Chemistry Reference
In-Depth Information
3.2.10
Alkoxides of Later 3d Metals
Iron trialkoxides are volatile compounds, except the methoxide which decomposed on
attempted distillation
in vacuo
.
282
Thus ethoxide,
n
-propoxide,
n
-butoxide, isoprop-
oxide, and
tert
-butoxide distilled at 155, 162, 171, 149, and 136
Ž
C, respectively under
0.1 mm pressure.
282
,
283
The ebullioscopic
283
and cryoscopic
474
molecular weight deter-
minations in benzene indicated all these normal alkoxides to be trimeric including
the methoxide, whereas the isopropoxide and
tert
-butoxide are dimeric in boiling
benzene.
283
A comparision of the properties of all the alkoxides of trivalent iron with those of
aluminium should be interesting as these metals have almost the same covalent radii
(1.22 and 1.26 A, respectively).
It is evident from Table 2.14 that the normal alkoxides of aluminium have generally
high boiling points and higher molecular complexities than the iron(
III
) analogues.
However, the boiling points are not all quoted at the same pressure and are therefore
not strictly comparable. These differences in molecular complexity are doubtless due
to structural differences between aluminium and iron(
III
) alkoxides and this may be a
reflection of stronger intermolecular bonding in the aluminium derivatives.
Alkoxides of divalent Mn, Fe, Co, Ni, and Cu are insoluble and nonvolatile,
7
due
to extensive molecular association and hence are not included here.
3.2.11
Alkoxides of Group 13 Elements
The lower alkoxides of boron are highly volatile compounds
475
which can be distilled
at atmospheric pressure; these derivatives are essentially monomeric in nature.
238
In comparing the volatilities of primary, secondary and tertiary alkoxides of boron,
it is noteworthy that the primary and secondary butoxides distil unchanged at 128 and
195
Ž
C, respectively
292
,
476
whereas the corresponding
tert
-butoxide undergoes decom-
position to produce olefin.
477
Table 2.14 Boiling points and molecular complexities of iron
and aluminium trialkoxides, M(OR)
3
B.p. (
Ž
C/mm)
Molecular complexity
R
Fe
Al
Fe
Al
240/10
5
Me
-
2.9
-
Et
155/0.1
162/1.3
2.9
4.1
Pr
n
162/0.1
205/1.0
3.0
4.0
Bu
n
171/0.1
242/0.7
2.9
3.9
Bu
i
173/0.1
-
3.0
-
Bu
s
159/0.1
172/0.5
1.7
2.4
Bu
t
136/0.1
-
1.5
1.95
Bu
n
CH
2
178/0.1
255/1.0
3.0
4.0
Bu
i
CH
2
200/0.1 (dec)
195/0.1
3.0
4.0
Bu
s
CH
2
178/0.1
200/0.6
3.0
4.1
Bu
t
CH
2
159/0.1
180/0.8
2.0
2.1
Pr
i
MeCH
162/0.1
162/0.6
1.9
2.0
dec D decomposes.
