Chemistry 253

Answer Key

Third Examination

1. Select the correct order of acidity for the following compounds:

The pKa values of 1, 2, and 3 are approximately 16, 45, and 10, respectively. The conjugate base of compound 1 is an aromatic ion.

2. Select the group that is most likely to have an electron donating inductive effect and resonance effect as a substituent on an aromatic ring.

A. -CH2CH3

B. -OCH3

C. -NHCH3

D. -SCH3

E. -PHCH3

To have an electron donating inductive effect the atom attached to the ring must be less electronegative than carbon. To have a resonance donating effect, it must also have non-bonded electrons. Phosphorous is the only element that fits both of these criteria.

3. Select the group G that will make the following reaction most likely:

This reaction involves a nucleophilic aromatic substitution. The rules are reversed in comparison to those we have discussed for electrophilic aromatic substitution. Deactivating groups in electrophilic aromatic substitution reactions are activators in nucleophilic aromatic substitutions.

4. Select the compound that you would expect to be least reactive in electrophilic aromatic substitution reactions.

Choice D. This compound has 38 nuclear protons holding on to 6 pi electrons. All the others have a smaller proton/electron ratio. Technically compound B has the largest proton/electron ratio, but sulfur is a larger atom than oxygen. If you compare A and B, you should conclude that A is less reactive than B because O is more electronegative than S.

5. Select the best definition of an aromatic compound

A. A compound that contains 4n + 2 pi electrons.

B. A compound that has a filled valence shell.

C. A compound that displays unusual stability compared to an alkene.

D. A compound that undergoes electrophilic substitution.

E. A compound that burns with a sooty flame.

Choices A and B describe necessary, but not sufficient conditions for a compound to display aromatic character.

6. How many of the following structures are aromatic?

A. 0

B. 1

C. 2

D. 3

E. 4

In the heterocyclic structure, the lone pair of electrons on the nitrogen are in the same plane as the sigma-bonded framework.

7.

Choice B is the best option.

8. Which of the following compounds would react with Cl₂/FeCl₃ the fastest?

Choice B. Of the various activating groups, methoxy is the strongest. Of the various deactivating groups, chloro is the least deactivating.

9. Select the compound that would be most reactive towards electrophilic aromatic substitution reactions.

Choice B. Structures A and B are the only two where both substituents are activators. In choice B the activating influence of the methyl group occurs at the same carbons as the activating effect of the OH group. This is not the case in choice A.

10. How many of the following compounds are a source of electrophilic bromine? NaBr, MgBr₂, CH₃Br, Br₂, AlBr₃

A. 1

B. 2

C. 3

D. 4

E. 5

Only Br₂ is a source of electrophilic bromine.

11. Select the best synthesis of.

Choice D.

12. Al Kane, Al Keene, Polly Ester, and Polly Merrick were invited to a costume party. "Hey,", Al Kane exclaimed, "I've got a great idea. Let's all go as cyclohexadienyl cations!" "What a great idea", Al and the pair of Pollys resonated. So they did, and they had a great time. One of them even won a prize for best costume. Who was it?

Al Kane won. His "costume" is the only one that represents a valid resonance structure for the cation formed during the chlorination of acetophenone. Choice C is less favorable than A because a positive charge is less stable on an oxygen than it is on a carbon when neither atom has a filled shell.

13.

Choice A. Hydrogen bonding should outweigh steric factors here.

14. Select the order of reactivity of compounds 1-4 with dibromine.

Choice A. Compound 1 is an alkene. It reacts essentially instantaneously with dibromine. The remaining three compounds are all aromatic; the electron/proton ratio increases on going from 2 to 3 to 4, i.e. 2 is electron rich while 4 is electron poor relative to benzene.

15.

The left hand ring is deactivated by the carbonyl group, so substitution will occur preferentially on the other ring. It will occur para to the methylene group for steric reasons.

16. How many of the following structures would have this MO diagram ?

The best answer was B, since it is the occupied orbitals that are important, and only the cyclobutadiene and the cyclopentadienyl cation have 4 electrons in a cyclic array. However, I allowed both choices A and B since I made an error in drawing the MO levels.

17. Which of the following transformations will not work?

Choice A. FeBr₃ is a catalyst. You need Br₂ to make the reaction go.

18. In the ¹H-NMR spectrum of phenol, the hydrogen atoms ortho to the OH group resonate at 6.77 ppm, 0.5 ppm higher than the hydrogen atoms in benzene. You want to know how much of this chemical shift difference is due to the inductive effect of the OH group and how much is due to its resonance effect . You decide to compare ¹H-NMR spectra of 2-butanol and butane because the chemical shift differences can only be due to the inductive effect of the OH group. The chemical shifts for these two molecules are

How much of the 0.5 ppm shift is due to the inductive effect of the OH group?

Inductive effects are transmitted through sigma bonds. To get an accurate comparison of the inductive effect of the OH group in the benzene/phenol pair with that in butane/2-butanol pair, you have to be certain the number of sigma bonds between the OH group and the H atom it is affecting is the same in both cases.

19. What is the correct order of the rate of bromination of the following compounds?

This is another example of where the electron/proton ratio determines the relative reactivities. It is 6/31 in 3, 6/32 in 2, 6/33 in 4, and 6/36 in 1.

20.

Choice C is the best option. In terms of resonance, there's no difference between choices A and C. C is preferred for steric reasons. In B and D, you can draw resonance structures where the nitrogen atom bears the positive charge and does not have a filled shell.

21. Select the MO diagram that best describes the pi system in purine

Choice B. This is a 10 pi electron system.

22. Only one of the following reactions will proceed as indicated. Which one?

Choice D. In A the acid will protonate the nitrogen and convert it to a meta director. In B the fluorine atom is a para director. In C the nitro group is such a strong deactivating group that Friedel-Crafts acylation does not occur.

23. Which of the following is not a valid resonance contributor for the intermediate formed during the reaction of 2-methylfuran with an electrophilic reagent, E⁺?

Choice D.

24. How many of the following alkenes are classified as electron rich?

The first and third structures are electron rich. Groups that are activators in electrophilic aromatic substitution reactions are electron donating when attached to a double bond. Deactivators are electron withdrawing. They make an alkene electron poor.

25. Arrange the following ions in order of increasing stability.

Structure 4 is anti-aromatic. It should be the least stable. Both double bonds in structure 3 stabilize the positive charge by resonance. There is no resonance stabilization of ion 1.