Boranes and Diboranes

Boranes are a class of chemical compounds of boron and hydrogen. The simplest borane consists of a single boron atom bonded to hydrogen atoms. Diboranes are a subset of boranes that contain two boron atoms. These compounds are of interest both from a theoretical perspective, due to their unusual bonding, and from a practical perspective, due to their applications in organic synthesis and materials science.

General Characteristics of Boranes

Boranes are characterized by their electron-deficient bonding, which is different from the conventional covalent bonding seen in hydrocarbons. Boron has only three valence electrons, which means that it cannot form normal electron-pair bonds with hydrogen (which has one valence electron) without having additional electrons. As a result, boranes often exhibit multi-center bonding, where three or more atoms share a pair of electrons.

Structure and Bonding in Diboranes

Diborane (B2H6) is the most well-known member of the borane family and has a unique structure. It consists of two boron atoms bonded to six hydrogen atoms. Four of these hydrogen atoms are terminal, each bonded to one boron atom. The remaining two hydrogen atoms are bridging hydrogens, each bonded to both boron atoms.

The bonding in diborane can be described using the concept of three-center two-electron (3c-2e) bonds. In this model, the two bridging hydrogen atoms and the two boron atoms form a bond where the two electrons are shared among three atoms.

Here is a simplified representation of the structure of diborane:

  H   H
   \ /
    B
   / \
  H   H
 /     \
B-------B
 \     /
  H   H
   \ /
    H

Table of Differences and Important Points

Property	Boranes	Diboranes
Composition	BHx (x varies)	B2H6
Bonding	Electron-deficient, multi-center bonds	Electron-deficient, includes 3c-2e bonds
Structure	Varies with the number of BH units	B2H6 with two bridging hydrogens
Stability	Generally unstable at room temperature	More stable than smaller boranes, but still reactive
Synthesis	From boron and hydrogen at high temperatures	From boron hydrides or by reducing boron halides with hydrides
Applications	Limited due to instability	Used as a reducing agent and in hydroboration reactions

Examples and Applications

Synthesis of Diborane

Diborane can be synthesized by the reaction of sodium borohydride (NaBH4) with an acid such as boron trifluoride (BF3):

$$ \text{3 NaBH}_4 + \text{4 BF}_3 \rightarrow \text{2 B}_2\text{H}_6 + \text{3 NaBF}_4 $$

Hydroboration Reaction

One of the most important applications of diborane is in the hydroboration reaction, where an alkene is converted into an organoborane. This is a key step in the synthesis of alcohols:

$$ \text{B}_2\text{H}_6 + 3 \text{CH}_2=\text{CH}_2 \rightarrow 2 \text{BH}_3\cdot\text{CH}_2\text{CH}_3 $$

The organoborane can then be oxidized to produce an alcohol.

Reactions with Diborane

Diborane reacts with water and alcohols to produce hydrogen gas and boric acid or borate esters, respectively:

$$ \text{B}_2\text{H}_6 + 6 \text{H}_2\text{O} \rightarrow 2 \text{B(OH)}_3 + 6 \text{H}_2 $$ $$ \text{B}_2\text{H}_6 + 6 \text{ROH} \rightarrow 2 \text{B(OR)}_3 + 6 \text{H}_2 $$

Safety Considerations

Diborane is highly flammable and toxic. It can spontaneously ignite in air and form explosive mixtures. It is also a potent reducing agent, which can lead to dangerous reactions if not handled properly.

Conclusion

Boranes and diboranes represent an interesting class of compounds with unique bonding and reactivity. Their electron-deficient nature leads to unusual structures and the potential for complex chemistry. Diborane, in particular, is a valuable reagent in organic synthesis, despite its challenges in handling and safety. Understanding the chemistry of boranes is essential for chemists working in synthesis and materials science.