Alum family

The metals of the aluminium sub-group are permanent in the air at ordinary temperatures, but when heated in oxygen or the air they become coated with their oxide. The volatility of the metals increases with the atomic weights, and the heavier metals are more easily reduced than those of lower atomic weight. The metals are all malleable, fusible, have small atomic volumes and form hydroxides, M(OH)3, which are typically amphoteric in the first three elements of the sub-group and basic only in the case of thallium. The last four members of the family form alums, and both aluminium and thallium form organo-metallic compounds, resembling zinc in this respect. 

Aluminum potassium sulfate belongs to a family of compounds known collectively as the alums. The term alum refers to a double salt that consists of aluminum, the sulfate group (S04), and one other metal. The presence of two metals, aluminum plus one other metal, accounts for the name double salt. Other common alums are aluminum ammonium sulfate and aluminum sodium sulfate. 

Compared to the relatively young history of the pure metal, aluminium compounds have been known for ages from the above-cited alum class to the more exclusive transition metal-doped aluminium oxides like ruby and sapphire (corundum varieties with chromium for the former and titanium and iron impurities for the latter) or aluminosilicate-like emeralds (a beryl type with chromium and vanadium impurities). However, to the synthetic chemist, aluminium chloride, is de facto one of the first jewels of the aluminium family. Aluminium trichloride (together with titanium tetrachloride, tin tetrachloride and boron trifluoride) is an exemplary Lewis acid that finds many applications in organic synthesis It is extensively used for instance in Friedel-Crafts alkylations and acylations, in Diels-Alder-type cycloadditions and polymerisation reactions. Its involvement in a wide range of reactions has been documented in many reviews and book chapters. ... 

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