11th Chemistry NCERT notes

Anomalous Properties of Boron

Boron shows anomalous behavior with the other members of the group, due to the following reasons:

  • Smallest size in the group
  • High ionization energy
  • Highest electronegativity in the group.
  • Absence of vacant d-orbital

Some Important Compounds of Boron (B)

Borax

  • Borax is a white crystalline solid.
  • It contains tetranuclear units. [B4O5(OH)4]2-
  • It formula is Borax formula
  • Borax dissolves in water to give an alkaline solution

Borax reaction with water

  • Borax is used as a water softener and cleaning agent.

Orthoboric acid (H3BO3) or B(OH)3

Boric acid can be prepared by the acidification of aqueous solution of borax.

Orthoboric acid (H3BO¬3) or B(OH)3

It can also be prepared by the hydrolysis of boron compounds.

Orthoboric acid (H3BO¬3) or B(OH)3

Physical properties of Boric acid

  • It is a white crystalline solid.
  • It is soft soapy in touch.
  • It is sparingly soluble in cold water but fairly soluble in hot water.

Uses of Boric acid

  • In the manufacture of heat resistant borosilicate glazes.
  • As a preservative for milk and food stuffs.
  • In the manufacture of enamels and glazes in pottery.

Structure of Boric Acid

Structure of Boric Acid

Diborane (B2H6)

  • The series of compounds of boron with hydrogen is known as boranes.
  • Diborane is prepared by the reduction of boron trifluoride with LiAlH4 in diethyl ether.

Diborane

  • Laboratory method of preparation. In laboratory diborane is prepared by the oxidation of sodium borohydride with iodine.

Diborane

  • Industrial method of preparation. On industrial scale, diborane is prepared by reduction of BF3 with sodium hydride.

Diborane

Structure of Diborane

Structure of Diborane

Physical Properties of Diborane

  1. Diborane is a colourless, highly toxic gas with a b.p. of 180 K.
  2. Diborane catches fire spontaneously upon exposure to air.
  3. Higher boranes are spontaneously flammable in air.

Chemical Properties

  • Boranes are readily hydrolyzed by water to form boric acid.

Boranes are readily hydrolyzed by water to form boric acid

  • It burns in oxygen evolving an enormous amount of heat.

It burns in oxygen evolving an enormous amount of heat.

  • Reaction with Lewis base: Diborane on treatment with lewis bases undergo cleavage reactions to form borane which then reacts with Lewis bases to form adducts.

Reaction with Lewis base

 

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