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Chemical properties

(1)     Occurrence :  The important of this group elements are given below,

Boron :    

Borax (Tincal)                                (Na₂B₄O₇.10H₂O),

Colemanite                                     (Ca₂B₆O₁₁5H₂O)

Boracite                                            (2Mg₃B₈O₁₅.MgCl₂),

Boronatro calcite                        (CaB₄O₇.NaBO₂.8H₂ O),

Kernite                                            (Na₂B₄O_7.4H₂O), Boric acid (H₃BO₃)

Aluminium : Corundum (Al₂O₃), Diaspore (Al₂O₃.H₂O), Bauxite (Al₂O_3. 2H₂O), and Cryolite (Na₃AlF₆).

(2)     Hydrides

(i)      Elements of gp 13 do not react directly with hydrogen but a number of polymeric hydrides are known to exist.

(ii)     Boron forms a large no. of volatile covalent hydrides, known as boranes e.g. B₂ H₆, B₄H₁₀, B₅H₁₁, B₆H₁₀ Two series of borones with general formula B_nH_n+4 and B_nH_n+6  are more important.

(iii)    Boranes are electron deficient compounds. It is important to note that although BX₃ are well known, BH₃ is not known. This is due of the fact that hydrogen atoms in BH₃ have no free electrons to form pπ-pπ back bonding and thus boron has incomplete octet and hence BH₃ molecules dimerise to form B₆H₆ having covalent and three centre bonds.

(iv)    Al forms only one polymeric hydride (AlH₃)_n commonly known as alane It contains A1…..H……Al bridges.

(v)     Al and Ga forms anionic hydrides e.g. LiAlH₄ and Li Ga H₄,

          4LiH + AlCl₃      Li[AlH₄] + 3LiCl

(3)     Reactivity towards air

(i)      Pure boron is almost unreactive at ordinary temperature. It reacts with air to form B₂O₃ when heated It does react with water. Al burns in air with evolution of heat give Al₂O₃.

(ii)     Ga and In are not effected by air even when heated whereas Tl is little more reactive and also form an oxide film at surface. In moist air, a layer of Tl (OH) is formed. 

(iii)    Al decomposes H₂O and reacts readily in air at ordinary temperature to form a protective film of its oxides which protects it from further action.

(4)     Oxides and hydroxides

(i)      The members of boron family form oxide and hydroxides of the general formula M₂O₃ and M (OH)₃ respectively.

(ii)     The acidic nature of oxides and hydroxides changes from acidic to basic through amphoteric from B to Tl.

B₂O₃ and B(OH)₃  > Al₂O₃ and Al(OH)₃ > Ga₂O₃  and Ga(OH)₃  > In₂O₃ In (OH)₃  >  Tl₂O₃ Tl(OH)₃

(acidic)                         (amphoteric)                    (amphoteric)                              (basic)                    (strong basic)

Note :  B(OH)₃ or H₃BO₃ is weak monobasic Lewis acid.

(iii)    Boric acid, B(OH)₃ is soluble in water as it accepts as it accepts lone pair of electron to act as Lewis acid. Rest all hydroxides of group 13 are insoluble in water and form a gelatinous precipitate.

          B(OH)₃ + H₂O → B(OH)₄^1- + H⁺

(iv)    Al₂O₃ being amphoteric dissolves in acid and alkalies both.

Al₂O₃ + 3H₂SO₄ → Al₂ (SO₄)₃ + 3H₂O

Al₂O₃ + 2NaOH 2NaAlO₃ + H₂O

                              Sodium meta aluminate

(v)     One of the crystalline form of alumina (Al₂O₃) is called corrundum. It is very hard and used as abrasive. It is prepared by heating amorphous form of Al₂O₃ to 2000 K.

(5)     Action of Acids

(i)      Boron does not react with non oxidizing acids, however, it dissolves in nitric acid to form boric acids.

(ii)     Al, Ga and In dissolve in acids forming their trivalent cations; however, Al and Ga become passive due to the formation of protective film of oxides.

(iii)    Thallium dissolves in acids forming univalent cation and becomes passive in HCl due to the formation of water insoluble TICl.

(6)     Action of Alkalies

(i)      Boron dissolves only in fused alkalis, 2B + 6NaOH (fused) → 2Na₃BO₃ + 3H₂

(ii)     Al and Ga dissolves in fused as well as in aqueous alkalis, 2Al + 2 NaOH + 2H₂O → 2NAl O₂ + 3H₂

(iii)    Indium remains unaffected in alkalies even on heating.

(7)     Halides

(i)      All the group 13 elements from the trihalides, MX₃ on directly combining with halogens.

          M + X₂ → MX₃

(ii)     All the trihalides of group 13 elements are known except Tl (III) iodide.

(iii)    Due to small size and high electronegativity of boron, all boron halides are covalent and Lewis acids. These exist as monomeric molecules having plane triangular geometry (sp² hybridization).

(iv)    All Boron trihalides except BF₃ are hydrolysed to boric acid.

          BX₃+ 3H₂O → B(OH)₃ + 3HX;               [X=Cl,Br,I]

However, BF₃ forms as addition product with water,

BF₃ + H₂O → H⁺ [BF₃OH]^–  H₃O⁺ [BF₃OH]^– .

BF₃ having less tendency for hydrolysis as well as Lewis acid nature, is extensively used as a catalyst in organic reactions e.g. Friedel- Crafts reaction.

(v)     Boron atom, in BX₃, has six electrons in the outermost orbit and thus it can accept a pair of electrons form a donor molecule like NH₃ to complete its octet. Hence boron halides act as very efficient Lewis acids. The relative Lewis acid character of boron trihalides is found to obey the order ; BI₃ > BBr ₃ > BCl₃ > BF₃.

However, the above order is just the reverse of normally expected order on the basis relative electronegativities of the halogens. Fluorine, being the most electronegative, should create the greatest electron deficiency on boron and thus B in BF₃ should accept electron pair from a donor very rapidly than in other boron trihalides. But this is not true.

This anomalous behaviour has been explained on the basis of the relative tendency of the halogen atom to back-donate its unutilised electrons to the vacant p orbitals of boron atom. In boron trifluoride, each fluorine has completely filled unutilised 2p orbitals while boron has a vacant 2p orbital. Now since both of these orbitals belong to same energy level (2p) they can overlap effectively as a result of which fluorine electrons are transferred into the vacant 2p orbital of boron resulting in the formation of an additional pπ – pπ bond. This type of bond formation is known as back bonding or back donation. Thus the B- F bond has some double bond character. Back bonding may take place between boron and of the three fluorine atoms and thus boron trifluoride is regarded as a resonance hybrid of some structures.

Resonance in boron trifluoride is also evidenced by the fact that the three boron-fluorine bonds are indentical and are shorter than the usual single boron-fluorine bond As a result of back bonding, the electron deficiency of boron is reduced and hence Lewis acid nature is decreased. The tendency for the formation of back bonding (pπ- pπ bond) is maximum in BF₃ and decreases very rapidly from BF₃ to BI₃ This is probably due to the fact that overlapping of the vacant 2p orbitals of boron cannot take place easily with the p-orbitals of high energy levels (3p in Cl, 4p in Br and 5p in iodine). Thus BI₃ Br₃ and BCl₃ are stronger Lewis acids than the BF_3.

(vi)    Lewis acid character of halides of the group 13 elements decreases in the order, B > Al > Ga > In

(vii)   Boron halides form complex halides of the type, [BF₄^–], in which boron atom extends its coordination number to four by utilising empty p-orbital. It cannot extend its coordination number beyond four due to non availability of d-orbitals. However, the other trihalides of this group form complex halides of the type (AlF₆)^3-, (GaCl₆)^3- and (InCl₆)^3-, etc where the central atom extends its coordination number to 6 by the use of d-orbitals.

(viii)  The fluorides of Al, Ga In and Tl are ionic and have high melting points. The high melting points of metal fluorides can be explained on the basis that their cations are sufficiently large and have vacant d-orbitals for attaining a coordination number of six towards the relatively small fluorine atom.

(ix)    Other halides of Al, Ga, In and Tl are largely covalent in anhydrous state and possess low m.pt. These halides do not show backbonding because of increases in the size of the element. However, the make use of vacant p-orbitals by co-ordinate bond i.e. metal atoms complete their octet by forming dimers. Thus aluminium chloride, aluminium bromide and indium iodide exist as dimers, both in the vapour state and in non-polar solvents.

The dimer structure for Al₂Cl₆ is evidenced by the following facts,

(a)     Vapour density of aluminium chloride measured at 400⁰C corresponds to the formula Al₂ Cl₆.

(b)     Bond distance between aluminium chlorine bond forming bridge is greater (2.21A⁰) than the distance between aluminum-chlorine bond present in the end (2.06 A⁰). The dimeric structure  disappears when the halides are dissolved in water This is due to high heat of hydration which split the dimeric structure into [M(H₂O)₆]³⁺ and 3X^– ions and the solution becomes good conductor of electricity.

Al₂Cl₆ + 2H₂O → 2[Al(H₂O)₆]³⁺ + 6Cl^–  ; Therefore Al₂Cl₆ is ionic in water.

The dimeric structure may also split by reaction with donor molecules e.g. R₃N. This is due to the formation of complexes of the type R₃NAl Cl₃ The dimeric structure of Al₂ Cl₆ exist in vapour state below 473 K and at higher temperature it dissociates to trigonal planar AlCl₃ molecule.

Note :     Boron halides do not exist as dimer due to small size of boron atom which makes it unable to co-ordinate four large-sized halide ions.

(x)     BF₃ and AlCl₃ acts as catalyst and Lewis acid in many of the industrial process.

Boron Family : Anomalous Behaviour of Boron