Question

a) There are several reagents that can be used to effect addition to a double bond, including: acid and water, oxymercuration--demercuration reagents, and hydroboration--oxidation reagents. Inspect the final product and select all the reasons why oxymercuration--demercuration was chosen to effect the following transformation instead of the other reagents.

b) Complete the mechanism for the reaction by adding curved arrows. The reagents chosen for the transformation are: 1) Hg(OAc)2, THF, H2O and 2) NaBH4, OH

Answer 1

Question (a)

Oxymercuration, hydroboration, and addition of acid and water all carry out the same reaction--addition of an H and an OH (that is, H₂O) across a double bond. There are subtle differences between each of these methods.

Hydroboration yield the anti-Markovnikov product. This is a regioselective process--that is, the OH is added to the less subtituted carbon. Compare this to addition of acid/water or oxymercuration where you add the OH to the more substituted carbon.

That being said, the main difference between oxymercuration and acid/water is that oxymercuration does not form carbocations. This means re-arrangements cannot occur and you get exactly the Markovnikov product you would expect.

Thus choices (1), (4), (5), and (7).

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Question (b)

I've drawn the mechanism below. It involves the formation of a bridged mercury cation and then subsequent reduction using borohydride. Th mechanism you drew is basically correct, I just added arrows.