Boron Family: Borane - Online Tutor, Practice Problems & Exam Prep

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

Now, borains represent highly reactive covalent hydrides composed of only boron and hydrogen atoms. We're going to say the simplest form is diborane. In it, it's just two boranes connected by hydrogen bridges.

Now what's a hydrogen bridge or bridging hydrogens? These are hydrogen atoms that unite molecules by forming 2 polarized covalent bonds. So here we have borane as BH3 diborane. We have basically a hydrogen from each borane bridging together through these polarized bonds, one borane to another borane.

So we typically do not find borane, we find diborane when we're looking for our molecule. Now these are rare and in-depth discussions are beyond the scope of this course because at this point we've always told ourselves that hydrogen can only make one bond. Well, that's not true.

When you get to higher levels of chemistry, you'll see that there are bridging hydrogens that can form kind of like 2 bonds. It's a little bit more complex than that. But don't worry, you don't have to go over it in Gen. chem. Just realize that this is going to be one instance where you see two bonds connecting to a hydrogen atom. This is very rare. It is not a usual thing.

Boron Family: Borane Example

Now based on the structure of the dye borne molecule, determine the hybridization of the boron atoms. So remember we have boron here and it's connected to hydrogens, and these hydrogens are connected to another one.

If we just focus on one of the boron atoms, let's focus on this one. How many groups or elements is it connected to? It's connected to 1, 2, 3, 4 groups. Remember? If you're connected to four groups, meaning surrounding elements or lone pairs, then you are sp3 hybridized.

There had been at option C is our correct answer, right? So in this case our hybridization for each borne atom is sp3.

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

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.