Important Compounds of the Boron Family: Boron is the first element in the IIIA or Group \(13\) of the periodic table. The group-\(13\) elements form the first group of the periodic table, with Boron, the first member being a non-metal. Boron is a fairly rare element and highly reactive, occurring in combination with other elements in the form of compounds. The important compounds of Boron family are used for many industrial and chemical applications and are quite a significant study.

List of Compounds Formed by Boron

Boron forms several compounds with different elements. Some of the compounds formed by Boron include:

1. Borax or Sodium Tetraborate
2. Diborane (Boron Hydride)
3. Halides of Boron
4. Boric Acid or Ortho Boric acid
5. Boron Nitride

Each compound has a different significance with respect to its formation, applications, and use.

Borax

Borax is an important compound of boron with the formula \({\rm{N}}{{\rm{a}}_2}{{\rm{B}}_4}{{\rm{O}}_7} \cdot {{\rm{H}}_2}{\rm{O}}\).  It is a white crystalline solid with tetranuclear units, \({\left[ {{{\rm{B}}_4}{{\rm{O}}_5}{{\left( {{\rm{OH}}} \right)}_4}} \right]^{2 – }}\). Hence, the right formula is \({\rm{N}}{{\rm{a}}_2}\left[ {\;{{\rm{B}}_4}{{\rm{O}}_5}{{\left( {{\rm{OH}}} \right)}_4}} \right].8{{\rm{H}}_2}{\rm{O}}\).

Preparation of Borax

I. Borax is obtained naturally from TINCAL or Suhaga, a mineral found in Tibet, in Ladakh and Puga Valley in India, California, and Ceylon. Borax is extremely soluble in water at \(100\,^\circ {\rm{C}}\). (About \(101\) grams per \(100\,{\rm{ml}}\) of water) This property is taken into account to separate Borax from the mineral to purify it.

II. Preparation from Colemanite: Borax is also obtained from Colemanite mineral, \({\rm{C}}{{\rm{a}}_2}\;{{\rm{B}}_6}{\rm{O}}11\). The powdered Colemanite ore is boiled in Sodium Carbonate, and the resultant solution is filtered to obtain Borax.  

\({\rm{C}}{{\rm{a}}_2}\;{{\rm{B}}_6}{{\rm{O}}_{11}} + 2{\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + 2{\rm{NaB}}{{\rm{O}}_2} + 2{\rm{CaC}}{{\rm{O}}_3}\)

The sodium metaborate formed is passed through carbon dioxide to obtain borax.

\(4{\rm{NaB}}{{\rm{O}}_2} + {\rm{C}}{{\rm{O}}_2} \to {\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}\)

III. Preparation from Boric Acid: Boric acid, on reaction with sodium carbonate, gives Borax.

\(4{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + 6{{\rm{H}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2}\)

Properties of Borax

1. Borax exists in three different forms, and they vary on the number of water of hydration present in their molecules:

	Types of Borax	Formula	Crystalized at temperature
1.	Prismatic Borax (Decahydrate)	\({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}.10{{\rm{H}}_2}{\rm{O}}\)	Normal or coordinate temperature
2.	Octahedral Borax or Jeweler’s Borax (Pentahydrate)	\({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}.5{{\rm{H}}_2}{\rm{O}}\)	At \(333\;{\rm{K}}\) or \(60\,^\circ {\rm{C}}\)
3.	Borax Glass/ Anhydrous Borax (an unstable form that crystalizes to decahydrate by absorbing moisture in the air)	\({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}\)	At \(1013\;{\rm{K}}\) (above melting point) or \(740\,^\circ {\rm{C}}\)

2. Hydrolysis: Borax is basic in nature, and its alkalinity is shown in the way it undergoes hydrolysis to give a strong base and a weak acid.

\({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + 7{{\rm{H}}_2}{\rm{O}} \to 2{\rm{NaOH}} + 4{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3}\)

3. Effect of Heat: Borax on heating eliminates the water molecule from swelling up, and on further heating, melts into a liquid. This liquid then forms a glassy bead on cooling to form sodium metaborate and boric anhydride. The reaction is as follows:

\({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}.10{{\rm{H}}_2}{\rm{O}}\Delta \to {\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7}\Delta \to 2{\rm{NaB}}{{\rm{O}}_2} + {{\rm{B}}_2}{{\rm{O}}_3}\)

When the hot and glassy bead mass is brought in contact with colored salts such as those of \({\rm{Cu}},{\rm{ Co}},{\rm{ Fe}},{\rm{ Ni}}\), and \({\rm{Cr}}\), the boric anhydride displaces the basic salts to form colored metaborate (the color is typical to the salts).

Structure of Borax

The structure of Borax is as shown below:

The structure is an amalgamation of two tetrahedral units and two triangular units.

Uses of Borax

Borax is used in:

1. Pharmaceuticals as an antiseptic.
2. Metal extraction as a flux.
3. Qualitative analysis as bora bead test, as mentioned above.
4. In water treatment for softening of water.
5. Industries to make resistant and shockproof borosilicate glasses and also optical glasses.
6. Leather manufacturing for cleaning skin and hides.
7. In candle manufacturing to render stiffness to candles.

Hydrides of Boron

Boron forms hydrides in different combinations, with a general formula of \({{\rm{B}}_{\rm{n}}}{{\rm{H}}_{{\rm{n}} + 4}}\) & \({{\rm{B}}_{\rm{n}}}{{\rm{H}}_{{\rm{n}} + 6}}\), wherein the hydrides resemble the alkanes in their series. The hydrides of boron are therefore called ‘Boranes’ synonymous with ‘Alkanes.’ The common hydride of boron is diborane, with a formula \({{\rm{B}}_2}{{\rm{H}}_6}\).

Diborane

Diborane, also known as boron hydride, has a formula of \({{\rm{B}}_2}{{\rm{H}}_6}\). It is a pyrophoric gas or a gas which ignites on exposure to air or oxygen, with an obnoxiously sweet smell.

Preparation of Diborane

1. From lithium aluminium hydride: Boron fluoride, in the presence of diethyl ether, reacts with \({\rm{LiAl}}{{\rm{H}}_4}\) to form diborane.
   \(4{\rm{B}}{{\rm{F}}_3} + 3{\rm{LiAl}}{{\rm{H}}_4} \to 2\;{{\rm{B}}_2}{{\rm{H}}_6} + 3{\rm{LiF}} + 3{\rm{Al}}{{\rm{F}}_3}\)
2. Laboratory Preparation: Diborane is prepared in the laboratory by reacting sodium borohydride with iodine at a higher boiling temperature in the presence of polyether solvent \( – {\left( {{\rm{C}}{{\rm{H}}_3}{\rm{OC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{O}}\). Hydrogen is released in the reaction.
   \(2{\rm{NaB}}{{\rm{H}}_4} + {{\rm{I}}_2}{\rm{ Polyether }} \to {{\rm{B}}_2}{{\rm{H}}_6} + 2{\rm{NaI}} + {{\rm{H}}_2}\)
3. Industrial Preparation: Industrially, diborane is prepared by reacting boron trifluoride with sodium hydride at \(450\;{\rm{K}}\).
   \(2{\rm{B}}{{\rm{F}}_3} + 6{\rm{NaH}}450\;{\rm{K}} \to {{\rm{B}}_2}{{\rm{H}}_6} + 6{\rm{NaF}}\)

Properties of Diborane

1. Diborane is a colorless gas and is stable at low temperatures and in the absence of moisture or grease.
2. On heating at \(100 – 300\,^\circ {\rm{C}}\), it forms higher boranes, such as \({{\rm{B}}_3}{{\rm{H}}_9},\;{\mkern 1mu} {{\rm{B}}_5}{{\rm{H}}_{11}}\), etc. And, on heating to a red hot temperature, Diborane completely decomposes to boron.
   \({{\rm{B}}_2}{{\rm{H}}_6}{\rm{ Red Hot }} \to 2\;{\rm{B}} + 3{{\rm{H}}_2}\)
3. Diborane catches fire when it comes in contact with oxygen and liberates a lot of heat (exothermic reaction). Due to this property, Diborane is used as energy fuel.
   \({{\rm{B}}_2}{{\rm{H}}_6} + 3{{\rm{O}}_2} \to {{\rm{B}}_2}{{\rm{O}}_3} + 3{{\rm{H}}_2}{\rm{O}} + {\rm{Heat}}\)
4. Diborane also liberates heat when it comes in contact with water to give boric acid.
   \({{\rm{B}}_2}{{\rm{H}}_6} + 6{{\rm{H}}_2}{\rm{O}} \to 2{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} + 6{{\rm{H}}_2}\)
5. Diborane also gives reducing agents such as \({\rm{LiB}}{{\rm{H}}_4}\) and \({\rm{NaB}}{{\rm{H}}_4}\) when it reacts with the metal hydrides.
   \(2{\rm{LiH}} + {{\rm{B}}_2}{{\rm{H}}_6}{\rm{ Ether }} \to 2{\rm{LiB}}{{\rm{H}}_4}\)
   \(2{\rm{NaH}} + {{\rm{B}}_2}{{\rm{H}}_6}{\rm{ Ether }} \to {\rm{2NaB}}{{\rm{H}}_4}\)

Structure of Diborane

Dithey, in \(1921\), proposed a bridge structure for Diborane because of the electron-deficient nature of the compound. The structure also adheres to electron-diffraction studies. And with the two Boron atoms in Diborane in \({\rm{s}}{{\rm{p}}^3}\) hybridized state, the \(2\) bridging hydrogens form a banana bond, giving Diborane a unique structure as shown:

Uses of Diborane

1. Diborane is used as a reductant in organic reactions like hydroboration.
2. It is used for making higher energy fuel cells and is also used in the manufacture of propellants.
3. It is used as a catalyst in reactions such as polymerization.

Boron Nitride

Boron nitride is a synthetically produced crystalline compound of boron and has a formula of \({\rm{BN}}\).

Preparation of Boron Nitride

Boron nitride, like the fabrication of other ceramic materials, can be prepared by the chemical vapour deposition method. Other preparations include:

1. When boron is heated in the presence of atmospheric nitrogen or ammonia, Boron Nitride is formed.
   \(2{\rm{B}} + {{\rm{N}}_2}{\rm{Heat}} \to 2{\rm{BN}}\)
   \(2{\rm{B}} + 2{\rm{N}}{{\rm{H}}_3} \to 2{\rm{BN}} + 3{{\rm{H}}_2}\)
2. Borax, on heating with ammonium chloride (till red hot), gives Boron Nitride.
   \({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + 2{\rm{N}}{{\rm{H}}_4}{\rm{Cl}} \to 2{\rm{BN}} + 2{\rm{NaCl}} + {{\rm{B}}_2}{{\rm{O}}_3} + 4{{\rm{H}}_2}{\rm{O}}\)
   The boron nitride is separated from other compounds in the product mixture when it is treated with \({\rm{HCl}}\). \({\rm{BN}}\) remains insoluble, while the others dissolve and can be separated out.
3. Ammonia, on reaction with boron trichloride also gives Boron nitride in pure form. The boronamide and boronimide formed in the first and second steps respectively decompose to form boron nitride.
   \({\rm{BC}}{{\rm{l}}_3} + 6{\rm{N}}{{\rm{H}}_3} \to {\rm{B}}{\left( {{\rm{N}}{{\rm{H}}_2}} \right)_3} + 3{\rm{N}}{{\rm{H}}_4}{\rm{Cl}}\)
   \(2{\rm{B}}{\left( {{\rm{N}}{{\rm{H}}_2}} \right)_3}\Delta \to {{\rm{B}}_2}{({\rm{NH}})_3} + 3{\rm{N}}{{\rm{H}}_3}\)
   \({{\rm{B}}_2}{({\rm{NH}})_3} \to 2{\rm{BN}} + {\rm{N}}{{\rm{H}}_3}\)

Properties of Boron Nitride

1. Boron Nitride, a white powder with a very high melting point.
2. It decomposes when heated in the presence of steam under pressure.
   \(2{\rm{BN}} + 3{{\rm{H}}_2}{\rm{O}} \to {{\rm{B}}_2}{{\rm{O}}_3} + 2{\rm{N}}{{\rm{H}}_3}\)
3. Boron Nitride is chemically very stable and completely unreactive to the attack from alkalis and mineral acids. It reacts with hydrogen fluoride \(\left( {{\rm{HF}}} \right)\) to form ammonium borofluoride.
   \({\rm{BN}} + 4{\rm{HF}} \to {\rm{N}}{{\rm{H}}_4}{\rm{B}}{{\rm{F}}_4}\)

Structure of Boron Nitride

The structure of boron nitride is very similar to graphite, except the \(^'{{\rm{C}}^’}\) atoms in the corners are replaced by Boron and nitrogen. It has interconnecting hexagonal ring structures. At very high temperatures and pressure, the shape takes on a new form called Borazon, which is harder than diamond.

Boric Acid

Boric acid or orthoboric acid, or hydrogen borate is a monobasic acid of boron and is a weak acid.

Preparation of Boric Acid

1. Using Borax: An aqueous solution of borax is made to react with hot and concentrated mineral acids such as \({\rm{HCl}}\) and \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\). Boric acid crystals are formed in the process.
   \({\rm{N}}{{\rm{a}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} + 5{{\rm{H}}_2}{\rm{O}} \to {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4} + 4{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3}\)
2. Extraction from Colemanite: Boric acid can also be extracted from Colemanite by mixing the powdered mineral with boiling water. \({{\rm{S}}{{\rm{O}}_2}}\) is circulated through suspension of Colemanite mineral to extract crystalized boric acid.
   \({\rm{C}}{{\rm{a}}_2}\;{{\rm{B}}_6}{{\rm{O}}_{11}} + 4{\rm{S}}{{\rm{O}}_2} + 11{{\rm{H}}_2}{\rm{O}} \to 2{\rm{Ca}}{\left( {{\rm{HS}}{{\rm{O}}_3}} \right)_2} + 6{{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3}\)
3. Boric acid also occurs naturally in Soffioni, the jets of steam present underground in the volcanic regions of Tuscany. When the jets are caught in the large vats or tanks of water, concentrated and cooled, boric acid separates out.

Properties of Boric Acid

1. Boric acid is a soapy, soft, and white needle-like crystal.
2. Acidity: Although it is an acid, it is not a proton donor. Instead, it acts as a Lewis acid. So, it accepts an electron pair from \({\rm{O}}{{\rm{H}}^ – }\) ion.
   \({{\rm{H}}_2}{\rm{O}} + {{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} \to {\left[ {{\rm{B}}{{({\rm{OH}})}_4}} \right]^ – } + {{\rm{H}}^ + }\)
3. Dehydration Reaction: On heating up to \({100^{\rm{o}}}{\rm{C}}\), Boric acid loses water to form metaboric acid, which on further heating to \({160^{\rm{o}}}{\rm{C}}\) forms tetraboric acid.
   \({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3}{\rm{100}}\,^\circ {\rm{C}} \to {\rm{HB}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}}\)
   \(4{\rm{HB}}{{\rm{O}}_2}160\,^\circ {\rm{C}} \to {{\rm{H}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} + {{\rm{H}}_2}{\rm{O}}\)
   On strong heating, the tetraboric acid formed decomposes to boron trioxide.
   \({{\rm{H}}_2}\;{{\rm{B}}_4}{{\rm{O}}_7} \to 2\;{{\rm{B}}_2}{{\rm{O}}_3} + {{\rm{H}}_2}{\rm{O}}\)
4. On heating with ethyl alcohol, a volatile compound, triethyl borate, is formed, which burns with a green flame.
   \({{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} + 3{{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}} \to {\rm{B}}{\left( {{\rm{O}}{{\rm{C}}_2}{{\rm{H}}_5}} \right)_3} + 3{{\rm{H}}_2}{\rm{O}}\)

Structure of Boric Acid

The planar \({\rm{BO}}_3^{3 – }\) in Boric acid is joined by three hydrogen atoms to form the structure of the acid. The structure of Boric acid is as shown below:

Uses of Boric Acid

1. In medicine – as eyewash and as an antiseptic
2. In the food industry, as a preservative
3. As an additive in glass manufacturing

Summary

Boron belongs to the \({13^{{\rm{th}}}}\)  group of the periodic table and is the first element in the P block. It forms many industrially important compounds. Some important compounds of Boron are Borax, hydrides of Boron, Boron Nitride, and Boric Acid.

These compounds are significant in the manufacturing of things in various fields. Both boron and compounds of boron are industrially useful and are employed extensively for several purposes as mentioned under their uses.

FAQs on Important Compounds of the Boron Family

Q.1. What types of compounds are formed by boron?
Ans: Boron forms covalent compounds with other elements because of its smaller size and non-metallic nature. Boron reacts with metals, acids, and water at a very high temperature to form various compounds.

Q.2. What are 3 uses for boron?
Ans: The \(3\) uses of boron are as follows:
1. Boron is incorporated in steel to form special rods which are used in atomic reactors.
2. Boron is used as a catalytic reagent in chemical reactions.
3. Boron is used in the casting of copper as a deoxidizer.

Q.4. What are 5 chemical properties of boron?
Ans: 1. It reacts with hot steam to form Boron trioxide.
\(2{\rm{B}} + 3{{\rm{H}}_2}{\rm{O}} \to {{\rm{B}}_2}{{\rm{O}}_3} + 3{{\rm{H}}_2} \uparrow \)
2. Boron reacts with metals to form hard borides.
\(3{\rm{Mg}} + {{\rm{B}}_2} \to {\rm{M}}{{\rm{g}}_3}{{\rm{B}}_2}\)
3. Boron reacts with air and nitrogen at a very high temperature \(\left( {{{700}^{\rm{o}}}{\rm{C}}} \right)\) to form Boron nitride.
\(2{\rm{B}} + {{\rm{N}}_2} \to 2{\rm{BN}}\)
4. Boron only reacts with oxidizing acids, such as \({\rm{HN}}{{\rm{O}}_3}\) and \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)_, to form Boric acid.
\({\rm{B}} + 3{\rm{HN}}{{\rm{O}}_3} \to {{\rm{H}}_3}{\rm{B}}{{\rm{O}}_3} + 3{\rm{N}}{{\rm{O}}_2} \uparrow \)
5. It dissolves in fused alkalis such as \({\rm{NaOH}}\) to liberate hydrogen from them.
\({\rm{2B}} + 6{\rm{NaOH}} \to 2{\rm{N}}{{\rm{a}}_3}{\rm{B}}{{\rm{O}}_3} + 3{{\rm{H}}_2} \uparrow \)

Q.5. What are 3 physical properties of boron?
Ans: 1. It is an inert and very hard element.
2. Boron exists in two allotropic forms – a grey-black, non-metallic crystalline solid and a dark brown amorphous powder.
3. The melting point of Boron is \({2300^{\rm{o}}}{\rm{C}}\), and the boiling point is \(2550\,^\circ {\rm{C}}\)

Q.5. Is boron flexible or brittle?
Ans: Among the two allotropic forms, the crystalline Boron is hard and brittle, while the other one is a brown and amorphous powder.

We hope this detailed article on the important compounds of the Boron family helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!