[tyler059]

Additions such as hydroboration, oxymercuration-demercuration and acid catalyzed hydration add (H) and (OH) to alkenes. How do you discern the reagents neccessary when all 3 of these reactions yield the sought after product? For example, the question states "Provide the reagents neccessary to accomplish the additon of H and OH to 1-ethyl-cyclopentene" and the answer is the reagents for an oxymercuration-demercuration reaction (Hg(OAc2) and NaBH4), which gives a markovnikov product.  Is it possible to also accomplish this reaction through an acid-catalyzed hydration of 1-ethyl-cylcopentene?

PLEASE HELP I HAVE AN ORGANIC QUIZZ TOMORROW !

[discodermolide]

You left it a bit late!
Look here http://www.chem.memphis.edu/parrill/EMOC/roadmap/hyd.html

[tyler059]

I understand I left it a bit late. Thanks for the link, but I'm just wondering if there is more than one possiblity for a reagent to yield the same products, or if there is a preferential reagent due to the kind of substrate? For instance, acid catalyzed hydration adds H and OH in a markovnikov fashion, following syn addition. An oxymercuration-demercuration reaction also adds H and OH in a markovnikov fashion, following syn addition. Therefore, is there a preferred one over the choice of reagent/addition pathway, or should either answer for reagents suffice?

[discodermolide]

Either answer should suffice, however, one of the reactions employs Hg, which of course is toxic and creates a waste problem. So I personally would not use it.

[tyler059]

Great! Thank-you for your help

[orgopete]

There should be more method than madness to electrophilic additions to alkenes. If you were to perform an acid catalyzed addition to an alkene, you may find rearrangement reactions accompanying the addition. For example, the acid catalyzed hydration of 3-methylbut-1-ene will give a mixture 2- and 3-methylbutan-2-ol. Mercury has non-bonded electrons than can interact with the carbocation to compete or block the migration. Although this requires a reduction step to remove the mercury, it gives a cleaner product.

I have not performed an acid catalyzed hydration of an alkene, but I had always wondered about the generality of this reaction. What conditions are required? Why do I ask? An alcohol is a lot more basic than an alkene. The success of the reaction depends upon protonation of an alkene to generate a carbonation which can then react with water. What prevents the dehydration reaction from also occurring?

The acid catalyzed hydration of a mono-substituted alkene seems to work if it forms a monosulfate intermediate. The monosulfate is less basic and should resist elimination. It simply need an hydrolysis to recover the alcohol. 1,1-Disubstituted alkenes seems to react efficiently. I presume this may be because the additional carbon substituent increases the basicity of the alkene, increases the reaction rate, and allows the reaction to proceed in a more dilute acid.

If I needed to hydrate an alkene, I think I would prefer doing an acid catalyzed addition of acetic acid. The acetate will be less basic than an alcohol so the addition should be more general. Organic Synthesis has a prep of an acetate from an alkene. This too requires a saponification reaction to get an alcohol.