Hydroboration Question

[dreamaloud]

In Hydroboration of an alkene, do you consider BH3 a lewis acid or a lewis base? I have one book that says acid b/c it is electron deficient and it forms a pi complex with the pi bonds of the alkene, while another says that it acts as a lewis base. Help - so which one is it????

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[medicomel]

it's a lewis acid, and can accept a pair of electrons to gain a positive charge. BH4+

somebody correct me if i'm wrong.

 

[aamartin81]

it's a lewis acid, and can accept a pair of electrons to gain a positive charge. BH4+

somebody correct me if i'm wrong.

That is right, just like AlCl3 is a Lewis acid catalyst.

Adam

 

[Prophecies]

it's a lewis acid, and can accept a pair of electrons to gain a positive charge. BH4+

somebody correct me if i'm wrong.

In the reaction, however, BH3 binds to a carbon to become BH2-, so perhaps it is acting as a Lewis base? Once the BH3 loses the proton, it is then a good leaving group, and the OH can replace it...giving the anti-Markovnikov orientation observed in hydroboration-oxidation.

Either way, to the OP--you don't have to know if BH3 acts as a Lewis acid or base...in fact, you don't even *really* have to know the mechanism of this reaction. All you have to know is that if you see BH3/THF then you know that the reaction is hydroboration and oxidation, and it is used to make alcohols. Since alkene reactions are no longer a part of the MCAT, it would be useless to learn the mechanism unless you are really interested, in which case I explained it above 😉 Good luck to you!

 

[medicomel]

all you have to remember for the hydroboration oxidation reaction is that the alcohol is added in an anti-markovnikov fashion.

oops! i see the above poster has said that too. bears repeating! 😉

 

[ryche22]

all you have to remember for the hydroboration oxidation reaction is that the alcohol is added in an anti-markovnikov fashion.

oops! i see the above poster has said that too. bears repeating! 😉

and repeating again: hydroboration = the anti-markovnikov, syn-addition of water.

late
rick

 

[QofQuimica]

it's a lewis acid, and can accept a pair of electrons to gain a positive charge. BH4+

somebody correct me if i'm wrong.

You're partly right and partly wrong. OP, I think that you are confusing BH3 with BH4-. BH3 is a Lewis acid. The definition of a Lewis acid is that it can accept a pair of electrons. Boron typically only has six electrons, and therefore it can accept two more to gain a NEGATIVE charge and form BH4- (not BH4+ !!!) BH4- is a lone pair donor, which makes it a Lewis base. It is actually a hydride (H-) that does the donating, not the boron itself.