CHEM 125a - Lecture 24 - Determining Chemical Structure by Isomer Counting (1869)

Half a century before direct experimental observation became possible, most structures of organic molecules were assigned by inspired guessing based on plausibility. But Wilhelm Körner developed a strictly logical system for proving the structure of benzene and its derivatives based on isomer counting and chemical transformation. His proof that the six hydrogen positions in benzene are equivalent is the outstanding example of this chemical logic but was widely ignored because, in Palermo, he was far from the seats of chemical authority.

CHEM 125a - Lecture 23 - Valence Theory and Constitutional Structure (1858)

Youthful chemists Couper and Kekulé replaced radical and type theories with a new approach involving atomic valence and molecular structure, and based on the tetravalence and self-linking of carbon. Valence structures offered the first explanation for isomerism, and led to the invention of nomenclature, notation, and molecular models closely related to those in use today.

CHEM 125a - Lecture 22 - Radical and Type Theories (1832-1850)

Work by Wöhler and Liebig on benzaldehyde inspired a general theory of organic chemistry focusing on so-called radicals, collections of atoms which appeared to behave as elements and persist unchanged through organic reactions. Liebig’s French rival, Dumas, temporarily advocated radicals, but converted to the competing theory of types which could accommodate substitution reactions.

CHEM 125a - Lecture 21 - Berzelius to Liebig and Wöhler (1805-1832)

The most prominent chemist in the generation following Lavoisier was Berzelius in Sweden. Together with Gay-Lussac in Paris and Davy in London, he discovered new elements, and improved atomic weights and combustion analysis for organic compounds. Invention of electrolysis led not only to new elements but also to the theory of dualism, with elements being held together by electrostatic attraction. Wöhler’s report on the synthesis of urea revealed isomerism but also persistent naiveté about treating quantitative data.

CHEM 125a - Lecture 20 - Rise of the Atomic Theory (1790-1805)

This lecture traces the development of elemental analysis as a technique for the determination of the composition of organic compounds beginning with Lavoisier’s early combustion and fermentation experiments, which showed a new, if naïve, attitude toward handling experimental data. Dalton’s atomic theory was consistent with the empirical laws of definite, equivalent, and multiple proportions.

CHEM 125a - Lecture 19 - Oxygen and the Chemical Revolution (Beginning to 1789)

This lecture begins a series describing the development of organic chemistry in chronological order, beginning with the father of modern chemistry, Lavoisier. The focus is to understand the logic of the development of modern theory, technique and nomenclature so as to use them more effectively. Chemistry begins before Lavoisier’s “Chemical Revolution,” with the practice of ancient technology and alchemy, and with discoveries like those of Scheele, the Swedish apothecary who discovered oxygen and prepared the first pure samples of organic acids.

CHEM 125a - Lecture 18 - Amide, Carboxylic Acid and Alkyl Lithium

This lecture completes the first half of the semester by analyzing three functional groups in terms of the interaction of localized atomic or pairwise orbitals. Many key properties of biological polypeptides derive from the mixing of such localized orbitals that we associate with “resonance” of the amide group. The acidity of carboxylic acids and the aggregation of methyl lithium into solvated tetramers can be understood in analogous terms.

CHEM 125a - Lecture 17 - Reaction Analogies and Carbonyl Reactivity

Continuing the examination of molecular orbital theory as a predictor of chemical reactivity, this lecture focuses on the close analogy among seemingly disparate organic chemistry reactions: acid-base, SN2 substitution, and E2 elimination. All these reactions involve breaking existing bonds where LUMOs have antibonding nodes while new bonds are being formed. The three-stage oxidation of ammonia by elemental chlorine is analyzed in the same terms. The analysis is extended to the reactivity of the carbonyl group and predicts the trajectory for attack by a high HOMO.

CHEM 125a - Lecture 16 - Recognizing Functional Groups

This lecture continues the discussion of the HOMO/LUMO view of chemical reactivity by focusing on ways of recognizing whether a particular HOMO should be unusually high in energy (basic), or a particular LUMO should be unusually low (acidic). The approach is illustrated with BH3, which is both acidic and basic and thus dimerizes by forming unusual “Y” bonds. The low LUMOs that make both HF and CH3F acidic are analyzed and compared underlining the distinction between MO nodes that derive from atomic orbitals nodes (AON) and those that are antibonding (ABN).

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