Question

Which reaction cannot give anhydrous $AlC{l_3}$?
A.Heating of $AlC{l_3}.6{H_2}O$
B.Passing dry $HCl$ over heated aluminium powder
C.Passing dry $C{l_2}$ over heated aluminium powder
D.Heating a mixture of alumina and coke in a current of dry $C{l_2}$

Answer 1

If a substance is termed as anhydrous, it signifies that it does not have water. We get anhydrous compounds from only crystalline ionic compounds when the water of crystallization is removed. Covalent compounds react with the water of crystallization during heating.

Complete step by step solution:
In case of option A, when $AlC{l_3}.6{H_2}O$ is heated, the water of crystallization evaporates. This method is commonly used for dehydration of ionic salts. But since $AlC{l_3}$ is a covalent compound, it reacts with the evaporated water and forms aluminium hydroxide. Thus it cannot be used to prepare anhydrous aluminium chloride.
Anhydrous aluminium chloride is prepared by passing dry hydrochloride gas over heated aluminum metal surfaces. The chemical reaction for this preparation of anhydrous aluminium chloride is given by:
$2Al + 6HCl \to 2AlC{l_3} + 3{H_2}$
Thus, option B can be used to prepare anhydrous $AlC{l_3}$.
Considering option C, if we pass dry chlorine gas over powdered and heated aluminium, we get aluminium chloride anhydrous salt.
Thus, option C can also be used to prepare anhydrous aluminium chloride.
Considering option D, when we heat a mixture of alumina and coke with a flowing current of dry chlorine gas, it also yields anhydrous salt of aluminium chloride.
The chemical reaction can be written as: $A{l_2}{O_3} + 3C + 3C{l_2} \to 2AlC{l_3} + 3CO$
Thus, we can also use option D for the preparation of anhydrous aluminium chloride.

Hence, the correct option is A.

Note:
Anhydrous aluminium chloride is a very good oxidising agent. It is widely used in a number of reactions due to its Lewis acidic properties. Most commonly, it is used as a catalyst in Friedel- Craft alkylation reaction.