P5_conformations

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Unformatted text preview: ion to the two (equivalent) chair conformers, cyclohexane possesses a third stable conformer, commonly known as a twist-boat. It is known experimentally to be ~25 kJ/mol higher in energy than chair cyclohexane, meaning that it makes up only 0.xx% of the total sample at room temperature. The temperature would need to lowered to xx K in order for twist-boat cyclohexane to make up 10% of a sample and thus be easily observed. One plausible mechanism for interconversion among chair conformers of cyclohexane involves the twist-boat conformer as an intermediate. An “animated” energy profile obtained from HF/6-31G* calculations shows what is going on. At the outset, the two ends (CH2 groups) of the chair conformer point in opposite directions. The end that points down moves upward, until five of the six carbons roughly lie in a single plane. This is the transition state (a so-called half-chair conformer). Further upward motion leads to the twist-boat intermediate. The process is reversed with the other CH2 group (starting from the twist boat). It moves down through a second half-chair structure and then to the other chair conformer. half chairs twist boat chair c hai r 23 Open the document chair-chair interconversion in cyclohexane and step through (or animate) the sequence of structures. Identify the two stable conformers (chair and twistboat) and the transition state (half-chair). Note that the overall process appears to occur in a stepwise fashion. Proton NMR Spectrum of Cyclohexane: Calculations can be used to assist in the assignment of experimental NMR spectra. A simple example is provided by lowtemperature proton NMR spectrum of cyclohexane, which as previously indicated shows resonances at 1.12 and 1.60 ppm. Use the HF/6-31G* model to say which resonance arises from the equatorial hydrogens and which arises from the axial hydrogens. Twist-Boat Cyclohexanes: Obtain equilibrium geometries for both chair and twist boat conformers of cyclohexane using the HF/6-31G* model. Is the energy difference between the two conformers consistent with the experimental estimate? Calculate the room-temperature equilibrium distribution of chair and twist-boat conformers. Is the higher-energy conformer likely to be seen? Elaborate. Repeat your calculations for 1,1-dimethylcyclohexane and 1,2-difluorocyclohexane. Is either likely to show a greater percentage of the twist-boat conformer than cyclohexane itself? Dipole Moments in Fluorocyclohexane: Fluorocyclohexane can adopt a conformation which either places the fluorine in an equatorial or axial position. Use the HF/6-31G* model to obtain equilibrium geometries for both conformers, and the use the Boltzmann equation to calculate an average dipole moment. Is the Boltzmann distribution dominated by one conformer or do both conformers contribute significantly? Distribution of Conformers at Equilibrium: Calculate the room-temperature equilibrium distribution of equatorial and axial conformers of methylcyclohexane and tert-butylcyclohexane. Use the HF/6-31G* model. What temperature would provide a 90:10 distribution of lower:higher energy conformer for each? Boat Cyclohexane: The actual process by which chair conformers of cyclohexane interconvert may actually be more complicated than that described above, and involve two (equivalent) twist-boat intermediates connected by a boat transition state. This picture suggests that the transition state connecting chair cyclohexane to the twistboat intermediate is higher in energy than the transition state connecting twist-boat intermediates. You already have obtained energies for chair and twist-boat conformers 24 from the HF/6-31G* model. Now, obtain data for the two transition states. For halfchair cyclohexane, start the structure with a structure with five carbons roughly in one plane; for boat cyclohexane, start with a structure in which opposing CH2 groups point in the same direction. Are your data consistent with this picture? Specifically, is the energy of the boat transition state lower than that of the half-chair transition state? CH Eclipsing Interactions in Cycloalkanes (no calculations required): The CH bonds in the chair form of cyclohexane are nearly perfectly staggered. On the other hand, it is not possible to completely stagger the CH bonds in seven-membered and larger cycloalkanes. As a consequence, it might be expected that hydrogenation of these compounds leading to the corresponding n-alkanes would be more exothermic than hydrogenation of cyclohexane. CnH2n + CH3(CH2)4CH3 CH3(CH2)n-2CH3 + C6H12 Build cyclohexane to cyclodecane and n-hexane to n-decane in a single document, and replace your structures with the corresponding T1 entries from the Spartan Molecular Database. Is hydrogenation of cycloheptane and larger cycloalkanes more exothermic than hydrogenation of cyclohexane? Is there a correlation between the energy of hydrogenation and the number of CH eclipsing interactions. (The T1 structures that you have employed correspond to the lowest-energy conformer.) Inversion Inversion is the name given to a process whereby a three-coordinate pyramidal center, most commonly a nitrogen or phosphorus center, p...
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This note was uploaded on 02/22/2010 for the course CHEM N/A taught by Professor Head-gordon during the Spring '09 term at University of California, Berkeley.

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