Chapter 15: Group 3 Practice Exercises
1. Write the detailed mechanism for the conversion of a hemiacetal to an acetal.Why does this reaction require acid, i.e., why doesn't it proceed in base? Why does iti proceed via an SN1-like mechanism?
2. Draw the structure of the hemiacetal intermediate formed from a carbonyl compound and ethylene glycol.
3. Explain why a Grignard reagent can not be prepared from 3-bromopropanal.
4.Explain why a molecule having both a ketone and an ester function cannot be selectively reduced to a primary alcohol using lithium aluminum hydride. Provide equations which illustrate how this result can be accomplished indirectly. What is the name of this strategy.
5. Explain why, even though it is a neutral molecule (not negatively charged) , triphenylphosphine is a very good nucleophile. Consider in your answer the character of the SN2 transition state and the factros that it is sensitive to.
6. Explain why the methyl protons of methylltriphenylphosphonium iodide are easily abstracted. As part of your answer, provide two resonance structures for the Wittig reagent. What is an "ylide" structure. Explain why phosphorous and not nitrogen is uniquely suitable for the Wittig reaction.
7.Is the conversion of a carbonyl group to an alkene double bond thermodynamically favorable? Why or why not. Explain why the Wittig reaction works, i.e., what is the thermodynamic "driving force"?
8. Write the stepwise mechanism for the Wittig reaction.