Hydroboration
- “Initial Tails” and “Final Heads”
- 3 Ways To Make OH A Better Leaving Group
- A Simple Formula For 7 Important Aldehyde/Ketone Reactions
- Acetoacetic
- Acids (Again!)
- Activating and Deactivating
- Actors In Every Acid Base Reaction
- Addition – Elimination
- Addition Pattern 1 – Carbocations
- Addition pattern 2 – 3 membered rings
- Addition Reactions
- Aldehydes And Ketones – Addition
- Alkene Pattern #3 – The “Concerted” Pathway
- Alkyl Rearrangements
- Alkynes – 3 Patterns
- Alkynes: Deprotonation and SN2
- Amines
- Aromaticity: Lone Pairs
- Avoid These Resonance Mistakes
- Best Way To Form Amines
- Bulky Bases
- Carbocation Stability
- Carbocation Stability Revisited
- Carboxylic Acids are Acids
- Chair Flips
- Cis and Trans
- Conformations
- Conjugate Addition
- Curved Arrow Refresher
- Curved Arrows
- Decarboxylation
- Determining Aromaticity
- Diels Alder Reaction – 1
- Dipoles: Polar vs. Covalent Bonding
- E2 Reactions
- Electronegativity Is Greed For Electrons
- Electrophilic Aromatic Substitution – Directing Groups
- Elimination Reactions
- Enantiocats and Diastereocats
- Enolates
- Epoxides – Basic and Acidic
- Evaluating Resonance Forms
- Figuring Out The Fischer
- Find That Which Is Hidden
- Formal Charge
- Frost Circles
- Gabriel Synthesis
- Grignards
- Hofmann Elimination
- How Acidity and Basicity Are Related
- How Are These Molecules Related?
- How Stereochemistry matters
- How To Stabilize Negative Charge
- How To Tell Enantiomers From Diastereomers
- Hybridization
- Hybridization Shortcut
- Hydroboration
- Imines and Enamines
- Importance of Stereochemistry
- Intermolecular Forces
- Intro to Resonance
- Ketones on Acid
- Kinetic Thermodynamic
- Making Alcohols Into Good Leaving Groups
- Markovnikov’s rule
- Mechanisms Like Chords
- Mish Mashamine
- More On The E2
- Newman Projections
- Nucleophiles & Electrophiles
- Nucleophilic Aromatic Substitution
- Nucleophilic Aromatic Substitution 2
- Order of Operations!
- Oxidation And Reduction
- Oxidative Cleavage
- Paped
- Pi Donation
- Pointers on Free Radical Reactions
- Protecting Groups
- Protecting Groups
- Proton Transfer
- Putting it together (1)
- Putting it together (2)
- Putting it together (3)
- Putting the Newman into ACTION
- Reaction Maps
- Rearrangements
- Recognizing Endo and Exo
- Redraw / Modify
- Robinson Annulation
- Robinson Annulation Mech
- Sigma and Pi Bonding
- SN1 vs SN2
- sn1/sn2 – Putting It Together
- sn1/sn2/e1/e2 – Exceptions
- sn1/sn2/e1/e2 – Nucleophile
- sn1/sn2/e1/e2 – Solvent
- sn1/sn2/e1/e2 – Substrate
- sn1/sn2/e1/e2 – Temperature
- Stereochemistry
- Strong Acid Strong Base
- Strong And Weak Oxidants
- Strong and Weak Reductants
- Stronger Donor Wins
- Substitution
- Sugars (2)
- Synthesis (1) – “What’s Different?”
- Synthesis (2) – What Reactions?
- Synthesis (3) – Figuring Out The Order
- Synthesis Part 1
- Synthesis Study Buddy
- Synthesis: Walkthrough of A Sample Problem
- Synthesis: Working Backwards
- t-butyl
- Tautomerism
- The 4 Actors In Every Acid-Base Reaction
- The Claisen Condensation
- The E1 Reaction
- The Inflection Point
- The Meso Trap
- The Michael Reaction
- The Nucleophile Adds Twice (to the ester)
- The One-Sentence Summary Of Chemistry
- The Second Most Important Carbonyl Mechanism
- The Single Swap Rule
- The SN1 Reaction
- The SN2 Reaction
- The Wittig Reaction
- Three Exam Tips
- Tips On Building Molecular Orbitals
- Top 10 Skills
- Try The Acid-Base Reaction First
- Two Key Reactions of Enolates
- What makes a good leaving group?
- What Makes A Good Nucleophile?
- What to expect in Org 2
- Work Backwards
- Zaitsev’s Rule
Of all the reactions in organic chemistry 1, the ones that give students the most trouble are probably rearrangement reactions. Since I already talked about that, I should also talk about the second most problematic reaction: hydroboration.
Hydroboration – or to be more complete, hydroboration-oxidation – where we addhydrogen and boron to an alkene. It’s notable because it’s the most prominent “exception” in alkene addition reactions.
It’s the reaction that goes “anti-Markovnikoff”. After we add BH3, addition of hydrogen peroxide (H2O2) and a base (usually NaOH) results in formation of an alcohol at the least substituted carbon of the alkene.
So we’re breaking C-C (pi) and forming C-H and C-OH
It might help to understand “why” hydroboration is weird. Because when you look into it, it’s not really such an exception at all. It actually makes sense.
First of all, let’s quickly review why the addition of an acid like H-Br is “Markovnikoff” in the first place. Bromine, being more electronegative than hydrogen (3.0 vs. 2.2) polarizes the H-Br bond so that the H is partially positive (electrophilic). So the alkene (nucleophile) ends up attacking the hydrogen (electrophile), leading to formation of a carbocation at the most stable position (i.e. the most substituted), which is then attacked by bromine.
The polarization of the bonds in BH3 actually goes the other way. The electronegativity of H is 2.2 and that of boron is 2.0. So in this case it’s boronthat’s partially positive (electrophilic) and hydrogen that’s partially negative (nucleophilic). So the alkene (nucleophile) will attack the boron (electrophile) so as to put positive charge on the most stable position (i.e. the most substituted) which forms a bond with the hydrogen.
It’s the same principle! The less substituted end of the alkene becomes attached to the electrophile.
One key difference here is that it doesn’t go through a carbocation. Instead, everything happens at once: the B-H and C-C Pi bonds break, and the C-H and C-B bonds form.
The next step – oxidation – also gives students trouble. Here’s some key points:
- in the end, the C-B bond is exchanged for a C-OH bond
- the O-O bond is weak. Breaking the O-O bond is the “driving force” for this reaction
- the purpose of the NaOH is to deprotonate the H2O2, which then becomes a better nucleophile (“the conjugate base is a stronger nucleophile”)
- The stereochemistry of C-B is preserved in the C-O
- The reaction mechanism is a cousin of rearrangement reactions – we’re breaking a C-B bond and forming a C-O bond, while breaking O-O
When drawing the mechanism, it might help to draw the ugly version first. Worry about making it look pretty later.
Tomorrow – let’s think about how to look at reactions backwards.
Thanks for reading! James
P.S. To see the hydroboration reaction in step-by-step detail, here’s its entry in the Reaction Guide.
P.P.S. Rearrangment mechanism:
