Mechanisms and Concerted Reactions Practice Problems

Fall Term, 2008

The following are problems chosen to make you think about mechanisms and the Woodward Hoffman rules. Work with the other students in the class to solve them. Remember, if you already know the answer to a problem, it is not a problem but an exercise (George Bodner).

1. The following reactions have been observed. For each reaction
   1. Come up with a reasonable mechanism to explain the transformation of starting material to product.
   2. Decide which step is the rate-determining step and, if it is the first step, write a rate law for the reaction. Ignore any initial step in which a strong acid, such as sulfuric acid, is used as a catalyst to generate hydronium ion.
   3. Draw an energy diagram for the reaction.

      

      

      

      

      

      

      7)  Ethyl acetoacetate can easily be prepared by a Claisen condensation of two molecules of ethyl acetate. If a strong acid, such as dilute sulfuric acid, is used in place of the acetic acid, and care is not taken to keep the reaction mixture cold, acetone, instead of ethyl acetoacetate, is the isolated product. Write mechanisms to explain the two different results.

      

2. The basicity of a compound depends on the availability of an electron pair to bond to a proton. The less available the electron pair is, the lower the basicity of the base will be, and the larger the acidity of the corresponding conjugate acid. Several factors can affect the availability: electronegativity of the atom with the electron pair; inductive effects of groups attached to the basic atom; resonance factors; and steric factors. Explain the differences in basicity for the three nitrogen bases shown below,
3. The following disrotatory electrocyclic ring-opening reaction has been observed. Is this reaction allowed or forbidden in the ground state (thermal) and allowed or forbidden in the excited state (photochemical). Explain your answer using correlation diagrams.

    

4. The following is a photochemical cycloaddition of a 4n system, with n = 2. Show, using the molecular orbitals for the appropriate HOMO and LUMO why this reaction can occur.
5. The anion shown below will undergo an electrocyclic ring closure under both thermal and photochemical conditions. One set of conditions leads to cis methyl groups and the other leads to trans methyl groups. Draw correlation diagrams for both conditions and decide which will lead to the cis and which the trans anion.