Q. Show the Halides of Aluminium?

All the four trihalides of aluminium, i.e. AIF₃, AICI₃,, AlBr₃, and AlI₃, are known. AIF₃,  is made by treating Al₂0₃ with HF gas at 1000 K and the other trihalides are prepared  by the direct exothermic reaction of the elements, e.g:

AlCl₃ is also obtained by heating a mixture of alumina and coke in a current of Cl₂:

Al₂0₃ + 3C + 3Cl₂-------------------------------> 2AlCl₃ + 3C0

Aluminium trifluoride differs from the other trihalides of Al in being ionic and nonvolatile in nature. Other halides of Al, as also of Ga and In, are covalent in nature when anhydrous and are relatively more volatile. AlCI₃, AlBr₃, and AlI₃, exist as dimeric species formed by pairing of two AIX₃ units as shown in Fig, 6.5. The pairing occurs by formation of a coordinate covalent bond from the halogen on one AlX₃ unit to the Al atom of another. Thus, for AlCl₃, the species Al₂CI₆, is formed. This is similar to the linking together of BeCI₂, units in solid BeCl₂, which you have already studied in the preceding unit.

The dimidiation of AlX₃, occurs because these halides are electron deficient. By  dimidiation, the halides attain' an octet of electrons. You have studied that the trihalides, BX₃ are also electron deficient and attain an octet by p?-p? bonding. This is not possible in case of A1 and other larger elements because-of lack of efficient.  ?-overlap, and hence, they dimerise. This dimidiation is retained when the halides dissolve in non-polar solvents such as C6H6 and CCl₃. In coordinating solvents, such  as, diethyl ether, trimethyl amine and phosphorus oxochloride, AlCl₃, forms  complexes like AlCl₃.0Et₃, AlCI₃, .NMe₃, and AlCl₃.OPCl₃, e.g.:

Al₂Cl₆ + 2(C₂H₅)₂O----------------------------------> 2 (C₂H₅)₂0AlCl₃

Alkyl and acyl chlorides, RCI and RCOCl, react with AlCl₃ to form complexes of the type R⁺AICI^-₄, and RCO⁺AICI^-₄, respectively; these are formed as intermediates in Friedel-Crafts alkylation and acylation reactions.