carbon Skeleton
      According to the suggested approach to planning a synthesis, the primary consideration is how to construct the target carbon skeleton starting with smaller molecules (or, alternatively, to reconstruct an existing skeleton). Construction of a skeleton from smaller molecules almost always will involve formation of carbon-carbon bonds. Up to this point we have discussed only a few reactions in which carbon-carbon bonds are formed.

    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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