Aluminum is the commonest metallic element in the earth's crust - as silicates, micas, and feldspar. The main commercial source is bauxite. The other elements are not widely found in nature.
Gallium has a melting point of 30�C and a boiling point of 2070�C, the largest liquid range of any substance. This makes it the best liquid for thermometers.
In general aluminum and to some extend gallium are more important in industries, so this lesson will focus on these two elements. When there is a distinct difference in chemistry for the other elements it will be noted accordingly.
OXIDES and HYDROXIDE
Aluminum is readily oxidized in air to give Al2O3. This forms a coating to protect the bulk of the aluminum against attack by acids. (Aluminum itself is readily attacked by mineral acids and can even react with water. It is also attacked by hot alkali hydroxides and halogens.) This resistant property of the oxide is attributed to its compact crystal structure. The non-compact oxide, known as gamma-form, readily dissolves in acids and water.
The oxides and hydroxides of aluminum and gallium are amphoteric compounds.
| With acid: | Al(OH)3 + 3 H3O+ � [Al(H2O)6]+� |
| With base: | Al(OH)3 + OH‾ � AlO2‾ + 2 H2O |
AlO2‾ is known as an aluminate ion while GaO2‾ as a gallate ion.
The oxides and hydroxides of indium and thallium are strictly basic.
HYDRIDES
Like borane, the hydrides are not stable and are isolated as adducts. One of the important class of adducts is the alkali metal salts. The stability of the alkali metal salts are in the order: B > Al > Ga.
If you take a careful look at aluminum hydride you will notice that the valence electronic shell has only six electrons, two short of the required eight to satisfy the octet rule. If we react lithium hydride with aluminum hydride, we would get Li[AlH4]. This is a smart partnership. Lithium is happy with the electron configuration of helium (1s�). The H‾, which is very unstable, "generously donate" the extra electron it was forced to accept from lithium to the aluminum atom. Aluminum is happy with its electron configuration of neon.
In [AlH4], the valence electrons spends most of its time with aluminum, so the hydrogen can enjoy the H+ status "some of the time".
The lithium adduct, LiAlH4, is a non-volatile, crystalline solid and can be prepared via the reaction.
Li[AlH4] is an important hydrogenation agent. It is soluble in diethylether or tetrahydrofuran (THF), and such solutions have been commonly used to hydrogenate (add hydrogen atom to) an organic acid (RCOOH) to an alcohol (RCH2OH), or olefins to alkanes.
The sodium adduct can be prepared as follow:
Note: reactions involving gases are usually conducted at high pressure.
All hydrides and hydride complexes are sensitive to water or protonated acids and the reaction can be explosive. So they must be handled with care, not with fear.
SALTS
Aluminum is soluble in mineral acids, hot alkali hydroxides, halogens and various nonmetals. Aluminum, gallium, indium and thallium form metallic ions [M(H2O)6]+� in aqueous solution, whereas boron does not. The ions tend to be slightly acidic.
HALIDES
The halides are Lewis acids. They dissolve in water to form hydrates, as discussed above, giving acidic solutions. As Lewis acid it can react with many organic compounds. For example;
Beside being a Lewis acid the halides can also react as a salt to give important organic complexes. For example,
ALUM
The Group IIIB elements (except boron) form a compound M(I)M(III)(SO4)2.12H2O, known as alums. M(I) is an alkaline cation (the lithium ion, which is too small in size, will not give an alum) and M(III) is a Group IIIB element or other transition metal that can exist in the trivalent state. (I will clarify this when we reach these elements). The alum for aluminum is better referred to as aluminum alum.
