Water acts as a nucleophile, attacking the carbocation to lead to the formation of alcohols.
Reagents: BH₃ and THF
BH₃ (Boron Hydride):
Structure: Boron is electron-deficient with only 6 valence electrons.
Requires stabilization by a solvent, typically THF (tetrahydrofuran).
THF:
Acts as a solvent to stabilize BH₃ by weakly binding to boron using lone pairs, though it's not a true covalent bond.
Mechanism of Reaction with BH₃
When BH₃ interacts with an alkene:
The hydrogen (H) is less electronegative than carbon, leading to a polar bond.
A partial positive charge (δ⁺) develops on boron, and a partial negative charge (δ⁻) on hydrogen.
Electron-rich species (like a pi bond) will attack the electron-poor boron.
As the alkene forms a bond with boron, a positive charge (carbocation) forms on the adjacent carbon.
The hydrogen from BH₃ helps stabilize the carbocation, leading to the more substituted alkene (Markovnikov addition).
Regioselectivity and Stereoselectivity with BH₃ = H₂O
First step: BH₃ adds across the double bond to form the more substituted carbocation.
Second step: The reaction converts BH₂ into OH when water is introduced.
Result: Hydroboration-oxidation results in the OH being located at the least substituted carbon (due to anti-Markovnikov addition).
Stereochemistry of Reactions
The reaction mechanism with BH₃ and H₂O is stereoselective, producing only syn addition of H and OH on the same face of the double bond.
Important Detail: During hydrogenation (like with H₂ and a metal catalyst), both hydrogens will add to the same side (syn addition).
If considering more complex reactions:
Reacting with a compound like H₃O⁺ would yield OH on the more substituted carbon instead (Markovnikov addition).
Stability and Hydrogenation Process
In the presence of a metal catalyst (like platinum or palladium), H₂ will be added across the double bond, transforming it into an alkane.
Notable Point: This reaction is very efficient and requires the proper metal catalyst to work at normal pressures.
Hydrogenation will generally cause the two hydrogens to add on the same side, leading to a syn product.
Bromination of Alkenes
In the lab:
Alkene reacts with Br₂ (brown-red), leading to the rapid consumption of the brown color, indicating reaction.
The outcome is formation of an anti product due to the mechanism of bromine addition:
The first bromine attaches to the alkene, forming a cyclic bromonium ion.
The second bromine attacks from the opposite side, resulting in an anti addition (trans stereochemistry between bromines).
This is explained by the need for bromide to attack the less hindered carbon, leading to anti orientation.
Notable products are usually a pair of enantiomers due to this anti addition mechanism.
Electrophilic Addition and Stereochemistry
Key Mechanism:
If an alkene approaches bromine, the electrons will polarize, enabling the alkene to attack the more positive site.
Stability of the generated carbocation governs the product outcome:
Important to note that only anti addition occurs with bromine due to the back-attack mechanism on the carbocation.
Summary of Key Takeaways
Hydration via BH₃ results in syn addition, with OH on the least substituted carbon, contradicting typical Markovnikov behavior seen with H⁃X (like H₃O⁺).
Alkene reactivity with Br₂ will yield anti addition products due to formation of a cyclic bromonium ion leading to backside attack by bromide (Br⁻).
The presence of a metal catalyst and sufficient reactants is required for effective hydrogenation and reduction of alkenes.
Observing the structure and connectivity during transformations will yield insights into resulting stereochemistry and regioselectivity across various electrophilic additions.