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Bringing Order to Organic Chemistry

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Radicals and Types: Bringing Order to Organic Chemistry Organic Chemistry can now make you completely mad. It seems like a primeval forest in a tropical country where we hate to venture, full of the most peculiar things, an enormous thicket with no end and no way out. Friedrich Wöhler to Jakob Berzelius, January 28, 1835 Monday, October 4, 2010 Transformation of Organic Chemistry, 1820-1850 Reasons for this transformation: • Recognition of isomerism. • Explanation of isomerism by “arrangement.” • The rapid adoption of Berzelian notation as “paper tools” • Justus Liebig’s invention of the Kaliapparat for organic analysis Monday, October 4, 2010 Justus von Liebig (1803-1873) Friedrich Wöhler (1800-1882) Monday, October 4, 2010 Compounds with Identical Molecular Formulas • Liebig: Silver fulminate: 77.53% AgO, 22.47% cyanic acid • Wöhler: Silver cyanate: 77.23% AgO, 22.77% cyanic acid • Wöhler, 1828: • Cyanic acid + ammonia –––> ammonium cyanate –––> urea • Berzelius: • Isomers: compounds with different properties, but identical elemental composition. Monday, October 4, 2010 Liebig, Wöhler and the Oil of Bitter Almonds (1834) • C14H12O2 + oxidant --> C14H12O4 (benzoic acid) • C14H12O2 + chlorine --> C14H12O2Cl2 • C14H12O2 + bromine --> C14H12O2Br2 • (Many other reactions with iodine, ammonia, etc.) • Benzoyl Hydrogen: C14H10O2 • H2 • Benzoyl chloride: C14H10O2 • Cl2 • Benzoyl iodide: C14H10O2 • I2 • Benzoic acid: C14H10O2 • OH2 • Constant set of atoms: C14H12O2 Benzoyl radical Monday, October 4, 2010 Liebig, Wöhler and the Oil of Bitter Almonds (1834) Role of Berzelian formulas in creating the concept of the benzoyl radical • Elemental analysis results must be converted into integral numbers of “atoms” (C14H10O2 • H2 for oil of bitter almonds) • Formulas represent the benzoyl radical, but are also the means of “discovering” it, by manipulating symbols on paper. • Further, elemental analysis of each derivative results in slightly different percentages of elements (carbon, for example), and cannot define the appearance of the benzoyl radical. • Only transformation of these percentages into the integral numbers of “atoms” makes the benzoyl radical apparent. Monday, October 4, 2010 Jean-Baptiste Dumas (1800-1884) Monday, October 4, 2010 Dumas and the Substitution Theory, 1834 Using modern formulas: C2H4O2 + chlorine –––> C2Cl3HO2 Acetic acid Choroacetic acid If alcohol has the formula C8H8•H4O2 chlorine can withdraw H4 without replacing it, transforming alcohol into acetic acid ether C8H8O2, which is what really happens. From this point in time, each atom of hydrogen that has been withdrawn will be replaced by one atom of chlorine, and without being concerned here with intermediary compounds, we state that chloral C8H2O2Ch6 is formed. Monday, October 4, 2010 Dumas and the Substitution Theory, 1834 If alcohol has the formula C8H8•H4O2 chlorine can withdraw H4 without replacing it, transforming alcohol into acetic acid ether C8H8O2, which is what really happens. From this point in time, each atom of hydrogen that has been withdrawn will be replaced by one atom of chlorine, and without being concerned here with intermediary compounds, we state that chloral C8H2O2Ch6 is formed. • one portion (“atom”) of hydrogen replaced with one portion (“atom”) of chlorine. • Stepwise replacement of hydrogen atoms. • The formulas indicate the stepwise replacement. • Nothing is observed in the reaction except the steady, continuous production of hydrochloric acid. Monday, October 4, 2010 Charles Gerhardt (1816-1856) Auguste Laurent (1807-1853) Monday, October 4, 2010 Laurent and Gerhardt Laurent • “Arrangement” of atoms in molecules must determine its properties • Crystallography provides clues to geometrical arrangement Gerhardt • Analogous properties mean analogous formulas/arrangements • Organic compound derived from three fundamental “types”: hydrogen, water, ammonia. • Radical conventionalist Both • Organic chemistry provides the model for all chemistry • Formulas must represent all known reactions of substance. • Atomic weights must be revised, and chemical formulas derived from a consistent set of magnitudes • Many formulas based on electrochemical dualism are impossible. Monday, October 4, 2010 Alexander Williamson (1824-1904) Monday, October 4, 2010 The Formation of Ether Alcohol + sulfuric acid –––> ether Liebig and Dumas (separately): C4H10O H2O –––> C4H10 + water Gerhardt/Laurent: EtOH + EtOH –––> EtOEt + H20 Monday, October 4, 2010 Williamson’s “Asymmetric synthesis” Argument for Ether’s Formula, 1850 • Liebig and Dumas were successful, because of a fortunate symmetry in the ether molecule. • In forming ether, according to Laurent and Gerhardt, two identical molecules were brought together. • Crucial Experiment: C4H10•KO + C2H6I2 –––> C4H10 + C2H6O (Liebig and Dumas) EtOK + CH3I –––> EtOMe (Williamson) “In this experiment, the two theories cross one another, and must lead to different results.” • A molecule of ether has exactly 4 carbons, 10 hydrogens and one oxygen; A molecule of alcohol has exactly 2 carbons, 5 hydrogens, and one oxygen. • methyl-ethyl, methyl-amyl, and ethyl-amyl ethers Monday, October 4, 2010 Use of Williamson’s Argument in Analogous Syntheses • Williamson: Acetone (CH3COCH3) and mixed ketones. • Gerhardt, 1852: Acetic anydrides and mixed anhydrides of acetic, benzoic, and salicylic acids. • Wurtz and alkanes, 1855 (Wurtz coupling): ethyl-butyl, ethyl-amyl, butyl- amyl, etc. Monday, October 4, 2010 Edward Frankland (1825-1899) Monday, October 4, 2010 Further Reading • Ursula Klein, Experiments, Models, Paper Tools: Cultures of Organic Chemistry in the Nineteenth Century, Stanford, CA: Stanford University Press, 2003. • J. H. Brooke, “Wöhler’s Urea, and Its Vital Force: A Verdict From the Chemists,” Ambix, 15 (1968): 84 • J. H. Brooke, “Laurent, Gerhardt, and the Philosophy of Chemistry,” Historical Studies in the Physical Sciences, 6 (1975): 405-29 • Peter J. Ramberg, “The Death of Vitalism and the Birth of Organic Chemistry: Wöhler’s Urea Synthesis in Textbooks of Organic Chemistry,” Ambix, 47 (2000): 170-95 Monday, October 4, 2010.
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