BORON FOR UNDERGRADUATES

BORON FOR UNDERGRADUATES

Dr. Anand S. Burange

Department of Chemistry, Wilson College, Mumbai

Introduction

You believe it or not, but we consume boron containing compound almost every day. For the safety storage of rice, it is sprinkled with boric acid (H₃BO₃). Therefore, I personally would suggest washing rice multiple times before its consumption. How boron exists in nature? In nature it exists as a 'Tincal' which is a crude form of borax. Various boron containing compounds can be obtained from the borax. Borax, its preparation and reactions are discussed below.

Borax (Na₂B₄O₇.10 H₂O)

Borax is known as sodium borate, sodium tetraborate and disodium tetraborate. It is white, soft and easily soluble in water.

Borax is generally obtained from the mineral Colemanite (Ca₂B₆O₁₁). Colemanite on boiling with sodium carbonate gives borax, calcium carbonate and sodium metaborate.

Reaction:

Reactions of Borax

Boric acid can be prepared by the hydrochloric acid to hot saturated borax solution until the solution becomes strongly acidic.

Borax on reaction with ammonium chloride forms boron nitride and boron oxide.

Boron (Symbol:B) stands first in group 13 with atomic no. 5. It is found in nature in the form of Borax.

Ground state electronic configuration          1S² 2S² 2P¹

Excited state electronic configuration          1S² 2S¹ 2Px¹ 2Py¹ 2Pz⁰

Pure Boron

Pure boron is prepared by passing mixture of hydrogen
(H₂) and boron tribomide (BBr₃) gas over an electrically heated tantalum filament. Boron generally reacts with many metals to form borides. In case of pure boron preparation, the temperature of tantalum filament is maintained between in 600-1600⁰ to avoid reaction between tantalum and boron. Tantalum reacts with boron above 1800⁰ (refer Fig 1).

Pure crystalline boron is black and has great metallic lustre and opacity. The electric properties of pure crystalline boron match with the semi-metals.

Figure 1. Preparation of pure boron

Moissan’s Boron

 Moissan’s boron is impure boron with 98% purity. It can be prepared mostly in two ways either by the reaction of potassium fluoroborate or boric oxide with metals (refer Fig 2). Moissan's boron is also called as impure boron since it is difficult to get pure boron by these methods.

Figure 2. Boron preparation methods

Boron compounds

Boron reacts with various elements to form boron compounds; some of them are well tabulated in the following table (refer Table 1).

Table 1. Boron compounds

Compound	Formula	Remark/Information
Boron carbide	B12C3	It is made by heating boron with carbon in electric furnace.       It is black, very hard substance and has significant electrical conductivity.
Boron sulphide	B2S3	Sublimes at 200⁰       It is prepared by the action of H2S with BBr3.
Boric oxide	B2O3	Amorphous boron on air oxidation; and boric acid when heated to a low red heat give boric oxide.       Boric oxide combines with water to produce boric acid.       It forms borates when dissolved in alkali.       It reduces to boron when heated with certain metals.
Metal borides	MxB M = metal	Metals react with boron to form metal borides which may be present as single atoms (Mo2B, W2B), chains (CrB, MoB, WB), hexagonal sheets (AlB2, CrB2) and three dimensional network (CaB6, LaB6)

Boron halides and their Lewis acidity

The Lewis acidity is the tendency to accept electron. Boron compounds are known for their incomplete octet. Generally boron forms SP² hybridization and has vacant 2P orbital which can accommodate electrons from any electron rich species/anion/ligand. The strength of Lewis acidity      depends upon the electron deficient nature of the boron in a given compound.

     Among BF₃, BBr₃ and BI₃ which has more Lewis acidity?

    Among boron halides, since we know that fluorine is more electronegative, it will have more –I            effect.

Electro negativity trends in halogens: F >  Cl  >  Br > I

Usually anyone think that BF₃ will be more acidic among others but this isn’t true.

Among BF₃, BBr₃ and BI₃, boron has vacant 2P orbital and all these halogens acquire three lone pairs. Electron pairs in case of F, Br and I is present is 2p, 4P and 5P orbital respectively. In case of boron trihalides, electron pairs on halogens interact with vacant 2P orbital of boron. 2P-2P interaction stronger compared among others (2P-3P, 2P-4P, 2P-5P) since orbital from same shell have nearly same energy (See Fig 3).   

Figure 3. Interactions in boron trihalides

In halogens, electron pairs on fluorine are in 2P orbital and hence make the strongest interaction among all the halogens and strength of interaction decreases from fluorine to iodine. The strong interaction of fluorine leads to the formation of partial bond between them. This interaction is also called as back bonding or pπ-pπbonding. More interaction with electron pairs from halogens decreases the electrophilic nature of boron. Therefore BF₃ is least Lewis acidic.

Order of Lewis acidity

BI₃   >   BCl₃   >   BBr₃   > BF₃

Frustrated Lewis Acid-Base pairs

When any Lewis acid comes in contact with Lewis base, it forms Lewis acid-base adducts. Its classical example is well depicted below (Fig. 4).

Figure 4. Lewis acid-base adduct formation

Imagine the Lewis acid in which boron atoms is attached with three bulky groups (say phenyl) and Lewis base (phosphine) also bearing large groups. On mixing of such Lewis acid and base, though there is presence of lone pair (of lewis base) and vacant orbital (of lewis acid), because of presence of bulky groups, they can’t form adduct and it creates frustration among the molecules. Such Lewis acid-base pairs are called as Frustrated Lewis a acid-base pair which is well represented below (Fig. 5).

Figure 5. Frustrated Lewis acid-base pairs

Doug Stephan from McMaster University, explored the reactivity of main group systems, queried the impact of systems in which Lewis acid and Lewis base functions were incorporated into the same molecule and sterically prevented from quenching each other.

This frustration has several other applications in organic chemistry. One of the application is the breaking of hydrogen molecule in presence of this frustrated Lewis acid base pairs and followed by hydrogenation of various substrates.

(Reference: Angew. Chem. Int. Ed. 2010, 49, 46–76.)

Boric acid: Monobasic, dibasic, tribasic?????

Boric acid (B(OH)₃) has three hydroxyl (-OH) groups in its structure. Though it seems to have three protons directly attached to oxygen atom, it is not tribasic acid. Actually boric acid is monobasic acid i.e. it gives only one proton or one mole of boric acid neutralizes one mole of mono acidic base. In reality boron never donates its own proton.

Then how boric acid is monobasic acid?

Boric acid shows unusual breakdown of water molecule when dissolved in it. Typically boric acid breaks the water molecule to form [B(OH)₄]^- and proton (responsible for acidity).

Diborane (B₂H₆)

It is expected that reduction of boron halide to produce borane (BH₃). However, it is quite difficult to isolate the monomer. All the reported syntheses result in the formation of diborane (B₂H₆). Few reactions of diborane synthesis are shown below.

(Exception: Trimethylboron doesn’t show tendency to dimerize alike BH₃.)

Reactions:

Diborane is the simplest boron hydride and falls in class of electron deficient compounds.

Diboranes are good reducing agents and show no tendency to accept electrons from other reducing agents. After synthesis of diborane, it took almost thirty years to predict its exact structure. In case of diborane, each boron atom undergoes SP³ hybridization which forms four hybrid orbital having three unpaired electron and one vacant orbital. Two BH₃ molecules form bond between them by 3c-2e bond which is also known as banana bonding. Specifically, vacant orbital of one boron, orbital with unpaired electron from another boron atom and S orbital of hydrogen atom overlap to form new molecular orbital i.e. 3c-2e bond (Fig.6).

Figure 6. Molecular orbital and electron contribution in diborane

In diborane, four hydrogen atoms which form direct covalent bond with boron called as terminal hydrogen atoms. Two boron atoms are four terminal hydrogen atoms lie in a plane while two hydrogen atoms forming banana of 3c-2e bond is present above and below the plane named as bridging hydrogen. Molecular structure of diborane is shown below (Figure 7).

Figure 7. Molecular structure of diborane 

Professor Lipscomb for his work in borane chemistry was awarded with Nobel Prize in 1976.

Bibliography

1. Inorganic Chemistry by James E Huheey

2. Angew. Chem. Int. Ed. 2010, 49, 46–76.

3. Chemistry- R. J. Gillespie, D. A. Humphreys, N. C. Baird and E. A. Robinson

4. Chemistry- Zumdahl

5.Inorganic Chemistry- Durrant 

(Tip: This blog is especially written for undergraduate students)