Advanced Organic Chemistry Problem Set 5 DUE: Friday April 30 @ 1:00

Advanced Organic Chemistry

Problem Set 5

DUE: Friday April 30 @ 1:00 pm

I hereby certify that the work contained in this problem set is exclusively my own, with the understanding that I am able to use my course notes and CheMagic as the only sources of reference.

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1. What can be deduced about the aromaticity of the following molecule from the chemical shift of its protons? Explain your rationale. Does the prediction from 1H NMR spectroscopy agree with your assessment based on Huckel number of pi electrons?

Hi Ho

Hi = 10.4 ppm Ho = 5.4 ppm

2. The cyclobutenyl carbocation is suggested to benefit from homoaromaticity. Explain how this might be possible using appropriate structures.

3. Consider the following two triple bond–containing compounds: i. The compound below was determined to be aromatic. What does this mean with regards to the extent to which the triple bond can contributes to aromaticity? Why can only some of the electrons be contributed? (Note: there is a geometrical reason).

ii. The following compound is curious in that at first glance, based on Huckel number of pi electrons, it would appear to be anti-aromatic (why??). However, it was demonstrated to be aromatic. Explainb how this is possible. (HINT: consider carefully the implications of the P-orbital arrangement of the allene C=C=C bonding situation. CheMagic has a problem if you try to draw the entire molecule, so just view the P-orbitals for the bottom C=C=C=C bond [click on sp and sp2 in the CheMagic model window to view]).

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4. Comment on the anticipated aromaticity of the compound below:

5. The two compounds below are expected to be aromatic. The 1H NMR chemical shifts of the protons on the periphery of the rings are quite similar. However, the methyl resonances are quite different. This was attributed to a difference in shape of the molecules. What do the differing CH3 resonance values imply about aromaticity? Explain how the shape must influence the diamagnetic ring current differently at the center of the ring than at the periphery to rationalize the difference.

δH = 8.04–8.77 ppm δH = 8.11–8.64 ppm CH = –2.06 δCH3 = –4.25 δ 3

6. Predict the pi-molecular orbitals and number of nodes present for the cyclopentadienyl carbocation like we did for benzene and cyclobutadiene. Is the compound predicted to be aromatic, anti-aromatic or non-aromatic? Explain.

7. Predict the MOs for the pi system of octatetraene below (i.e., shade the orbitals). Under the proper column, provide the number of nodes for each MO and then fill the MOs with the proper number of electrons. Label the HOMO and the LUMO

# of nodes electrons

octatetraene

i. Will the thermal electrocyclic ring closure of octatetraene be a conrotatory process or a disrotatory process? Explain using the appropriate structures

ii. Based on your answer in part i, predict the electrocyclic product (including stereochemistry) of the following reaction:

100 °C

iii. Will the photochemical electrocyclic ring closure of octatetraene be a conrotatory process or a disrotatory process? Explain using the appropriate structures

iv. Based on your answer in part iii, predict the electrocyclic product (including stereochemistry) of the following reaction:

8. What is the predicted product of the following Diels Alder reaction? Include the stereochemistry that is expected of the major product.

NO2 heat

NO2

9. In class we discussed that 5-methylcyclopentadiene undergoes rapid isomerization to form mixtures of methylated cyclopentadiene compounds. Interestingly, 7- Methylcycloheptatriene is stable to such isomerization. Explain this difference in behavior (include both electronic and geometric reasons).