Experiment 4- Hydroboration Oxidation Flashcards
What happens in the hydration of alkenes reaction?
The alkene reacts with the catalyzed acid to produce a stable carbocation. From there, water does nucleophilic attack to yield the Markovnikov product —-> OH on the MORE substituted carbon of the double bond.
If the molecule is acid-sensitive, what is another way that HYDRATION can be achieved?
Through oxymercuration-reduction.
What happens with oxymercuration-reduction reactions?
Mercury is added to the LESS substituted side of the alkene. From there, NA happens from water. Lastly, NaBH4 is used to reduce the C-Hg bond to a C-H bond. MARKOVNIKOV addition of alkenes.
What is the purpose of reduction reactions?
To make new C-H bonds
Is oxymercuration often used? Why or why not?
No, because mercury is severely toxic, rendering the reaction obsolete.
What is the product of hydroboration-oxidation of alkenes ?
Anti-Mark addition of OH.
What is used in the Hydroboration-Oxidation reaction of alkenes?
Boron based Lewis acid: borane or 9BBN. From there, the borane reagent adds to the LESS substituted side of the alkene. Hydrogen peroxide and a strong base are used to oxidize the C-B bond into a C-O bond, leaving boric acid as a byproduct of the reaction. ANTI-MARK addition of OH.
How many valence electrons does Boron have?
3, makes 3 bonds
How many bonds to trivalent boron and aluminum based compounds make?
3.
Explain what is significant about trivalent aluminum and boron based compounds.
Since they have 3 bonds with no lone pair, they are highly unstable, making them extremely reactive. THERE’S an empty p orbital.
What is the purpose of using THF and SMe2 with Hydroboration-Oxidation?
Since the borane reagent has an empty p-orbital and is highly unstable, THF and SMe2 are used to donate lone pair electrons to stabilize the compound. They bond strongly enough to stabilize it but weak enough to be able to be removed.
THF and SMe2 are merely lending their lone pair of electrons to boron, resulting in _____.
Dative bonds (bonds that are formed when electrons are donated from a neutral nucelophile).
Are charges included with dative bonds? What’s the significance of this?
No, they’re not. Since they aren’t included and there is a difference in boning between BH3.THF and NaBH4, for example, BH3.THF is able to be reactive with alkenes while the latter is not.
Which of the following about BH3 is false?
1) contains an empty p-orbital
2) it is a strong nucleophile
3) it is trivalent
4) it is stabilized by a Lewis-base donor, such as SMe2.
5) it’s a good electrophile.
ANSWER: 2
Why can BH3.THF and BH3.SMe2 readily react with alkenes?
Because the THF and SMe2 ligands can readily dissociate and be replaced by the alkenes.
