Chemistry 251/253

Answer Key

Homework 8

1. Draw the structure of Compound A in the following transformation. CCCC(=O)CC(=O)OC

2. Draw the structure of compound B in the following transformation.COC(=O)CC(=O)OC

 

3. Draw the structure of compound C in the following transformation. The * indicates a labeled carbon atom.CCC(=O)C(C)C(=O)CC

4. Draw the structure of compound D in the following transformation.CN(C)CC1CCCC1=O

5. Draw the structure of compound E in the following transformation.CC[N+](=Cc1ccccc1)CC

6. How many of the following compounds would react with acetone in the presence of a catalytic quantity of acid to form an enamine? 1 Enamines are formed by the reaction of aldehydes and ketones with secondary amines. Only the last structure is a secondary amine.

7. How many of the following compounds are enamines?0

8. Draw the structure of compound F in the following transformation.

9.The following reaction produced a product that gave the 13C-NMR spectra shown below. Draw the structure of the product of the reaction.CC(CC#N)C1CCCC1=O This is a Michael addition followed by an acid catalysed hydrolysis reaction that regenerates the ketone from which the enamine was prepared originally.

 

10. Imagine deprotonating each of the compounds shown below. How many of those deprotonation reactions produce an aromatic anion? 1 Deprotonation of cyclopentadiene generates a resonance stabilized anion that contains 6 pi electrons.

11. How many pi electrons are there in ? 14

12. How many of the following compounds obey Huckel's rule?3 The first compound has 10 pi electrons, while the second and third have 14. The last structure has 12 pi electrons.

Questions 13-14 involve compounds that contain only carbon and hydrogen. Be careful. Interpreting the spectra is tricky.
13. The 13C-NMR data for this compound is shown below. The 1H-NMR spectrum is attached. The molecular weight of the compound is 202. Draw the structure of this compound. c1cc2ccc3cccc4ccc(c1)c2c34

14. The 13C-NMR data for this compound is shown below. The 1H-NMR spectrum is attached. The molecular weight of the compound is 176. Draw the structure of this compound.The H-NMR makes it clear that there are only two types of hydrogens. The lack of splitting indicates that the hydrogen atoms of one type must be insulated from those of the other type by an intervening atom that contains no attached hydrogen atoms, i.e. a fragment such as Ha-C-C-C-Hb. The C-NMR indicates there are three types of carbons. Furthermore, the DEPT spectra establish that two of the carbons are CH while the third is quaternary. While this molecule is not aromatic according to Huckel's rule, the two 6-membered rings are, as attested to by the fact that Hb resonates in the aromatic region. The signal at 8.18 is not indicative of aromatic protons, but rather of a vinylic proton that is shifted well downfield by virtue of the fact that the double bond is conjugated to both 6-membered rings. c1cc2ccc3ccc4ccc1c2c43

 

15. Select the picture that does not represent a valid MO of 1,3,5-hexatriene, H2C=CH-CH=CH-CH=CH2.C The MOs should contain an element of symmetry.

We have seen that the solvolysis of alkyl bromides leads to the formation of carbocations. Conversely, treatment of alkyl bromides with lithium metal generates carbanions. We have also considered various factors that affect the relative stabilities of these ions.
16. Arrange the following isomeric alkyl bromides in order of increasing rate of solvolysis in a solution of aqueous ethanol.C<A<B Ionization of compound C would produce a carbocation with "anti-aromatic" properties because it would contain a cyclic pi system with 4 pi electrons in it.

17. Arrange the following isomeric alkyl bromides in order of increasing rate of reaction with lithium metal in THF.C<B<A The ion formed from compound C would be "anti-aromatic", while the one formed from compound A would possess aromatic properties, i.e. it should be the most stable ion.

18. The dianion produced in the following reaction is stable.Select the MO diagram that is most consistent with this observation. C The pi system of this dianion contains 10 electrons. It conforms to Huckel's rule where n = 2.

19. How many of the following molecules would be described by MO diagram ?1 You have to look at the pattern of the orbitals and the number of electron in those orbitals. The pattern indicates that the pi system is delocalized over 4 atoms. The occupancy says there are four electrons.

20. Select the MO diagram that best describes.D The structure shown contains 8 pi electrons and the pi system is spread over 7 atoms.

21. Draw the structure of the product of the following reaction.I accepted either chlorobenzene or bromobenzene for this question. I originally thought the reaction should proceed as shown below because the electrons on the bromine are more available than those on chlorine. That is why bromine is a better nucleophile than chlorine in an Sn2 reaction, for example. However, since chlorine is more electronegative than bromine, it is not clear which atom should be more nucleophilic in the present case. Clc1ccccc1 or Brc1ccccc1

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22. Polycyclic aromatic hydrocarbons, PAHs, are suspected human carcinogens. They are also responsible, in part, for the flavor of a charcoal broiled steak since they are components of the smoke that is produced when charcoal burns. Yum. A representative PAH is benzo[a]pyrene-. Since it is classified as a PAH, benzo[a]pyrene must conform to Huckel's rule. What is the value of n for this compound? 8 While this molecule contains 20 pi electrons, only 18 of them are arrayed around the perimeter of the structure. 4n + 2 = 18 when n = 8.

23. When Al Kane attempted to perform the synthesis shown below, the 1H-NMR spectrum of the compound he actually obtained indicated clearly that it was not the product he had hoped for. Draw the structure of the product Al made. The initially formed carbocation underwent a 1,2-hydride ion shift to produce the more stable tertiary carbocation. This intramolecular reaction occurs faster than the alternative intermolecular reaction with benzene. CC(C)C(C)(C)c1ccccc1 

24. Ethyl Ester planned the following synthesis. Draw the structure of compound F. ClC(=O)c1ccccc1

25. Draw the structure of the cyclohexadienyl cation intermediate that is formed in the following reaction. Show the charge on the atom where it first develops. OC1=C[CH+]C(Br)C=C1

26. The transformationrequires, among others, two of the reagents shown below. Enter the letters corresponding to those reagents.

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27. Draw the structure of the product of the following reaction:F[B-](F)(F)[Cl+]Cl

28. Draw the resonance structure that makes the most important contribution to the cyclohexadienyl cation BrC1C=CC=CC1=[Cl+]

29. Select the major product of the following reaction:D This is a simple electrophilic addition reaction of an alkene. The pi system of the isolated double bond is the most react part of theis molecule when it is treated with electrophiles such as dibromine.

30. Draw the structure of the product of the following reaction. Cc1cccc2C(=O)CCc21 This is an intramolecular Friedle-Crafts acylation.