Preperation of Borax, Organometallic compound of Triethyl aluminium and formation of halide.

Borax: Na₂B4O_7.10H₂O.

Preperation.

1. Borax is prepared by boiling solution of Na₂CO₃ with orthoboric acid (H₃BO₃).

$$4Na_2CO_3+H_3BO_4\xrightarrow[]{boil}Na_2B_4O_7+6H_2O+CO_2↑$$

$$H_3BO_4=Orthoboric\, acid$$

2. It is also prepared by passing CO₂ gas through the solution of sodium meta-borate.

$$4NaBO_2+CO_2→Na_2B_4O_7+Na_2CO_3$$

$$4NaBO_2=Sodium\, metaborate$$

3.Na₂B₄O₇ is crystallised on solution to get Na₂B₄O₇.10H₂O

$$Na_2B_4O_7(aq)\xrightarrow[]{crystallise}Na_2B_4O_7.10H_2O$$

$$Na_2B_4O_7.10H_2O=borax$$

Properties.

It reacts with acid quantitatively.Hence, borax is used as the primary standard substance in titration against acid.

$$Na_2B_4O_7.10H_2O+HCl→NaCl+4H_3BO_3+5H_2O$$

One of the product in this reaction is orthoboric acid.Hence, the indicator used in this titration should be unaffected by the acid. Normally, methyl orange is used in this titration which changes its colour in the P.H. range 3.1-4.4

Structure.

Uses.

1. It is used in the preparation of the glass industry.
2. It is used in the identification of basic radical by dry tests in a salt analysis.
3. It is used in softening of water.
4. It is used as the flux,in welding and soldering.

The organometallic compound of Aluminium alkyls: AlET3(Trialkyl aluminium).

Preperation.

1.Triethyl Aluminium (AlEt₃) is prepared by treating Al with HgEt2.

$$2Al+3HgEt_2→2AlEt_3+3Hg$$

.2It is also prepared by treating Grignard reagent with AlCl₃.

$$3C_2H_5MgCl+AgCl_3→Al(C_2H_5)_3+3MgCL_2$$

AlEt₃ act as a polymerization catalyst in the polymerization of ethane into polyethene.At 90 to 120degreeC and under 120 atm pressure C₂H₄(ethane)molecules insert between Al-Et bond for a long chain. This long chain derivative is then hydrolysed to get polyethene.

As many as100 ethene molecules inserted in this polymerization.The polymer obtained in this way of low molecular weight and have no commercial use.Furthermore, this process is slower and required high temperature and high pressure.However, if transition metal catalyst (TiCl₄) which is called Zeigler-Natalcatalyst.Furthermore, the reaction does not require high pressure and high temperature and the polymer with high molecular weight (20,000-30,000) is produced.

Formations of halides.

These elements form halides of type MX₃. All these halides electron deficient and hence are lewis acid this halide remove electron deficient either by polymerization or resonance or complex formation.All these halides can form complexes in the presence of a base.

$$MX_3+X^{-}→MX_4^{-}$$

$$BF_3+F^{-}→BF_4^{-}$$

$$AlCl_3+Cl^{-}→AlCl_4^{-}$$

1. Boron halide BX₃ remove electron deficient by resonance.

Resonance in BX₃ arises due to the formation of dative π-bond between the Boron and X-atom.This π bond remains delocalized in the whole molecule. It means B-X bonds in BX₃ have double bond characters. That's why B-X bond length in BX₃ is intermediate between single and double bonds.In other words , B-X bonds in BX₃ are significantly shorter than the single bond length ie; the sum of the covalent radii of Boron and F.

2. AlCl₃ removes electron deficient by polymerization (dimerization).

Dimerisation takes place by the formation of halogen bridge in which a covalently bonded halogen atom to one Al atom donates a lone pair of the electron to another Al atom to form the coordinate bond.

Q. BeCL₃ exist as monomer while AlCl₃ exists as a dimer.

Group IIIA elements (B, Al) have only three valence electron.When these are used to form three covalent bond, the atom has a share in only six electrons.The compounds are therefore electron deficient.In BeCl₃ being B atom very small can get effective π overlap and attain octet by π bonding with any one of the three Cl atoms.So it doesn't need to polymerise to remedy electron deficient.But in AlCl₃. Al is larger and cannot get effective π bonds, so they polymerise to remedy the electron deficiency.In this way, AlCl₃ exist as a dimer.

Q. The P π-P π back bonding occurs in the halide of Boron but not halide of Aluminium.

BF₃ is an electron deficient compound.In BF₃ being a B-atom very small can get effective π-overlaps and attain octed by π-bonding with any one of the three halogen atom.This bonding is called P π-P π back bonding.This back bonding is formed between the empty 2P_z orbital on B-atom, which is not involved in hybridization and P-orbital of any of the three halogen atom.

The halide of Al like AlCl₃ is also electron deficient.The size of Al is large, so it can not get effective π-overlap with halogen and therefore P π-P π back bonding does not occur in halide of Al.So, halides of Al are dimerized to fulfil electron deficient.

Cement.

Cement is an important building material which is employed in the construction of building, bridge.It was first to introduce in England in 1824 by Joseph Aspdin.To form such cement are used namely portland cement and high alumina cement.

Portland cement is made by heating the correct mixture of lime,stone, (CaCO₃) with sand (SiO₂) and clay (alumina silicate) at a temperature of 1450-1600 degreeC and 2-3% of gypsum salt (CaSO₄.2H₂O). Gipson salt is also added to slow the setting process.The composition of cement usually given in terms of oxides.A typical composition of portland cement is as below.

CaO= 70%

SiO₂=20 %

Fe₂O₃=3%

Al₂O₃= 5%

CaSO₄.2H₂O=2%

High alumina cement is made by fusing limestone and bauxite with the small amount of SiO₂ at 1400-1500 degree C.A typical composition of high alumina cement is as below.

CaO= 40%

SiO2=10%

Al2O3= 40%

Fe2O3= 10%

It is the most expensive than portland cement.It has advantages over the portland cement is that it sets much faster and develop high strength within a day.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.