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CHE 118A
Vahid Eskandari

S LievensJanuary 2012 University of California Davis For use with UCD Chemistry 8 and 118 series Approaching Synthesis ProblemsIntroduction Synthesis can be challenging to study for as each problem is unique and tricks that work for one question may not work for another It is helpful to think of studying synthesis as analogous to writing a poem To write a poem we need to have a good vocabulary know many reactions a solid understanding of grammar mechanisms and basic principles and a certain amount of inspiration Like poetry some synthesis can be elegant 100 step convergent epics while others may be short but serviceable limericks A synthetic chemist is always learning new reactions and looking for ways to make their synthesis simpler and more elegant however even the best epic poet had to start writing somewhere With practice particular patterns will show up as familiar and while there is no magic process there are ways to approach a problem that give a high likelihood of finding the necessary connections Flexibility of thinking is as important knowing the chemistry NOTE Writing a synthesis of a large molecule may take an afternoon or even a few days but doing the same synthesis in a lab may take years It is important for a synthetic chemist to be flexible in the approach to a target molecule and no synthesis written on paper is every truly correct until its been done with actual materialsLinear vs Convergent SynthesisThe goal of synthesis is to make as much as possible of the target molecule with minimal waste and expense To that end synthesis may be linear or convergent Typically convergent syntheses are preferred as material is lost at each step and convergent synthesis allows portions of the molecule to be built independently resulting in fewer steps directly between starting material and product and better overall yields Most syntheses seen in undergraduate classes will be linear synthesis but the principle of convergence becomes more important as syntheses get longer and more complex ABCDA Linear SynthesisEFA 5 step linear synthesis If all steps are 90 yield total yield90 x 90 x 90 x 90 x 90 x 10053ABCFA Convergent SynthesisDEA convergent synthesis 3 step longest linear sequence If all steps are 90 yield total yield90 x 90 x 90 x 10073 1
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