Questions: Anti-Markovnikov Addition and Hydroboration

Question 1

What is the major product when 1-butene (CH₂=CHCH₂CH₃) undergoes hydroboration-oxidation with BH₃ followed by H₂O₂/NaOH?

• A. 2-butanol, because the OH adds to the more substituted carbon as in acid-catalyzed hydration
• B. 1-butanol, because the OH ends up on the less substituted carbon via the concerted mechanism
• C. A mixture of 1-butanol and 2-butanol, because carbocation rearrangement is possible
• D. Butyraldehyde, because boron is oxidized before the C–O bond forms

Answer: (B)

Hydroboration-oxidation gives anti-Markovnikov alcohol: the OH ends up on the less substituted (terminal) carbon, giving 1-butanol. Boron adds to the less hindered carbon (C-1) in the concerted step, and oxidation replaces boron with OH at the same carbon. Option A describes acid-catalyzed hydration (Markovnikov), which gives 2-butanol. Option C is wrong because there is no carbocation intermediate — the concerted mechanism prevents rearrangement entirely.

Question 2

Why does hydroboration give boron on the less substituted carbon rather than the more substituted carbon?

• A. Because the more substituted carbon has higher electron density, which repels the electron-deficient boron
• B. Because the carbocation intermediate forms preferentially at the less substituted position
• C. Because the concerted transition state places the bulky boron at the less sterically hindered carbon
• D. Because boron is electronegative and prefers to bond to less substituted carbons due to inductive effects

Answer: (C)

In the concerted, four-centered transition state, both B–C and C–H bonds form simultaneously. There is no carbocation intermediate. Boron is electron-deficient (empty p orbital) and is the electrophile, but its bulk controls where it attaches: the less substituted carbon is more accessible. Option B is wrong — it describes a carbocation pathway, which does NOT occur in hydroboration. Option D is wrong about the reason; it is sterics, not inductive effects, that direct boron.

Question 3

In hydroboration-oxidation, both the boron and the hydrogen add to the same face of the double bond in the concerted step.

• T. True
• F. False

Answer: True

Because hydroboration is a concerted reaction — both B and H add simultaneously through a cyclic transition state — both atoms must approach the same face of the π bond at the same time. This is called syn addition. The stereochemical consequence is that the resulting alcohol retains the relative configuration of the addition (both OH and H on the same face), which matters when the alkene has stereocenters or substituents that differentiate the faces.

Question 4

Hydroboration-oxidation and acid-catalyzed hydration of an alkene give the same alcohol product, just by different mechanisms.

• T. True
• F. False

Answer: False

These two reactions give opposite regiochemistry. Acid-catalyzed hydration follows Markovnikov's rule: OH adds to the more substituted carbon, because the reaction proceeds through the more stable carbocation intermediate. Hydroboration-oxidation gives the anti-Markovnikov product: OH adds to the less substituted carbon, because the concerted mechanism is controlled by steric access rather than carbocation stability. For any unsymmetrical alkene, these two methods yield different alcohol products.

Question 5

Why does the mechanism of a reaction determine its regiochemistry, using hydroboration as your example?

Model answer: Regiochemistry is determined by which intermediate or transition state controls bond formation. Acid-catalyzed HX addition proceeds through a carbocation intermediate, and the more stable (more substituted) carbocation forms preferentially — directing X to the more substituted carbon (Markovnikov). Hydroboration has no intermediate at all; the cyclic transition state forms in a single concerted step where sterics determine which carbon boron attacks. Since the less substituted carbon is less hindered, boron goes there — giving anti-Markovnikov regiochemistry. The reagent changed, but the deeper reason for the different outcome is the change in mechanism.

This is the organizing principle for all addition reactions: asking 'does this go through a carbocation?' immediately tells you whether to expect Markovnikov or non-Markovnikov products. Carbocation mechanisms favor the more substituted position because higher substitution stabilizes positive charge. Concerted or radical mechanisms can favor the less substituted position for steric or SOMO-energy reasons. Mechanism is not just a description of how — it is the explanation of where.