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Freshman Organic Chemistry 2

Freshman Organic Chemistry 2

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33. Green Chemistry. Acids and Acid Derivatives

Green chemistry needs new asymmetric reactions and safer, more environmental Mitsunobu reactions. The Mitsunobu mechanism is general and reliable, but atom inefficient, generating almost 30 times as much weight of by-products as of the water it is designed to eliminate. Admirably green processes include autoxidation of aldehydes to carboxylic acids using only O2, and oxidation of alcohols by loss of H2 using a ruthenium catalyst. Relative pKa values of carboxylic acids provide insight into the role of inductive and resonance effects in organic transformations. One analysis suggests that the special acidity of carboxylic acids owes four times as much to inductive as to resonance effects. Carboxylic acids can be prepared both by oxidation and by reduction.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

49mins

2 Apr 2012

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35. Acyl Insertions and [gr]α-Reactivity

When a nucleophilic atom bearing a good leaving group attacks a carbonyl group, an adjacent R group can migrate to the new atom, inserting it into the R-acyl bond. This mechanism can insert O, NH, or CH2 groups into the acyl bond with informative stereospecificity in the case of the Beckmann rearrangement of oximes. Although the migrating groups are formally anionic, relative migratory aptitudes show that they give up electron density during rearrangement. Acid dissociation of protons [gr]α to a carbonyl group to form enolates, and the ease of forming enols, gives [gr]α-carbons nucleophilic reactivity under both basic and acidic conditions. This explains H/D exchange and racemization as well as halogenation and alkylation of [gr]α-carbons.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

48mins

2 Apr 2012

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36. [gr]α-Reactivity and Condensation Reactions

As in many synthetic procedures, an important challenge in ketone alkylation is choosing reagents and conditions that allow control of isomerism and of single vs. multiple substitution. [gr]β-Dicarbonyl compounds allow convenient alkylation and preparation of ketones and carboxylic acids. The aldol condensation, in which an [gr]a-position adds to a carbonyl group to generate a [gr]β-hydroxy- or an [gr]α,β-unsaturated carbonyl compound, can be driven to completion by removal of water. The Robinson annulation reaction is an important example of conjugate addition to [gr]α,β-unsaturated carbonyl compounds. [gr]α-Acylation of esters as in the Claisen condensation is a key step in the biosynthesis of fatty acids. Determining the constitutional structure of sugars posed a daunting challenge to early carbohydrate chemists.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

53mins

2 Apr 2012

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32. Measuring Bond Energies: Guest Lecture by Prof. G. Barney Ellison

Spectroscopic determination of bond dissociation energies is relatively straightforward for many diatomic molecules, but for polyatomic molecules it requires merging the results from a variety of challenging experiments. Professor Ellison describes how such techniques as flowing-afterglow mass spectroscopy and negative-ion photoelectron spectroscopy together with data on free-radical kinetics and heats of formation have allowed precise determination of the O-H, C-H, and C-O bonds in methanol and other compounds. Interpreting these reliable data provides new insight into the nature of chemical bonding and “resonance”.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

47mins

2 Apr 2012

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30. Oxidation States and Mechanisms

A difficult exam question shows how visible and NMR spectroscopy related to long-term misassignment of the structure for the triphenylmethyl dimer. Evidence from 1970 shows that Friedel-Crafts propylation involves an SN2 mechanism, not a protonated cyclopropane. Assigning oxidation states from -4 to +4 to the carbon atoms of proposed starting material and product allows choosing whether a reagent that is oxidizing or reducing or neither is appropriate. Beyond belonging to the appropriate redox class, the reagent must have an appropriate mechanism. Alcohol oxidations by elemental bromine and by Cr+6 reagents are shown to involve familiar substitution, elimination, and addition mechanisms. Mechanistic understanding allows adjusting conditions to make oxidation selective.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

46mins

2 Apr 2012

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29. Imines and Enamines. Oxidation and Reduction

Imines are pervasive in chemistry and biology, playing key roles both the in artificial Strecker synthesis of amino acids and their biosynthesis by L-glutamate dehydrogenase and by transamination. Imines are also involved in Stork’s [gr]α-alkylation and acylation of ketones by way of enamine intermediates. Oxidation and reduction in organic chemistry can involve actual electron transfer, when ion-radical intermediates are involved as in the formation of Grignard reagents or in the pinacol reduction. But more often in treating the covalent molecules of organic chemistry atomic oxidation states are used as an artificial bookkeeping device that helps suggest reagent choice for transformations that do not involve literal electron transfer. Oxidation states are assigned by pretending that covalent bonds between different atoms are purely ionic.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

50mins

2 Apr 2012

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28. Mechanism and Equilibrium of Carbonyl Reactions

This lecture aims at developing facility with devising plausible mechanisms for acid- and base-catalyzed reactions of carbonyl compounds, carboxylic acids, and their derivatives. When steric hindrance inhibits the A/D mechanism of Fischer esterification, an acid-catalyzed D/A mechanism can still occur. Substituent influence on the equilibrium constants for carbonyl hydration demonstrates four effects: bond strength, steric, electron withdrawal, and conjugation. Cyclic acetals play an important role in protecting the carbonyl groups of sugars, but acetals also can be used to protect alcohols, as can silyl ethers. Using amines instead of alcohols allows converting carbonyl compounds to imines via carbinolamines.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

49mins

2 Apr 2012

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26. Aromatic Substitution in Synthesis: Friedel-Crafts and Moses Gomberg

The Friedel-Crafts reaction creates new alkyl- or acyl-aromatic bonds, with or without cation rearrangement. Designing reaction sequences, especially those involving diazonium intermediates, so as to access a wide variety of substituted benzenes provides a good introduction to the challenges of synthetic organic chemistry. Aromatic rings with strong electron withdrawal can undergo nucleophilic aromatic substitution, which plays an important role in biochemistry. The special properties of phenyl-substituted alkanes, especially benzylic reactivity and steric hindrance, played an important role in the development of organic chemistry a century ago.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

50mins

2 Apr 2012

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25. C-13 and 2D NMR. Electrophilic Aromatic Substitution

Proton decoupling simplifies C-13 NMR spectra. Dilute double labeling with C-13 confirmed the complex rearrangement scheme in steroid biosynthesis. Two-dimensional NMR yields correlations between NMR signals that underlie structural determination of proteins and identification of the mechanism of a rapid carbocation rearrangement. Substitution of an electrophile for a proton on an aromatic ring proceeds by a two-step association-dissociation mechanism involving a cyclohexadienyl cation intermediate. The relative rates of forming various products from substituted benzenes correlates with the substituents’ influences on the stability of the various cyclohexadienyl cation intermediates. The spectrum of electrophile reactivities is very broad. Important contributions for activating electrophiles were made by Friedel and Crafts working in Paris.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

50mins

2 Apr 2012

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24. Higher-Order Effects, Dynamics, and the NMR Time Scale

Because spin-spin splitting depends on electron spin precisely at a nucleus, splitting by a C-13 depends on its orbital’s hybridization. “Higher-order effects” that give complex multiplets for nuclei with similar chemical shifts can be understood in terms of the mixing of wave functions of similar energy. Averaging of chemical shifts or spin-spin splitting may be used to measure the rate of rapid changes in molecular structure, such as changes in conformation or hydrogen bonding. Since the spectroscopic time scale depends on frequency differences, averaging is easier in NMR than in IR. A typical problem involves predicting the NMR spectrum of a compound with diastereotopic groups. In proton decoupling radio frequency irradiation of a particular proton can make it cease to split the NMR signals from nearby protons.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

51mins

2 Apr 2012

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23. Diamagnetic Anisotropy and Spin-Spin Splitting

Through-space interaction between magnets of fixed strength and orientation averages to zero during random molecular tumbling, suggesting that the local field about a proton should be sensitive only to electrons that orbit about itself. The chemical shift can be sensitive to electrons orbiting elsewhere if the amount of orbiting varies with molecular orientation. This “diamagnetic anisotropy” is commonly used to rationalize the unusual chemical shifts of protons in acetylene and in aromatic and antiaromatic compounds. The other source of a proton’s local field is nearby magnetic nuclei, which can be counted by the splitting multiplicity. Unlike chemical shift, which is measured in fractional units because it depends on the strength of the applied field, this spin-spin splitting (J), measured in Hz, is dependent only on molecular structure. J depends not on spatial proximity, but on orbital overlap, which, remarkably, is larger for anti- than for eclipsed conformations.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

48mins

2 Apr 2012

Rank #11

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22. Medical MRI and Chemical NMR

Magnetic resonance imaging (MRI) requires gradients in the applied magnetic field, while chemical nuclear magnetic resonance (NMR) requires a highly uniform field. When protons in different parts of the body can be driven to broadcast different frequencies, tomography allows reconstructing a three-dimensional image showing water location. Dependence of the signal intensity on relaxation allows BOLD functional MRI that shows brain activity. When the applied magnetic field is sufficiently uniform, chemical NMR spectra differentiate proton signals according to local field variations within molecules. Modern research in a chemical laboratory like Yale’s depends on the availability of many magnetic resonance spectrometers. Peak integrals show the relative number of protons in different molecular environments, while peak frequencies or “chemical shifts” show the bonding environment of groups of protons. Often downfield (deshielded) or upfield (shielded) shifts are correlated with local electron density.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

49mins

2 Apr 2012

Rank #12

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21. Functional Groups and Fingerprints in IR Spectroscopy. Precession of Magnetic Nuclei

Infrared spectroscopy provides information for analyzing molecular structure and for understanding bonding and dynamics. Although the normal modes of alkanes involve complex coordinated vibration of many atoms, the unusual strengths of multiple bonds give alkenes and alkynes distinctive stretching frequencies. The intensity of characteristic out-of-plane C-H bending peaks allows assignment of alkene configuration. Characteristic carbonyl stretching peaks in various functional groups demonstrate the importance of pi- and sigma-conjugation. The complex fingerprint region of IR spectra differentiates the subtle isomerism of polymorphic crystalline pharmaceuticals. A 90° phase lag between force and velocity explains the precession of tops and of magnetic nuclei in a magnetic field. Nuclear precession in the combination of a stationary magnet and a pulsed radio-frequency field can be visualized by means of the “rotating frame.”Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

53mins

2 Apr 2012

Rank #13

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20. Electronic and Vibrational Spectroscopy

Time-dependent quantum mechanics shows how mixing orbitals of different energy causes electrons to vibrate. Mixing 1s with 2p causes a vibration that can absorb or generate light, while mixing 1s with 2s causes “breathing” that does not interact with light. Many natural organic chromophores involve mixing an unshared electron pair with a vacant pi orbital, whose conjugation determines color. Infrared spectra reveal atomic vibration frequencies, which are related by Hooke’s law to bond strengths and “reduced” masses. Infrared spectra are complicated by the coupling of local oscillators of similar frequency to give “normal” modes. Alkane chains possess characteristic stretching and bending modes, with descriptive names, that may, or may not, absorb infrared light.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

49mins

2 Apr 2012

Rank #14

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19. Aromatic Transition States: Cycloaddition and Electrocyclic Reactions

Cyclic conjugation that arises when p-orbitals touch one another can be as important for transition states as aromaticity is for stable molecules. It is the controlling factor in “pericyclic” reactions. Regiochemistry, stereochemistry, and kinetics show that two new sigma bonds are being formed simultaneously, if not symmetrically, in the 6-electron Diels-Alder cycloaddition. Although thermal dimerization of thymine residues in DNA is forbidden, photochemistry allows the 4-electron cycloaddition. “Electrocyclic” ring opening or closing chooses a conrotatory Möbius pathway, or a disrotatory Hückel pathway, according to the number of electron pairs involved and whether light is used in the process. Dewar benzene provides an example of a very unstable molecule that can be formed photochemically and then persists because of unfavorable orbital overlap in the transition state for ring opening.Complete course materials are available at the Open Yale Courses website: http://oyc.yale.eduThis course was recorded in Spring 2011.

49mins

2 Apr 2012

Rank #15