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Stereoisomeric Relationships

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Z. Afrer an aqueous solution containing Co(OH)3 and three equivalents of alanine, H2NCH(CH3)C02H2, is heated four neutral, six-coordinate compounds having the formula {Co[(H2NCH(CH3)CO2]3} can be isolated. Two of these compounds are red and have nearly identical absorption spectra, which suggests they have the same stereochemical arrangement of donor atoms about the cobalt ion. Two of these compounds are red and have nearly identical absorption spectra, which suggests they have the same stereochemical arrangement of donor atoms about the cobalt ion. The two red isomers each exhibit a single meythl resonance in their proton NMR spectra. (The methyl resonance is a doublet as a result of coupling to the methine proton). The other two compounds are violet and have nearly identical absorption spectra again indicating that the stereochemical arrangement of donor atoms about the cobalt ion is the same in both isomers. However, both of the violet compounds exhibit three methyl resonances(als odoublets) in their proton NMR spectra.

a) Structures of two of the four isomers of tris(alaninato)cobalt(III) are shown below. Expand on this generic to indicate the absolute configuration (A or delta) of the metal center, the stereo-chemistry of the donor set (fac or mer) and the absolute configuration (R or S) of the chirality centers in the bidentate ligand.
b) What is the stereoisomeric relationship between these structures, i.e., constitutional isomers, diastereoisomers, enantiomers, etc.
c) Name the two stereoisomers whose structures are not shown in the same rnanner as done in a)?
d) What would be be the name of the enantiomer of L-mer-tris(S-alaninato)colbalt(Ill)?
e) Indicate which isomers have a single methyl doublet in their proton NMR specta and which display three methyl doublets.

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a) The isomers are redrawn below in the same order as given. Facial isomers are when two similar ligands are 90 degrees from each other while in meridian they are 180 from each other. Check this file for explanation of the metal center ...

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The solution discusses stereoisomeric relationships.

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Stereochemical Analysis on Molecules

CHEM 433

Turn your work in on a separate set of paper. Your sheets must be stapled, and if torn from a spiral-bound notebook, must be free of the little paper frills that are present if you just rip it out of the book without using the perforations.

1. Label the hybridization of the non-hydrogen atoms in the following structures.

2. Draw the σ and σ* molecular orbitals diagrams for the discrete and localized σ bond formed between the carbon and chlorine in chloroethane. Indicate any polarization this bond may have in your diagram by drawing the shapes of the discrete bonding and antibonding orbitals.

3. Draw the π and π* molecular orbitals for the discrete and localized π bond in acetone (propan-2-one). Indicate any polarization this bond may have in your diagram by drawing the shapes of the discrete bonding and antibonding orbitals.

4. Formaldehyde has a very large dipole moment (2.33 Debye) whereas carbon monoxide has a very small dipole moment (0.11 Debye). Use resonance and electronegativity arguments to explain these results.

5. Consider bond dipoles to predict which conformer of formic acid should have the higher dipole moment: A or B?

6. It is known that bulky substituents prefer the equatorial to the axial position in cyclohexane. Nevertheless, the equilibrium shown lies to the right. Provide an explanation for this:

7. Explain why the cyclohexane derivative shown prefers the conformation with the methyl group axial (B) rather than equatorial (A).

8. Sketch a Newman projection of what you think would be the preferred conformation of hydrazine (H2NNH2), and briefly explain your choice.

9. Bicyclopentyl shows a strong preference for the conformation with the highlighted hydrogens anti. Using any relevant chemical arguments, rationalize this result.

This next one might be a bit tough:
10. Usually cyclohexane A values are reported at ΔGo values, but for some substituents, ΔHo and ΔSo values are also available. Such values are shown below for methyl and isopropyl (equatorial to axial interconversion). Consider the two chair conformers of cis-1-methyl-4-isopropylcyclohexane. Calculate the percentage of each form present at (A) 300K, (B) 100K, and (C) 75K.

Subsitituent ΔHo (kcal/mol) ΔSo (cal/mol·K)
methyl 1.75 0
isopropyl 1.52 -2.31

11. Predict the preferred conformation of fluoromethanol, FCH2OH, around the carbon/oxygen bond and briefly rationalize your choice.

12. Provide an explanation as to why cyclopentene is less strained than cyclopentane.

13. Rationalize the trends in the table using the following equilibrium reaction:
R-group s-trans s-cis
Methyl 0.7 0.3
Ethyl 0.55 0.45
Isopropyl 0.3 0.7
Tert-butyl ~0 ~1

14. I know we didn't do anything like this in lecture but give it a try. Predict the most stable conformation of the following molecule (Hint: think intramolecular interactions).

This next one will be tough, too.
15. Given that the A value for a methyl group on cyclohexane is 1.8 kcal/mol, draw the energy diagram for the ring interconversion that takes the methyl group from an equatorial to an axial position. What is your prediction as the relative energies for the different twist-boat conformations, and how does this affect your prediction to the lowest energy pathway for the ring interconversion (use a model kit if you need to).

16. Do a full stereochemical analysis of the following molecule. Label all stereoisomeric relationships (enantiomers, diastereomers and/or meso/identical).

Bonus: what is the name of this molecule (trade name, not IUPAC).

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