The P-Block Elements (Important Compounds Of Boron)

Some Important Compounds Of Boron

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Boron forms several important compounds, notably Borax and Orthoboric acid, which exhibit unique chemical properties and applications.

Borax ($Na_2B_4O_5(OH)_4 \cdot 8H_2O$ or $Na_2B_4O_7 \cdot 10H_2O$)

Formula: Commonly known as sodium tetraborate decahydrate.

Preparation: Borax is obtained from natural deposits or prepared synthetically from boric acid or borate minerals reacting with sodium carbonate.

Properties:

• White crystalline solid.
• Soluble in water, forming a weakly alkaline solution due to hydrolysis.

$B_4O_7^{2-}(aq) + 7H_2O(l) \rightleftharpoons 4B(OH)_3(aq) + 2OH^-(aq)$

• Borax Bead Test: Borax, when heated, fuses to form a glassy bead of sodium metaborate ($NaBO_2$) and boric anhydride ($B_2O_3$). This bead reacts with metal oxides to form colored borate complexes, which is used for qualitative analysis of metals.

$Na_2B_4O_7 \cdot 10H_2O \xrightarrow{heat} Na_2B_4O_7 + 10H_2O$

$Na_2B_4O_7 \xrightarrow{heat} 2NaBO_2 + B_2O_3$

Example: $B_2O_3 + CuO \rightarrow CuB_2O_4$ (blue colored bead)

Uses:

• Used in the borax bead test for qualitative analysis.
• Used as a flux in metallurgy, soldering, and welding to remove oxides.
• Used as a water softener.
• Used in the manufacture of glass, enamels, and pottery.
• Has mild antiseptic properties and is used in some medicines.

Orthoboric Acid ($H_3BO_3$)

Formula: Boric acid.

Preparation: Prepared by acidifying aqueous solution of borax.

$$Na_2B_4O_7(aq) + 2HCl(aq) + 5H_2O(l) \rightarrow 2NaCl(aq) + 4H_3BO_3(aq)$$

Properties:

• White crystalline solid with a soapy touch.
• Sparingly soluble in cold water but moderately soluble in hot water.
• It is a weak monoprotic acid. It acts as a Lewis acid, accepting $OH^-$ ions from water rather than donating a proton directly.

$B(OH)_3(aq) + H_2O(l) \rightleftharpoons [B(OH)_4]^-(aq) + H^+(aq)$

or more accurately $B(OH)_3 + 2H_2O \rightleftharpoons [B(OH)_4]^- + H_3O^+$

• When heated, it first forms metaboric acid ($HBO_2$) and then boric anhydride ($B_2O_3$).

$H_3BO_3 \xrightarrow{413 K} HBO_2 + H_2O$

$2HBO_2 \xrightarrow{573 K} B_2O_3 + H_2O$

Uses:

• Used as an antiseptic and a mild astringent.
• Used in glass manufacturing and enamels.
• Used as a fire retardant.
• Acts as a flux in soldering.

Diborane, $B_2H_6$

Formula: Diborane is the simplest boron hydride.

Preparation:

• Laboratory: Prepared by the reaction of lithium aluminum hydride ($LiAlH_4$) with boron trifluoride ($BF_3$).

$8BF_3 + 6LiAlH_4 \rightarrow 4(BF_3 \cdot \text{etherate}) + Li_3AlF_6 + AlF_3 + 3B_2H_6(g)$ (Simplified view)

A more direct lab preparation uses sodium borohydride ($NaBH_4$) with iodine ($I_2$):

$2NaBH_4 + I_2 \rightarrow B_2H_6 + 2NaI + H_2$

• Commercial: Prepared by the reaction of sodium borohydride with boron trifluoride or iodine.

Properties:

• Colorless gas.
• Sweet smelling but highly toxic.
• Flammable and burns in air or oxygen with a very hot flame, producing boron oxide and water.

$B_2H_6(g) + 3O_2(g) \rightarrow B_2O_3(s) + 3H_2O(g)$

• It reacts readily with water to form boric acid and hydrogen gas.

$B_2H_6(g) + 6H_2O(l) \rightarrow 2B(OH)_3(aq) + 6H_2(g)$

• It reacts with Lewis bases (like ammonia) to form adducts.

$B_2H_6 + 2NH_3 \rightarrow 2BH_3 \cdot NH_3 \rightarrow 2(BN) + 6H_2$ (at higher temp)

Structure:

• Diborane has a unique structure with two boron atoms bridged by two hydrogen atoms (three-center-two-electron bonds). The remaining two hydrogen atoms are bonded normally to each boron atom.
• It is an electron-deficient molecule.

Uses:

• Used as a reducing agent in organic synthesis.
• Used in the production of high-purity boron.
• Used as a source of hydrogen in some applications.