CHM 746 EXAM 2 (Take Home) 100 Points Due March 15, 2004

CHM 746 EXAM 2 (Take Home) 100 Points Due March 15, 2004

Answer Key

1. (5) Propose a structure for an alkyl halide that gives (Z)-3-methyl-2-phenyl-2-pentene and none of the (E) isomer on E2 elimination. Indicate the stereochemistry.

2. (25) Show reasonable arrow-pushing mechanisms for each of the following transformations:

a)

b)

c)

d)

e)

3. (4) In the following question, choose the one best answer. Explain your answer briefly.

The strongest acid:

(a) p-nitrobenzoic acid (b) p-bromobenzoic acid the nitro group is the strongest

electron-withdrawing group -

c) m-methylbenzoic acid (d) m-methoxybenzoic acid enhancing the d+ on acid H

4. (30) Show the first step of an arrow-pushing mechanism between each of the following pairs of molecules.

5. (9) Show reasonable products or missing reactants from each of the following reactions. Indicate stereochemistry, as appropriate.

a)

b)

c)

6. (4) Why do you think that Ph3P is used for Wittig reactions, rather than, e.g., Me3P? What problems might be encountered with the latter?

The ylide prepared with trimethylphosphine has acidic protons on the methyls, one of which could be abstracted by the negative carbon atom of another ylide. This cannot happen with the ylide from triphenylphosphine.

7. (4) Ethyl vinyl ether reacts with ethanol in the presence of an acid to yield 1,1-diethoxyethane rather than 1,2-diethoxyethane. Why?

8. (6) In each of the following pairs, circle the compound which will react more readily with the reagent indicated. You need not explain why.

9. (4) The reaction shown below is a possible variation on the Chugaev reaction. Give reasons why this variation is not used.

Carbon dioxide is a gas – more difficult to work with than CS2; the O anion in the intermediate is a much poorer nucleophile than the S analog – slowing reaction with MeI; the final thermal elimination would have to be conducted at much higher temperature since the carbonate is more stable than the S analog.

10. (4) A student reacted benzamide with ethyl magnesium bromide. He / she observed reaction occurring (vigorous bubbling and gas evolution). However, after work-up with aqueous acid, the starting material was recovered in nearly quantitative yield. Why ? Which gas was evolved ?

The gas was ethane. The Grignard is acting as a base and abstracting an NH proton (giving a resonance stabilized ion). Addition of acid reprotonates the azaenolate giving back benzamide.

11. (5) A different student tried the reaction of N,N-diethylbenzamide with ethyl magnesium bromide. No gas evolution occurred. However, on work-up with aqueous acid, a good yield of PhCOEt was obtained. Give a mechanism and explain why the reaction took a different course.

There are no acidic hydrogens a to the C=O. Attack by the ethyl anion occurred at the C of the C=O breaking the pi bond. The anionic intermediate is sufficiently stable (and the N anion is sufficiently poor as a leaving group) to survive until addition of acid. The latter helps the leaving group to leave.