Group 13 Elements: Boron Family | Inorganic Chemistry PDF Download

p-Block Elements

The elements in which the last electron enters the outermost p orbital are called p-block elements. 

• As the maximum number of electrons that can be accommodated in a set of p-orbitals is six, therefore there are six groups of p-blocks in the periodic table.
• The elements from the group 13 to 18 in the periodic table are p-Block elements

Inert Pair Effect

Inert Pair Effect is a decrease in stability of the maximum oxidation state and an increase in the stability of the (maximum-2) state on descending the group. 

• The inert pair effect shows itself increasingly in the heavier members of the group.
• Ge(+II) is a strong reducing agent whereas Ge(+IV) is stable Sn(+II) exists as simple ions which are strongly reducing but Sn(+IV) is covalent and stable. 
• Pb(+II) is ionic, stable, and more common than Pb(+IV), which is oxidizing. 
• The lower valencies are more ionic because the radius of M²⁺ is greater than that of M⁴⁺ and according to Fajans rules, the smaller the ion the greater the tendency to covalency.

Group 13 Elements (Boron Family)

The Elements are B( Nonmetal), Al, Ga, In, Tl (metals) 

General electronic configuration [Noble gas] ns² np¹

Atomic And Physical Properties

(1) Atomic and Ionic radii

Atomic radii: B > Ga < Al < In < Tl

(2) Ionization Enthalpies.

B > Tl > Ga > Al > In

(3) Melting and Boiling points

M.P. B > Al > Tl > In > Ga 

B.P. B > Al > Ga > In > Tl

(4) Electropositive Character

Due to high IE they are less electropositive on moving down the group metallic character increases due to decreases in IE [\B is nonmetals and other elements are metals]

Properties of Boron

It exists in five forms four of which are crystalline and one is amorphous. All crystalline forms are very hard made up of clusters of B₁₂ units. All crystalline forms are black in appearance and chemically inert. Melting points are around 2300°C. But amorphous form is brown and chemically active.

Compounds Of Boron

Boron Trioxide(B₂O₃)

Preparation

Properties

It is a weakly acidic oxide and reacts with alkalies or bases to form borates

 ( Sodium orthoborate), It reacts with water slowly to form orthoboric acid. When heated with transition metal salts if forms coloured compounds.

 The oxyacids of boron are:

(a) Orthoboric acid (b) Metaboric acid (HBO₂) (c) Tetraboric acid (H₂B₄O₇) (d) Pyroboric acid (H₄B₄O₈)

Orthoboric Acid

Orthoboric acid H₃BO₃ behaves as a weak monobasic acid it does not donate protons but rather accepts OH^–. It is therefore a Lewis acid B(OH)₃. Due to sp² hybridization of boron, boric acid is a planar molecule and due to H-bonding between different molecules boric acid has a layer structure.

Preparation

H₃BO₃ is soluble in water and behaves as weak monobasic acid. It does not donate protons like most acids, but rather it accepts OH–. It is, therefore, a Lewis acid (B(OH)₃)

Properties

• It is a weak monobasic Lewis acid and in aqueous solution, the boron atom completes its octet by removing. OH^– from water molecules.

• Since B(OH)₃ only partially reacts with water to form H₃O⁺ and [B(OH)₄]^– it behaves as a weak acid. 
• Thus it cannot be titrated satisfactorily with NaOH as a sharp endpoint is not obtained. If certain polyhydroxy compounds such as glycerol, mannitol, or sugar are added to the titration mixture then B(OH)₃ behaves as a strong monobasic acid. 
• And hence can now be titrated with NaOH and endpoint is detected using phenolphthalein as indicator.
• The added compound must be cis diol to enhance the acidic properties in this way the cis-diol forms very stable complexes with [B(OH) ₄]^– formed in forward direction above, thus effectively removing it from solution. 
• Hence reaction proceeds in forward direction (Le-Chatelier principle)

 Heating of boric acid:

 When boric acid is heated with ethyl alcohol, the evolved gas vapours of ethylborate is burned forming a green-edged flame.

Borax (Na₂B₄O₇.10H₂O)

Preparation of Borax:

Properties

• Its aqueous solution is alkaline because of its hydrolysis to weak acid H₃BO₃ and strong alkali NaOH

(ii) When borax powder is heated, it first swells due to loss of water in the form of steam but at 740°C it becomes converted into colourless transparent borax bead.

Boron Hydrides

Binary compounds of B with H are called boron hydrides of boranes. These compounds form following two types of series.

(Nido) B_nH_n+4 → B₂H₆, B₅H₉, B₆H₁₀, B₁₀H₁₄ 

(Arachno) B_nH_n+6 → B₄H₁₀, B₅H₁₁, B₆H_12, B₉H₁₅

Diborane (B₂H₆)

Structure of Diborane

Preparation

(i) 4BCl₃ + 3LiAlH₄ → 2B₂H₆ + 3LiCl + 3AlCl₃

Properties:

 B₂H₆ + 3O₂ →B₂O₃ + 2H₂O;    ΔH = -2137.7 kJ mol^-1

Pyrolysis of B₂H₆ in sealed vessels at temperatures above 375 K is an exceedingly complex process producing a mixture of various boranes e.g. B₄H₁₀, B₆H₁₂ and B₁₀H₁₄

 B₂H₆ + 6MeOH → 2B(OMe)₃ + 6H₂

 B₂H₆ + 2Me₃N → 2Me₃N  – BH₃

 B₂H₆ + 2Me₃N →2Me₃PBH₃

 B₂H₆ + 2CO  B₂H₆2NH₃B₃N₃H₆ (borazine)

 B₂H₆ + 2LiH → 2LiBH₄

 Carbaboranes are compounds of carbon, boron, and hydrogen in which carbon and born atoms occupy the vertices of a triangulated polyhedron. The compounds may be considered to have been derived from boranes by the replacement of BH^– units by CH units. The carboranes and the boranes having the same number of electrons in their bonding, framework have similar skeletal structures. Three structurally different types of carboranes, namely, closo, nido and arachno are known at present. 

Borazine Or Borazole, (BH)₃ or B₃N₃H₆

This compound is isoelectronic with benzene and hence has been called Inorganic Benzene.

Preparation

Borazine can be prepared by the following methods.

(i) By Stock and Pohland’s method (1926):  By the action of NH₃ on diborane (B₂H₆). The adduct, B₂H₆. 2NH₃ is first formed, which then gets decomposed by heating in a closed tube at 200°C. 

(ii) By heating BCl₃ with NH₄Cl:

(iii) By heating mixture of LiBH₄ and NH₄Cl (Laboratory method):

Chemical Properties

(i) Addition reactions: 

(a) One molecule of B₃N₃H₆ add three molecules of HCl or HBr in the cold, without a catalyst. These molecules get attached with all the three B -atoms, of B₃N₃H₆ molecule, since B -atom is more negative than N-atom in B—N or B=N bond and hydrogen chloride derivative (B₃N₃H₉Cl) is obtained. This addition reaction is not shown by benzene.

When this derivative is heated to 50 -100 °C, it looses, three H₂ molecules, to give B, B, B -trichloroborazine, B₃N₃H₃Cl₃.

(b)  

(ii) Hydrolysis:

(a) Borazine gets slowly hydrolyzed by water to produce boric acid [B(OH)₃ or H₃BO₃], NH₃, and H₂. Hydrolysis is favoured by the increase in temperature.

(b) It is reported that under proper conditions, borazine reacts with three molecules of water and gives B -trihydroxyl borazine, B₃N₃H3(OH)₃ (substitution reaction).

(iii) Pyrolysis: When borazine is pyrolyzed above 340°C, B₆N₆H₁₀ and  B₅N₅H₈ are produced. These products are boron-nitrogen analogs of diphenyl and naphthalene respectively

(iv) Hydrogenation: Hydrogenation of borazine produces polymeric materials of indefinite composition.

(v) Formation of adduct: Borazine forms an adduct with CH₃OH. This adduct undergoes pyrolysis with the elimination of H₂ and gives B-trimethoxy-borazine.

(vi) Reaction with aniline: Borazine undergoes a strongly exothermic reaction with aniline produce tri -aminoborine.

Structure of borazine molecule: 

• In the structure of borazine, both B and N atoms are sp² hybridized. Each N-atom has one lone pair of electrons, while each B-atom has an empty p-orbital. (B—N) π-bond in borazine is a dative bond, which arises from the sidewise overlap, between the filled p-orbitals of N-atom and empty p-orbitals of B-atom.
• Since borazine is isoelectronic with benzene, both the compounds have aromatic π-electron cloud (electrons in π-orbitals). 
• Due to greater differences in the electronegativity values of B and N-atoms, the π-electron cloud in B₃N₃ ring of borazine molecule is partially delocalized, while in case of the benzene ring, the π-electron cloud is completely delocalized.
•  In fact, complete delocalization of π-electron cloud in B₃N₃ ring in borazine molecule, cannot be expected, since N -π orbitals are of lower energy than the B -π orbitals. 
• Molecular orbital calculations have indicated that π-electron drift from N to B is less than the σ-electron drift from B to N, due to greater electronegative of N-atom. 
• In C₆H₆ molecule, C=C bonds are nonpolar while in the case of B₃N₃H₆ molecule due to the difference in electronegativities between B and N atoms, B—N bond is polar.
• The ring structure of borazine molecule is the same as the layer lattice structure of boron nitride, (BN)_n.
• It is due to the partial delocalisation of the π-electron cloud that π-bonding in B₃N₃ ring weakened. In addition, N-atom retains some of its basicity and the boron atom retains some of its acidity. 
• Polar species like HCl, therefore, attack the double bond between N and B. Thus, borazine in contrast to C₆H_6, readily undergoes addition reactions.
• In these reactions, more electronegative atom (e.g., Cl in HCl molecule) is generally attached with B-atom, which is less electronegative then N in B—N bond.
• In borazine, B—N bond length is equal to 1.44 Å, which is between the calculated single B—N bond (=1.54 Å) and double bond, B=N(=1.36 Å) distances. The angles are equal to 120°. IN benzene C—C bond length is equal to 1.42 Å.

B-trimethyl borazine, [B(CH₃)]₃(NH)₃

It is prepared by heating B(CH₃)₃ with NH₃, at 320-340° atm for 2 hours.

At 100°C, water replaces the NH groups by O-atoms and gives B-trimethyl boraxime, [B(CH₃)]₃O₃.

Boraxine,(BH)₃O₃

This compound is isoelectronic with borazine, B₃N₃H₆(B₃O₃H₃) = 3 × 3 + 3 × 6 + 3 × 1 = 30, B₃N₃H₆ = 3 × 3 + 5 × 3 + 6 × 1 = 30. It is produced by the explosive oxidation of B₂H₆ or B₅H₉. This compound decomposes at room temperature to diborane (B₂H₆) and boron trioxide (B₂O₃).

2B₃O₃H₃ →  2B₂O₃ + B₂H₆

Boraxine exhibits the aromatic properties of benzene. Boraxine is even less stable and presumably has less p-delocalization than borazine. In this molecule, B—O bond distance is equal to 1.38 Å. The characteristic Roman frequency of the ring is at 8.7 cm^-1. B=O, double bond present in the structure is due to the donation of a lone pair of electrons from O-atom to boron atom. This results in the development of a formal negative charge on B-atom and equal formal positive charge on O-atom.

N-trimethyl borazine, (BH)₃[N(CH₃)]₃

It is obtained in 90% yield by heating a mixture of B₂H₆ and NH₂.(CH₃) in the correct proportions at 180-200°C for 2 hours.

This compound can also be prepared by reducing Monomethyl ammonium chloride. (CH₃)NH₃Cl with lithium borohydride LiBH₄.

3(CH₃)NH₃Cl + 3LiBH₄ →  (BH)₃[N(CH₃)]₃ + 3LiCl + 9H₂

Boron Halides

• The relative strength of Lewis acids of boron trihalides increases in the order BF₃ < BCl₃ < BBr₃. This order of Lewis acid strength is just reversed of that expected on the basis of the electronegativity of the halogen. This is explained on the basis of overlapping of halogens sidewise with empty 2p-orbital of B forming pp-pp back bonding. The tendency of back bonding decreases as BF₃ > BCl₃ > BBr₃ > Bl₃ due to the difference in the energy states of the orbitals involved.
• Both boron and aluminum halides are Lewis acid but only aluminum halides exist as dimers whereas boron halides exist only as monomers. This is due to the reason that boron atom is so small that it cannot accommodate four large-sized halide ions around it.
• BF₃ and AlCl₃ are widely used as Lewis acid catalysts in Friedel -Crafts reactions and many industrial processes.
• Solution of AlEt₃ and TiCl₄ in hydrocarbon solvent reacts endothermically to form a brown solid. This is the important Ziegler Natta catalyst for polymerizing ethene to form polythene.

Aluminum (Al)

Properties

(i) Action of air: Dry air has no action of Al. But moist air forms a thin layer of Al₂O₃ on its surface and it loses its luster. At very high temperatures it burns to form Al₂O₃ and AlN.

AlN reacts with hot water to form Al(OH)₃ and NH₃.

Compounds of Aluminium

(i) Aluminium Oxide (Al₂O₃) Alumina

Preparation

Properties

It is a white amorphous powder insoluble in water but soluble in acids (forming eg., AlCl₃) as well as alkalies (forming NaAlO₂). Thus amphoteric in nature. It is a polar covalent compound.

(ii) Aluminium Chloride (AlCl₃.6H₂O) 

It is a colorless crystalline solid, soluble in water. It is anhydrous AlCl ₃ is a deliquescent white solid.

Preparation

Properties

(iii) Al₂Cl₆, Al₂Me₆

Aluminum halides are dimers in the gas phase: aluminum chloride has the molecular formula Al₂Cl₆ in the vapour. Each Al atom acts as an acid towards a Cl atom initially belonging to the other Al atom. Aluminum chloride is widely used as a Lewis acid catalyst for organic reactions.

Alkyl aluminum dimers are similar in structure to the analogous dimeric halides but the bonding is different. In the halides, the bridging Al—Cl bond involves an electron pair. In the alkyl aluminum dimers, the Al—C—Al bonds are longer than the terminal Al—Cl bonds, which suggests that they are 3c—2e bonds, with one bonding pair shared across the Al—C—Al unit, somewhat analogous to the bonding in diborane, B₂H₆.

(iv) Alumns

M₂SO₄, M’₂(SO₄)₂.24H₂O
Where M = Na+, K⁺, Rb⁺, Cs⁺, As⁺, Tl⁺, NH₄⁺ 
M’ = Al⁺³, Cr⁺³, Fe⁺³, Mn⁺³, Co⁺³ 
K₂SO₄.Al₂(SO₄)₃.24H₂O                                  Potash alum
(NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O                           Ammoniumalum
K₂SO₄.Cr₂(SO₄)₂.24H₂O                                 Chromealum
(NH₄)₂SO₄.Fe₂(SO₄)₂.24H₂O                          Ferricalum

Preparation: Al₂O₃ + 3H₂SO₄ →  Al₂(SO₄)₃ + 3H₂O

Al₂(SO₄)₃ + K₂KO₄ + aq. Solution →  crystallize

Properties

It is a white amorphous powder insoluble in water but soluble in acids (forming eg., AlCl₃) as well as alkalies (forming NaAlO₂). Thus amphoteric in nature. It is a polar covalent compound.