CH 391: 4th Problem Set
1. Write the detailed mechanism for the reaction of benzene
with a general electrophile (E+). What is the generic name given to
the type of intermediate involved in this reaction? Provide three resonance
structures for the intermediate, summerize as a DL/PC stucture, and characterize the intermediate.
2. Since two of the above
canonical structures are equivalent and the third is nearly equivalent in
energy, what conclusions could be drawn, using resonance theory, concerning the
magnitude (qualitatively) of the resonance stabilization?
3.For the rds of this
reaction, use resonance theory to derive a TS model, summarize as a DL/PC, and
provide a characterization of the TS model. Then, use the Hammond Principle to
refine your characterization. Is the resonance stabilization of the TS large or
small? What effect would this stabilization, taken alone, have on the reaction
rate? In view of this, explain why reactions of electrophiles with benzene are
typically slower than with simple alkenes.
4. Adapt the general model to
the specific case of nitration. Then use this model to explain why nitration of
toluene occurs preferentially at the o, p positions. This will require you to
use the method of competing TS’s, i.e., showing the specific
DL/PC’s for ortho, meta, and para nitration of toluene, and comparing
their characters.
6. Write the detailed
mechanism for the base-catalyzed hydration of a carbonyl compound. Explain why
the first step is not an rds.
7. Use resonance theory to
derive a TS model for the slow step, summarize as a DL/PC, and characterize this model. Use this
model to explain why nucleophilic addition to carbonyl compounds is much more
facile than to simple alkenes, even though the carbonyl pi bond is much
stronger than the alkene pi bond. You should provide a comparative TS for the
alkene addition and provide at least two reasons for the faster addition to
carbonyl compounds.
8. Explain why 3-pentanone
reacts more slowly than acetone. Why does cyclohexanone react more rapidly than
acetone? Would you expect chloroacetone (i.e. chloromethyl methyl ketone) to
react more or less rapidly in the hydration reaction? Explain in terms of a TS
character.
9. Provide a detailed
mechanism for the acid-catalyzed hydration of a general carbonyl compound, and
derive and characterize a DL/PC model for the slow step. Explain why electronic
effects (EWG’s and EDG’s) are very small in acid-catalyzed
hydration.
Ester Hydrolysis.
10. Write the detailed mechanism for the acid-catalyzed hydrolysis of a general ester. What is the overall reaction type? What are the reaction types of each stage of the reaction? Explain how it was experimentally determined that acyl-oxygen rather than alkyl oxygen cleavage typically occurs. How can a direct displacement be ruled out?
11. Give the structure of a
specific ester which undergoes alkyl-oxygen cleavage via an SN1
–like mechanism; via an SN2 –like mechanism.