37 episodes

(CHEM 125) This is the first semester in a two-semester introductory course focused on current theories of structure and mechanism in organic chemistry, their historical development, and their basis in experimental observation. The course is open to freshmen with excellent preparation in chemistry and physics, and it aims to develop both taste for original science and intellectual skills necessary for creative research.

This course was recorded in Fall 2008.

Organic Chemistry - Audio J. Michael McBride

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(CHEM 125) This is the first semester in a two-semester introductory course focused on current theories of structure and mechanism in organic chemistry, their historical development, and their basis in experimental observation. The course is open to freshmen with excellent preparation in chemistry and physics, and it aims to develop both taste for original science and intellectual skills necessary for creative research.

This course was recorded in Fall 2008.

    37 - Potential Energy Surfaces, Transition State Theory and Reaction Mechanism

    37 - Potential Energy Surfaces, Transition State Theory and Reaction Mechanism

    After discussing the statistical basis of the law of mass action, the lecture turns to developing a framework for understanding reaction rates. A potential energy surface that associates energy with polyatomic geometry can be realized physically for a linear, triatomic system, but it is more practical to use collective energies for starting material, transition state, and product, together with Eyring theory, to predict rates. Free-radical chain halogenation provides examples of predicting reaction equilibria and rates from bond dissociation energies. The lecture concludes with a summary of the semester's topics from the perspective of physical-organic chemistry.

    • 2 sec
    36 - Bond Energies, the Boltzmann Factor and Entropy

    36 - Bond Energies, the Boltzmann Factor and Entropy

    After discussing the classic determination of the heat of atomization of graphite by Chupka and Inghram, the values of bond dissociation energies, and the utility of average bond energies, the lecture focuses on understanding equilibrium and rate processes through statistical mechanics. The Boltzmann factor favors minimal energy in order to provide the largest number of different arrangements of "bits" of energy. The slippery concept of disorder is illustrated using Couette flow. Entropy favors "disordered arrangements" because there are more of them than there are of recognizable ordered arrangements.

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    34 - Sharpless Oxidation Catalysts and the Conformation of Cycloalkanes

    34 - Sharpless Oxidation Catalysts and the Conformation of Cycloalkanes

    Professor Barry Sharpless of Scripps describes the Nobel-prizewinning development of titanium-based catalysts for stereoselective oxidation, the mechanism of their reactions, and their use in preparing esomeprazole. Conformational energy of cyclic alkanes illustrates the use of molecular mechanics.

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    35 - Understanding Molecular Structure and Energy through Standard Bonds

    35 - Understanding Molecular Structure and Energy through Standard Bonds

    Although molecular mechanics is imperfect, it is useful for discussing molecular structure and energy in terms of standard covalent bonds. Analysis of the Cambridge Structural Database shows that predicting bond distances to within 1% required detailed categorization of bond types. Early attempts to predict heats of combustion in terms of composition proved adequate for physiology, but not for chemistry. Group- or bond-additivity schemes are useful for understanding heats of formation, especially when corrected for strain. Heat of atomization is the natural target for bond energy schemes, but experimental measurement requires spectroscopic determination of the heat of atomization of elements in their standard states.

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    33 - Conformational Energy and Molecular Mechanics

    33 - Conformational Energy and Molecular Mechanics

    Understanding conformational relationships makes it easy to draw idealized chair structures for cyclohexane and to visualize axial-equatorial interconversion. After quantitative consideration of the conformational energies of ethane, propane, and butane, cyclohexane is used to illustrate the utility of molecular mechanics as an alternative to quantum mechanics for estimating such energies. To give useful accuracy this empirical scheme requires thousands of arbitrary parameters. Unlike quantum mechanics, it assigns strain to specific sources such as bond stretching, bending, and twisting, and van der Waals repulsion or attraction.

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    32 - Stereotopicity and Baeyer Strain Theory

    32 - Stereotopicity and Baeyer Strain Theory

    Why ethane has a rotational barrier is still debatable. Analyzing conformational and configurational stereotopicity relationships among constitutionally equivalent groups reveals a subtle discrimination in enzyme reactions. When Baeyer suggested strain-induced reactivity due to distorting bond angles away from those in an ideal tetrahedron, he assumed that the cyclohexane ring is flat. He was soon corrected by clever Sachse, but Sachse's weakness in rhetoric led to a quarter-century of confusion.

    • 2 sec

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