PRACTICE EXERCISES FOR CHAPTER 17
1. Arrange in order of increasing thermodynamic stability all of the acyl derviates which we consider in class ( esters, carboxylic acids, amides, anydrides, carboxylate anions, and acid chlorides). Explain this order of thermodynamic stability bases upon resonance effects.
2. What net effect does the Cl in an acid chloride have upon the thermodynamic stability of the carbonyl group, i.e., does it stabilize or destabilize? Explain. Explain why the oxygen atom of an ester does not have the same qualitative effect upon the stability of an ester function.
3.Classify the transformation of a carboxylic acid function to an acid chloride function as exergonic or endergonic (negative or positive standard free energy change). How can this transformation be accomplished conveniently? Why is this reagent successful in effecting this transformation.
4. Write the mechanism for the reaction of an acid chloride with an alochol. What is the product? Why is no acid or base catalysis needed here, whereas nearly all of the other acyl substitution reactions require either acid or bases catalysis or promotion or strong heating?
5.Write the detailed mechanism for the acid catalyzed hydrolysis of an ester. Compare this mechanism with that for the esterification of the same acid with the same alcohol (REALLY: DO THIS COMPARISON IN DETAIL AND VERIFY THAT ONE REACTION MECHANISM IS THE REVERSE OF THE OTHER WITH THE EXCEPTION OF THE USE OF HYDRONIUM ION AS THE CATALYST IN ONE CASE AND THE CONJUGATE ACID OF THE ALCOHOL IN THE OTHER CASE).
6.Now write the mechanism for the base promoted hydrolysis of an ester. What does "promoted" mean? Explain why the reaction is promoted and not catalyzed. What is the driving force for the reaction (which step and why)?
7.Provide a resonance treatment (three structures) for the conjugate acid of an amide. What is somewhat surprising about the position of the protonation? Explain in detail why this position is preferred.
8. Write the detailed mechanism for the acid promoted hydrolysis of an amide to a carboxylic acid. Is the conversion of an an amide to a carboxylic acid thermodynamically favorable? Why or why not? Explain why the reaction is promoted and not catlyzed.
9. Explain why the conversion of a carboxylic acid to an amide, although thermodynamically favorable, is not easy to bring about directly at room temperature.
10.How can amides be prepared from carboxylic acids under mild temperature conditions in an indirect fashion?
11.What is the hybridization state of nitrogen in amides and in ordinary amines? Explain the basis for the hybridization state of nitrogen in amides, using a an orbital depiction to assist your explanation.