Borane Reactions: Mechanisms and Acid-Base Behavior

Concept: Borane Reactions

Introduction to Borane Reactivity

Borane (BH3) is a highly reactive chemical species due to its electron deficiency. Its reactions are driven by its strong tendency to accept electrons, making it a strong Lewis acid. Borane commonly reacts with nucleophiles such as water and hydroxide ions, and its chemistry is foundational in understanding electron-deficient compounds.

• Electron-deficiency: Borane lacks a complete octet, making it highly reactive.

• Lewis acid: Borane acts as an electron pair acceptor in reactions.

• Types of reactions: Borane reacts with water (H2O) and with Lewis bases (such as OH-).

Reactions with Water

Hydrolysis of Borane

When borane is exposed to water, it undergoes a vigorous reaction due to its high reactivity. This process is called hydrolysis and results in the formation of boric acid and hydrogen gas.

• Hydrolysis reaction: Borane reacts with water to produce boric acid and hydrogen gas.

Equation:

$\mathrm{B_2H_6 + 6H_2O \rightarrow 2B(OH)_3 + 6H_2}$

• Products: Boric acid (B(OH)3) and hydrogen gas (H2).

• Application: This reaction is highly exothermic and is used in laboratory demonstrations of hydrogen gas evolution.

Example: If the enthalpy change ($\Delta H$) of energy is measured for every mole of diborane reacting with water, how much energy would be released when 100.0 g of diborane is submerged and excess water is used? (See calculation: -1310 kJ)

Lewis Acid-Base Reactions

Borane as a Lewis Acid

Borane's electron deficiency makes it a classic Lewis acid, meaning it can accept electron pairs from Lewis bases. In these reactions, borane interacts with electron-rich species such as hydroxide ions.

• Lewis acid: Electron pair acceptor (borane).

• Lewis base: Electron pair donor (hydroxide ion).

• Product: Formation of an adduct, where the base donates a pair of electrons to borane.

Equation:

$\mathrm{BH_3 + OH^- \rightarrow [BH_3OH]^-}$

• Adduct: The product formed is a borane-hydroxide complex, often written as [BH3OH]-.

• Application: This type of reaction is important in understanding the behavior of electron-deficient compounds and their interactions with bases.

Example: Draw the adduct product formed from the reaction between borane and a hydroxide ion: [BH3OH]-

Reactant	Type	Product
Borane (BH3)	Lewis Acid	[BH3OH]-
Hydroxide Ion (OH-)	Lewis Base	[BH3OH]-

Additional info: Borane's chemistry is a classic example of electron-deficient species in inorganic chemistry, and its reactions illustrate key concepts in acid-base theory and molecular structure.