Why Does BH2 Add to the Less Substituted Carbon in Hydroboration?

[icosane]

I'm a bit confused on why the BH2 adds to the less substituted carbon in this reaction. I have found this explanation online,

"The boron atom is highly electrophilic because of its empty p orbital (ie. it wants electrons), and forms a slight bonding interaction with the pi bond. Since some electron density from the double bond is going towards bonding with the boron, the carbon opposite the boron is slightly electron deficient, left with a slightly positive charge. Positive charges are best stabilized by more highly substituted carbons, so the carbon opposite the boron tends to be the most highly substituted."

To me, this explains why the boron would add to the more substituted carbon, not the other way around. If the boron is electron deficient, wouldn't it add to the more, not less, substituted carbon?

 

[Mtnbiker]

I'm a bit confused on why the BH2 adds to the less substituted carbon in this reaction. I have found this explanation online,

"The boron atom is highly electrophilic because of its empty p orbital (ie. it wants electrons), and forms a slight bonding interaction with the pi bond. Since some electron density from the double bond is going towards bonding with the boron, the carbon opposite the boron is slightly electron deficient, left with a slightly positive charge. Positive charges are best stabilized by more highly substituted carbons, so the carbon opposite the boron tends to be the most highly substituted."

To me, this explains why the boron would add to the more substituted carbon, not the other way around. If the boron is electron deficient, wouldn't it add to the more, not less, substituted carbon?

Hydroboration-oxidation is believed to be mainly a steric effect, although there is regioselectivity (hydrogen is the negative half of the B-H bond and boron is the positive half). The steric effect stems from the fact that boron (and its attached substituents) is MUCH larger than a hydrogen atom, so it bonds to the "less crowded" carbon of the double bond.

Regioselectivity, although less of a factor, supports the passage you quoted, since the more negative hydrogen will be attracted to the slight positive charge of the more substituted carbon, and boron to the more negative.

 

[Blueshift5]

Thank you for your question. I can understand your confusion about the direction of addition in hydroboration reactions. However, the explanation you found online is correct.

Firstly, it is important to note that the boron atom in BH2 is not completely electron deficient. It has a partial positive charge due to the presence of the empty p orbital, but it is still capable of forming bonds and acquiring electrons. In the hydroboration reaction, the BH2 group acts as an electrophile, seeking to form a bond with the alkene.

Now, let's consider the electron distribution in the alkene. The pi bond in the double bond is made up of two p orbitals, one from each carbon atom. These p orbitals are parallel to each other and have their electron density concentrated above and below the plane of the molecule. When the BH2 group approaches the alkene, the empty p orbital of boron interacts with the pi bond, causing a shift in the electron density. The electrons from the pi bond move towards the boron, creating a slight positive charge on the carbon atom opposite the boron.

As you mentioned, positive charges are stabilized by more substituted carbons. This means that the slightly positive carbon atom will be more stable if it has more alkyl groups attached to it. Therefore, the BH2 group will add to the less substituted carbon in order to form a more stable intermediate. This is also known as Markovnikov's rule, which states that in the addition of an electrophile to an alkene, the electrophile will add to the carbon atom with more hydrogen atoms attached.

I hope this explanation helps to clarify your confusion. If you have any further questions, please feel free to ask. it is important to question and understand the concepts we come across, so keep up the curiosity!