Diborane: Properties, Reactions, and Structure

Posted on Mar 17, 2024 in Chemistry

Diborane is a chemical compound that consists of boron and hydrogen atoms, and has a molecular formula B²H⁶. This substance is highly unstable at room temperature. The compounds consisting of boron and hydrogen atoms are called boranes. Diborane is one of the simplest borane hydrides.

Methods of Preparation:

  1. When boron trifluoride is treated with Li or sodium hydride in ethereal solution, B2H6 is formed. 2BF3 + 6LiH – ether – 6LiF + B2H6. 2BF3 + 6NaOH – ether – 6NaOH + B2H6.
  2. B2H6 is obtained from the reaction of BF3 & sodium borohydride in ethereal solution. BF3 + 3NaBH4 – ether – 2B2H6 + 3NOF.
  3. Diborane is formed when electric discharge is done in a mixture of hydrogen with boron trichloride or boron tribromide. 2BCl3 + 5H2 – B2H5Cl + 5HCl. 6B2H5Cl – 5B2H6 + 3BCl3.

Properties:

  • Diborane is a colorless gas, stable at low temperatures and in the absence of moisture.
  • It reacts with water and forms orthoboric acid: B2H6 + 6H2O – 3H3BO3 + 6H2.
  • Reacts with dry HCl in the presence of AlCl3 to give hydrogen.
  • At 120°C, combines with two molecules of NH3 to form diborane diammoniate (B2H6,2NH3) which is a white solid.
  • It releases a huge amount of energy when burnt in the presence of oxygen.
  • Its boiling point is about 180K, and it is a toxic gas.
  • Most diboranes are known to be flammable in air.
  • It smells sweet.

Structure:

It proved difficult to establish the structure of diborane as it is an electron-deficient compound. In order to form the required 7 bonds, the number of valence electrons needed is 14, but in B2H6, only 12 such electrons are available. Thus, the electrons to join the BH3 units (BH3-BH3) aren’t available between boron atoms. Although from its formula (similar to C2H6), it appears that boron, like carbon, may be in a tetrahedral structure, but boron has only three electrons in its outermost orbit, hence a tetrahedral structure isn’t possible. Sidgwick in 1927 suggested a singlet bond in the molecule.

Reaction with Ammonia:

  • With excess of ammonia at low temperature, an addition compound called diammoniate of diborane B2H6, 2NH3 is obtained.
  • With excess of ammonia at high temperature, a polymer of boron nitride (BN) is formed.
  • Diborane and ammonia in the ratio of 1:2 and at high temperature yield borazine.

Reaction with Chlorine: It reacts with chlorine and forms chloroborane B2H6 + 6Cl2 -> 2BCl3 + 6HCl.

Reaction with Sodium: It reacts with sodium to form a crystal of metal B2H6 + 2Na -> NaBH4 + B2H6.2Na (sodiumhybridoborate).

Arrhenius Equation:

It expresses the influence of temperature on the velocity of the reaction. The derivation of this equation is based on the following postulates.

  • All the molecules of the reactants are not taking part in the chemical reaction.
  • There are 2 types of molecules (active & passive molecules).
  • The molecules with sufficient energy are called active molecules, and molecules with pure energy are called passive molecules. There exists equilibrium between passive & active molecules.
  • Only active molecules undergo chemical change.

The rise in temperature shifts the equilibrium towards the right, so more active molecules are present, hence an increase in temperature increases the rate of reaction.

Derivation: Let us consider k1 – rate of forward reaction & k2 – rate of reversible reaction.

Rate of F.R – k1 [A] [B].

Rate of B.R – k2 [C] [D].

At equilibrium, the rate of F.R = Backward Reaction.

kc = k1/k2 = [C] [D] / [A] [B].

The equation explains that an increase in temperature exponentially increases the rate of reaction. This equation helps to calculate the energy of activation of the reaction.

Mathematical treatment of collision theory for a bimolecular reaction:

Let z – be the number of binary collisions per second between two identical molecules in 1ml of a gas. E – is the energy of activation for this process.

Transition State Theory:

Collision theory was not sufficient to explain the mechanism of most reactions. Therefore, a modern theory was developed known as the Transition State Theory or Activated Complex Theory or Absolute Reaction Rate Theory.

The derivation of this theory was based on the following postulates:

  • As the reactive molecules approach each other, there is continuous change in the bond distance. These changes are accompanied by energy change.
  • So, the reactant molecules are then transferred into an energy-rich intermediate called the activated complex or transition state.
  • The activated complex formed by rearrangement of atoms & redistribution of energy.
  • The activated complex which is formed is temporarily unstable and in equilibrium with the reacting molecules.
  • The activated complex decomposes to give the product. Reactant – activated complex – product.
  • The activation energy is defined as the additional energy the reacting molecule must acquire to form the activated complex.

Advantages of Transition State Theory over the Collision Theory:

  • The factor ‘p’ can be calculated, hence the equation derived by transition state theory. But in collision theory, P was introduced arbitrarily.
  • The concept of entropy of activation in transition state theory is very useful for quantitative purposes.
  • In collision theory, no account is taken of the internal motion of the molecule, whereas the Transition State Theory takes into account the degree of freedom of the molecule and changes of the degree of freedom which undergoes the reaction. Therefore, Transition State Theory is more advantageous and logical than the collision theory.

Order & Molecularity:

It is the sum of powers raised on concentration terms in the rate expression. It is the number of molecules of reactants taking part in the elementary step of the reaction. Experimental, theoretical, negative, positive, integer, fraction, zero, not assigned for the overall reaction, assigned for each elementary step of the mechanism, it depends on temperature and concentration, independent.

Types of Ores:

  1. Free/Native Ore – metals in a free state: Cu, Ag, Au, Hg.
  2. Sulphide Ore – some metals occur as sulphide ores: Zn, Cu, Ag, Fe.
  3. Oxide Ore – Al2O3.2H2O – bauxite, Fe3O4 – magnetite, SnO2 – tinstone, MnO2 – pyrolusite, Cu2O – cuprite, Fe2O3 – hematite.
  4. Sulphate Ore: MgSO4.7H2O – Epsom Salt, CaSO4.2H2O – Gypsum, CaSO4.½H2O – Plaster of Paris, BaSO4 – Barite, PbSO4 – Anglesite, SrSO4 – Celestite.
  5. Halide Ore – NaCl – Rock salt, KCl – Sylvite, AgCl – Horn silver. KAl.MgCl2.6H2O – carnallite.
  6. Silicate Ore: Zn2SiO4 – Willemite, LiAl(SiO3) – spodumene or triphane.
  7. Nitrate Ore: KNO4 – Indian salt peter, NaNO3 – Chile salt peter.
  8. Carbonate Ore: MgCO3 – magnesite, CaCO3 – limestone, ZnCO3 – calamine, CuCO3Cu(OH)2 – Malachite.