Chemistry of Boranes | Chemistry Optional Notes for UPSC PDF Download

Boranes

Preparation

• Diborane can also be obtained in small quantities by the reaction of iodine with sodium borohydride in diglyme.
  
  
• On heating magnesium boride with HCl a mixture of volatile boranes are obtained.
  
  

Physical Properties

• Colourless and highly inflammable gas at room temperature. 
• At high concentrations, it ignites rapidly in the presence of moist air at room temperature. 
• It smells sweet. 
• It has boiling point of about 180 K.
• It is toxic gas.
• It releases huge amount of energy when burnt in the presence of O₂.
• It readily hydrolysed in the water to give hydrogen gas and boric acid.

Chemical Properties

• Diboranes reacts with water and alkali to give boric acid and metaborates respectively.
  
• Action of air: At room temperature pure diborane does not react with air or oxygen but in impure form it gives B₂O₃ along with large amount of heat.
  
  ΔH = -2165 KJ mol^-1
• Diborane reacts with methyl alcohol to give trimethyl Borate.
  
• Hydroboration: Diborane adds on to alkenes and alkynes in ether solvent at room temperature. This reaction is called hydroboration and is highly used in synthetic organic chemistry, especially for anti Markovnikov addition.
  
• Reaction with ionic hydrides: When treated with metal hydrides it forms metal borohydrides
  
• Reaction with ammonia: When treated with excess ammonia at low temperatures diborane gives diboranediammonate. On heating at higher temperatures it gives borazole.
  

Structure and Bonding in diborane

• In diborane two BH₂ units are linked by two bridged hydrogens. Therefore, it has eight B-H bonds. However, diborane has only 12 valance electrons and are not sufficient to form normal covalent bonds. The four terminal B-H bonds are normal covalent bonds (two centre - two electron bond or 2c-2e bond). 
• The remaining four electrons have to be used for the bridged bonds. i.e. two three centred B-H-B bonds utilise two electrons each. Hence, these bonds are three centre- two electron bonds (3c-2e). The bridging hydrogen atoms are in a plane as shown in the figure 2.3. In diborne, the boron is sp³ hybridised.
  
• Three of the four sp³ hybridised orbitals contains single electron and the fourth orbital is empty. Two of the half filled hybridised orbitals of each boron overlap with the two hydrogens to form four terminal 2c-2e bonds, leaving one empty and one half filled hybridised orbitals on each boron. The Three centre - two electron bonds), B-H-B bond formation involves overlapping the half filled hybridised orbital of one boron, the empty hybridised orbital of the other boron and the half filled 1s orbital of hydrogen.

Uses of diborane

• Diborane is used as a high energy fuel for propellant 
• It is used as a reducing agent in organic chemist 
• It is used in welding torches

Borazine

• Borazine, also known as borazole. 
• It is a polar inorganic compound with the chemical formula B₃H₆N₃.
• In this cyclic compound, three BH units and three NH units alternate. 
• The compound is isoelectronic and isostructural with benzene. 
• Borazine has polar hexagonal structure containing 6 membered ring, in which B and N atoms are arranged alternately. 
• Because the similarity between the structures of borazine and benzene that borazine is called inorganic benzene. 
• In borazine, both Boron and Nitrogen are sp² hybridized. 
• Each N-atom has an empty p-orbital. 
• B-N bond in borazine is dative bond, while arises from the sidewise overlap between the filled porbitals of N-atom and empty p-orbitals of B-atom. 
• Since borazine is isoelectronic with benzene, both the compounds have aromatic electron cloud. 
• Due to greater difference in electronegativity values of B and N - atoms, the electron cloud in B3N3 ring of the borazine molecule is partially delocalized. While in the case of benzene ring, the electron cloud is completely delocalized. 
• MO calculations have indicated that electron drift from N to B is less than the electron drift from B to N due to the greater electronegativity of the N-atom. 
• In benzene molecules, C=C bonds are non-polar, while in case of B₃H₆N₃, due to difference in electronegativities between B and N atoms, the B-N bond is polar. 
• It is due to the partial delocalization of electron clouds that bonding in the B₃N₃ ring is weakened.
• N-atoms retain its basicity and B-atoms retain its acidity. 
• In borazine, B-N bond length is equal to 1.44 A^o, which is between calculated single B-N bonds (1.54 A^o). 
• B=N bond length is 1.36 A^o. 
• The angles are equal to 120^o.

Preparation

Properties

• Colorless liquid
• Volatile liquid
• Boiling point 84.5 ⁰C 
• Melting point -58 ⁰C 
• Decomposes at -80 ⁰C

Structure

Chemical properties

Uses

• Borazines are also starting materials for other potential ceramics such as boron carbonides.
• Borazine can also be used as precursor to growing boron nitride thin films on surfaces, such as nanomesh structure which is formed on rhodium.

Boron Halides

It is represented as BX₃ where X = F, Cl, Br, I

Synthesis

• When B₂O₃ is treated with halogens in presence of carbon, Boron halides are formed.
  
• It is synthesized by reactions of CaF₂ and boric oxide in the presence of H₂SO₄
  

Properties

• They are small in size with high charge density. 
• They are covalent in nature. 
• They act as non- electrolytes in liquid state. 
• The boiling point is ranging from 13-300°C and it decreases when the atomic number increases. 
• Boron halides are electron deficient because B atom has an incomplete octet structure.
• Boron atom accepts 6 electrons from 3 BX bonds and 2 more electrons from the donor compound.

Chemical Properties

• BCl₃ hydrolyzes readily to give hydrochloric acid and boric acid:
  
• Reduction of BCl₃ to elemental boron is conduct commercially (see below). In the laboratory, when boron trichloride can be converted to diboron tetrachloride by heating with copper metal: