2 Homometallic Alkoxides

34 Alkoxo and Aryloxo Derivatives of Metals
Pr i
O OBu
Al Al
O
t
OBut ButO ButO Pri (2-I)
resulting in a final product of the composition Al2(OEt)(OBu t )5. It appears, therefore,
that the corresponding [Al(OEt)(OBu t )2]2 may also have (2-II) as one possible
structure, 293 in which further replacement at the less shielded of the two aluminium
atoms may lead to further reaction leading to a structure of the type (2-III).
Bu t
Bu
Al
O
tO But O OEt
Al
O
Et
(2-II)
Bu t O
Bu t O
Al
Bu t
OBu t
O OBu
Al
O
Et
t
OBut (2-III)
2.7.4 Fractionation in the Presence of an Inert Solvent
In order to complete an alcoholysis reaction, an excess of the higher boiling reactant
alcohol would be required, so that the lower boiling alcohol could be fractionated out;
this can sometimes be achieved by use of a solvent which is higher boiling than even
the reactant as well as the product alcohols. When the original alkoxide is ethoxide or
isopropoxide, the use of an inert solvent such as benzene offers an added advantage
by virtue of the formation of a lower boiling azeotrope with ethyl alcohol or isopropyl
alcohol, which facilitates the removal of the liberated alcohol by fractional distillation.
For example, zirconium isopropoxide on treatment with alcohols was converted to pure
alkoxides by the removal of the isopropyl alcohol–benzene azeotrope continuously on
a fractionating column: 135
Zr(OPr i ⊳4.Pr i OH C xROH ! Zr(OPr i ⊳4 x ⊲OR⊳x C ⊲x C 1⊳Pr i OH " ⊲2.87⊳
Mehrotra 4,308–310 observed that benzene is a good solvent for the alcoholysis reactions
for the following reasons:
(a) The reactant alcohol can be used economically since only the stoichiometric
amount is required for complete replacement.