Chemistry Reference
In-Depth Information
not permit the additional ramified alcohol molecule to come close enough to the reacting
metal atom for the initiation of further alcoholysis reactivity. For example all the
isopropoxide groups in Mf
-OPr
i
2
Al
OPr
i
2
g
n
(where M is a bi-, tri- or tetravalent
metal atom) are easily replaced by
n
-butoxy groups when treated with excess
n
-butanol.
However, with tertiary butyl alcohol, only 3 out of 4 isopropoxide groups around
each aluminium atom can be replaced, as the product Mf
OPr
i
OBu
t
Al
OBu
t
2
g
n
appears to become immune to further reactivity with the next tertiary butyl alcohol
molecule. For example, a derivative BaAl
3
OPr
i
11
synthesized
91
by dissolving barium
in isopropanol in the presence of 3 moles of Al
OPr
i
3
(Eq. 3.30) has been assigned
the plausible structure (3-III) on the basis of NMR and alcoholysis studies:
OPr
i
(4)
Al
(6)
Pr
i
O
Pr
i
O
(1)
OPr
i
OPr
i
Al
Ba
Pr
i
Pr
i
O
OPr
i
(7)
(2)
Pr
i
O
OPr
i
(8)
(5)
Al
O
(3)
Pr
i
(3-III)
The proposed structure has been supported by the observation that only five
isopropoxide groups [(1) to (5)] are replaced by treatment with excess Et
3
COH, whereas
on treatment with the less sterically demanding species Me
3
COH, eight isopropoxide
groups, (1) to (5) initially, then (6), (7), and (8), are finally replaced, when each of the
three aluminium atoms is left bonded with three tertiary butoxy and one isopropoxy
group.
The gradual stepwise methanolysis and very mild hydrolysis reactions
197
represented
by Eqs (3.118) - (3.120) are highly interesting and confirm the suggested structures:
Pr
i
Pr
i
Pr
i
Pr
i
Me
Me
O
O
O
O
O
O
+
6MeOH
− 6Pr
i
OH
Ln
Al
Ln
Al
Ln
Al
Standing
O
O
O
O
Me
Pr
i
3
3
3
Pr
i
Pr
i
Pr
i
Me
Transient white precipitate
Clear solution
3
.
118
Pr
i
Pr
i
Pr
i
O
O
O
+ 6Hacac
− 6Pr
i
OH
Ln
Al
Ln
Al(acac)
2
(3.119)
O
Pr
i
O
3
3
Pr
i
Pr
i
Soluble volatile product
