The major factor affecting why boron and hydrogen add across a double bond in an apparent anti-Markovnikov manner (i.e. H adds to the more substituted side) in hydroboration is
Steric Hindrance
The major factor that affects why boron and hydrogen add across a double bond in an apparent anti-Markovnikov manner in hydroboration is the presence of a boron-hydrogen bond. In hydroboration, the initial step involves the formation of a complex between the alkene and a boron compound, usually borane (BH3). This complexation creates a lower-energy transition state compared to the traditional Markovnikov addition.
The boron-hydrogen bond in the complex is polarized, with the hydrogen slightly negatively charged and the boron slightly positively charged. The electron-rich alkene attacks the positively charged boron, resulting in the formation of a cyclic intermediate called a borinate.
This cyclic intermediate is then decomposed by hydrolysis, resulting in the syn addition of boron and hydrogen across the double bond. Since the alkene has a higher electron density at the less substituted carbon, it prefers to attack the boron, resulting in the anti-Markovnikov orientation.
Overall, the presence of the boron-hydrogen bond and the preference of the alkene to attack the boron rather than directly add hydrogen contribute to the apparent anti-Markovnikov addition observed in hydroboration.
The major factor affecting why boron and hydrogen add across a double bond in an apparent anti-Markovnikov manner in hydroboration is the presence of an empty p orbital on boron. This allows boron to form a bond with the double bond carbon while freeing up an electron pair on boron.
To understand this concept, we need to consider the nature of the boron-hydrogen bond and the stability of the resulting borane compound. In hydroboration, the reaction proceeds through a cyclic transition state known as a three-membered ring called a cyclic borane intermediate.
Here's how the reaction occurs step by step:
1. The empty p orbital on boron approaches the double bond, and one of the pi bonds in the double bond shifts towards boron.
2. As a result, both boron and carbon form a new bond, called a sigma bond, while simultaneously breaking one of the pi bonds in the double bond.
3. At the same time, the hydrogen atom attached to boron forms a bond with the carbon atom that has less substitution, thus adding in an apparent anti-Markovnikov fashion.
This process occurs due to the stability of the resulting borane compound that forms after the addition of hydrogen. The borane compound formed is relatively stable compared to other possible products. The empty p orbital on boron allows it to accept an electron pair, making it favorable to add hydrogen to the less substituted side of the double bond.
In summary, the major factor affecting why boron and hydrogen add across a double bond in an apparent anti-Markovnikov manner in hydroboration is the presence of an empty p orbital on boron, which allows boron to accept an electron pair and form a stable borane compound.