How does this reagent- Hg(OAc)2/H2O then NaBH4- react with an alkene?

[Lazerous]

And whatever the product is, will it be markovnikov or anti-markovnikov?

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[artanis]

Hey there,

This reaction is markovnikov. But more importantly, the reason it is markovnikov is due to the fact that there is a mercurinium intermediate in the mechanism and the water molecular attaches to the carbon that is more highly substituted, letting go the mercury-acetate. The NaBH4 is there to cleave the mercury acetate on the less substituted carbon for a hydrogen.

I hope this makes sense.

 

[PooyaH]

It adds cis-OH and H to the alkene and it's markovnikov.

 

[Lazerous]

Hey there,

This reaction is markovnikov. But more importantly, the reason it is markovnikov is due to the fact that there is a mercurinium intermediate in the mechanism and the water molecular attaches to the carbon that is more highly substituted, letting go the mercury-acetate. The NaBH4 is there to cleave the mercury acetate on the less substituted carbon for a hydrogen.

I hope this makes sense.

Yes it does. Thanks!

 

[artanis]

It adds cis-OH and H to the alkene and it's markovnikov.

it does not add cis. Anything that has a bromonium, chrolonium, in this case Mercurinium intermediate, it will be ANTI (not cis). The one that is "SYN" addition is the hydroboration which is known as the ANTI-markovnikov reaction. This is gonna be on there for sure so know it, either the anti-markovnikov addition to an alkene, or to an alkyne, but for the alkyne know that tautomorization occurs to produce an aldehyde.

 

[PooyaH]

it does not add cis. Anything that has a bromonium, chrolonium, in this case Mercurinium intermediate, it will be ANTI (not cis). The one that is "SYN" addition is the hydroboration which is known as the ANTI-markovnikov reaction. This is gonna be on there for sure so know it, either the anti-markovnikov addition to an alkene, or to an alkyne, but for the alkyne know that tautomorization occurs to produce an aldehyde.

You're right. I meant to say the overall product would be anti addition.

 

[Jessi4eve]

Markovnikov anti addition of water to double bond WITHOUT rearrangements. That's the correct answer in 10 words.😀

 

[Contach]

Markovnikov anti addition of water to double bond WITHOUT rearrangements. That's the correct answer in 10 words.😀

what type of rearrangement are u talking about?
and how does an alkyne addition tautomerize into an aldehyde???

 

[dentalknight]

what type of rearrangement are u talking about?
and how does an alkyne addition tautomerize into an aldehyde???

no rearrangements means no hydride or methyl shifts. and an alkyne tautomerizes into an aldehyde when the triple bond is between the second to last and the last hydrocarbon. The triple bond will reduce to a double bond with an OH group connected to the last carbon. This enol will tautomerize into an aldhyde with resonance causing a more stable carbonyl resulting in an aldehyde. hope that makes sense

 

[artanis]

no rearrangements means no hydride or methyl shifts. and an alkyne tautomerizes into an aldehyde when the triple bond is between the second to last and the last hydrocarbon. The triple bond will reduce to a double bond with an OH group connected to the last carbon. This enol will tautomerize into an aldhyde with resonance causing a more stable carbonyl resulting in an aldehyde. hope that makes sense

This is correct. To add to this, if you add an -OH group to the markovnikov position of a terminal alkyne (i.e. carbon 2), it will tautomerize into a KETONE not an aldehyde. So the carbon 2 participates in the carbonyl group. So if you want an aldehyde from a terminal alkyne, add hydroborate it with either 1. 9-BBN, THF 2. H2O2, NaOH

 

[artanis]

what type of rearrangement are u talking about?
and how does an alkyne addition tautomerize into an aldehyde???

Okay, for adding an -OH, the only time you can get hydride/alkyl shifts is when you straight up add water in presence of acid. The mechanism has a carbocation intermediate and therefore it will do necessary hyrdride/methyl shifts in order to form the more stable carbocation. The reason why oxymercuration for a means of adding an -OH in the markovnikov sense is that there is no shifts possible due to the mercurinium ion formed in the intermediate.

 

[Mamona]

..

 

[nixon13]

i always couple hydroboration and oxymercuration together. They are like reciprocals. Hydroboration adds anti-mark and the OH and H are cis. Oxymercuration adds markovnikov and OH and H are anti.