Boron Family: Borane: Videos & Practice Problems

Boranes, composed of boron and hydrogen atoms, are a class of highly reactive covalent hydrides. The simplest borane, diborane (B₂H₆), features unique bridging hydrogens that form polarized covalent bonds between two boron atoms. These bridging hydrogens challenge the common notion that hydrogen can form only one bond by demonstrating the capacity to participate in two bonds simultaneously. While the intricacies of boranes and hydrogen bridging are typically explored in advanced chemistry, it's essential to recognize that exceptions to general bonding rules exist, even if they are not a primary focus in general chemistry courses.

Boranes

Boranes are a class of highly reactive covalent hydrides consisting solely of boron and hydrogen atoms, with diborane (B₂H₆) being the simplest and most notable example. In diborane, two boron atoms are connected by hydrogen atoms known as bridging hydrogens. These bridging hydrogens form two polarized covalent bonds, effectively linking the two boron centers.

In terms of molecular structure, borane is represented as BH₃, but it is rarely found in isolation; instead, diborane is the more common form encountered. This phenomenon challenges the conventional understanding that hydrogen can only form a single bond. In advanced chemistry, it becomes evident that bridging hydrogens can create a scenario where a hydrogen atom appears to form two bonds, although this is an exception rather than the rule.

It is important to note that the presence of bridging hydrogens is a unique characteristic of certain boron compounds and is not typically addressed in general chemistry courses. Therefore, while the concept of hydrogen forming multiple bonds is intriguing, it remains a specialized topic beyond the foundational principles of chemistry.

Boron Family: Borane Example

In the diborane molecule (B₂H₆), the structure reveals that each boron atom is connected to four groups: two hydrogen atoms and two bridging hydrogen atoms that connect to the other boron atom. This arrangement leads to the conclusion that each boron atom undergoes hybridization to form four equivalent hybrid orbitals.To determine the hybridization, we consider the number of groups surrounding each boron atom. Since each boron is connected to four groups, the hybridization can be classified as sp³. This sp³ hybridization results in a tetrahedral geometry around each boron atom, which is essential for understanding the bonding and molecular shape of diborane.In summary, the hybridization of each boron atom in diborane is sp³, reflecting its connectivity to four groups, which is crucial for the molecule's overall structure and properties.

The formation of diborane from its elemental components is given below:

Determine the enthalpy value for the formation of diborane when given the enthalpy values for the following partial reactions:

• A. +36 kJ
• B. –3550 kJ
• C. +520 kJ
• D. +27 kJ
• E. –171 kJ

Answer: (A)

Here’s what students ask on this topic:

Why is borane unstable?

Borane, BH₃, is unstable due to its electron deficiency. In a stable molecule, atoms typically follow the octet rule, where they have eight electrons in their valence shell to be energetically stable. However, in borane, the boron atom only has six valence electrons after forming three B-H bonds. This electron deficiency makes the molecule highly reactive as it seeks to complete its octet.

Additionally, borane has an empty p-orbital which can easily accept a pair of electrons from a donor molecule, making it a good Lewis acid. This characteristic also contributes to its reactivity, as it tends to form adducts with Lewis bases.

The molecular geometry of borane is another factor. The BH₃ molecule is trigonal planar, and this geometry exposes the boron atom to potential attacking reagents, further contributing to its instability. For these reasons, borane is not commonly found in its monomeric form (BH₃) but rather exists in dimeric or complexed forms such as diborane (B₂H₆) or in complexes with other Lewis bases.

What is borane?

Borane is a compound that consists of boron and hydrogen with the chemical formula ${\mathrm{BH}}_3$. However, this formula represents the simplest unit of borane, which is not stable on its own. Borane molecules tend to form clusters or complexes to achieve stability. The most common and stable of these is diborane, with the chemical formula $B_2{H}_6$.

Diborane is a colorless, highly reactive gas at room temperature. It's part of a larger group of compounds known as boron hydrides, which vary in their structure and complexity. Boranes are known for their use in organic chemistry as reducing agents, especially in hydroboration reactions, where they add across carbon-carbon double bonds to give trialkylboranes. These trialkylboranes can then be converted into a variety of different organic compounds, making boranes useful intermediates in organic synthesis. Due to their reactivity, boranes must be handled with care, as they can be both toxic and flammable.

What elements make up borane?

Borane is a compound that consists of boron and hydrogen. The simplest unit of borane is BH₃, which is known as borane or borine. However, BH₃ is highly unstable and not found in a free state. Instead, it forms various stable structures through dimerization or by forming complexes with different organic substrates. A common form of borane that you might encounter is diborane, with the chemical formula B₂H₆. In diborane, two boron atoms are bonded to six hydrogen atoms, with four terminal H atoms and two bridging H atoms that connect the boron atoms in a three-center two-electron bond. This bonding structure is quite unique and is a characteristic feature of many boranes, which are collectively known as boron hydrides. Boranes are used in various chemical reactions, particularly in organic synthesis and as reducing agents.

Why is borane so reactive?

Borane, or boron hydride (BH₃), is highly reactive due to several factors. Firstly, boron has only three valence electrons, which makes it electron-deficient and thus it does not have a full octet in its outer shell. This electron deficiency makes borane an electrophile, meaning it is attracted to electron-rich species, such as bases or nucleophiles, to complete its octet.

Additionally, borane's reactivity is influenced by its molecular geometry. It has a trigonal planar shape, which allows for the easy approach of nucleophiles. The B-H bonds in borane are also polarized, with boron being less electronegative than hydrogen, which creates partial positive charges on the boron atom, further increasing its electrophilic character.

Lastly, the small size of the boron atom allows for orbital overlap with other reactants, facilitating chemical reactions. This combination of electron deficiency, polarized bonds, and favorable geometry makes borane highly reactive, especially in hydroboration reactions, where it adds across carbon-carbon double bonds in organic compounds.