Aluminium Chloride

Preparation

When aluminium metal or Al(OH)₃ is treated with HCl, solution of hydrated aluminium chloride, AlCl₃.6H₂O is obtained.

2Al + 6HCl 2AlCl₃ + 3H₂↑

Al(OH)₃ + 3HCl AlCl₃ + 3H₂O

On evaporating the solution, the crystals of AlCl₃.6H₂O are obtained. On heating these crystals, anhydrous aluminium chloride cannot be obtained due to the formation of Al₂O₃.

2(AlCl₃.6H₂O) Al₂O₃ + 6HCl + 9H₂O

The anhydrous aluminium chloride is however prepared as follows:

(i) By passing dry chlorine gas or vapours of hydrochloric acid gas over heated aluminium powder. The vapour of aluminium chloride is condensed in the receiver.

2Al + 3Cl₂ 2AlCl₃

2Al + 6HCl 2AlCl₃ + 3H₂↑

(ii) By heating a mixture of alumina and coke in a current of chlorine (Mac Affe process)

Al₂O₃ + 3C + 3Cl₂ 2AlCl₃ + 3CO

(iii) By heating alumina (Al₂O₃) in a current of carbonyl chloride (COCl₂).

Al₂O₃ + 3COCl₂ 2AlCl₃ + 3CO₂

(iv) By heating alumina in a current of S₂Cl₂ vapour and Cl₂. (Commercial preparation)

4Al₂O₃ + 3S₂Cl₂ + 9Cl₂ 8AlCl₃ + 6SO₂

Properties

(i) Anhydrous salt is a white crystalline solid. It is hygroscopic and fumes in moist air. It sublimes at 180°C. (Melting point = 193°C at 2 atm pressure).

(ii) It is a typical covalent compound as is shown by its volatility, solubility in organic solvents like C₆H₆, CS₂ etc and poor conductivity of the fused state.

(iii) An aqueous solution of the salt is acidic in nature because in water it undergoes hydrolysis and forms HCl.

AlCl₃ + 3H₂O      Al(OH)₃ + 3HCl

With moist air, the salt gives the fumes of HCl. The equation representing the reaction is the same as given above.

(iv) When NH₄OH is added to AlCl₃ solution, white precipitate of Al(OH)3 is obtained.

AlCl₃ + 3NH₄OH Al(OH)₃↓ + 3NH₄Cl

White

(v) With excess of NaOH, the solution of the salt gives sodium meta−aluminate, NaAlO₂.

AlCl₃ + 4NaOH (excess) NaAlO₂ + 2H₂O + 3NaCl

(soluble)

(vi) It forms addition compounds with a number of donor molecules like NH₃, PH₃,
COCl₂ etc.

AlCl₃ + 6NH₃ AlCl₃.6NH₃

Addition compound

Uses

(i) In organic chemistry for introducing alkyl radicals originally linked with halogen into benzene nucleus (Friedel Craft’s reaction).

(ii) In the manufacture of petrol by the cracking of mineral oils.

(iii) As a catalyst in the manufacture of dyes, drugs and perfumes.

Dimerization of AlCl₃

2AlCl₃ Al₂Cl₆