Chemistry Reference
In-Depth Information
Table 2.16 Boiling points, latent heats and entropies of vaporization, and molecular
complexities of aluminium alkoxides
Physical
H
S
5
[Al(OR)
3
]
n
state
B.p. (
T
5
.
0
)
a
b
kcal mol
1
cal deg
1
mol
1
n
Al(OEt)
3
White solid
184.5
11.8
5100
23.9
52.2
4.1
(189.0)
(20.2)
(43.7)
Al(OPr)
3
Colourless
232.5
12.6
6025
27.5
54.5
4.0
liquid
(222.7)
(22.3)
(45.0)
Al(OBu)
3
Colourless
270.0
12.7
6540
29.9
55.0
3.9
liquid
(259.6)
(24.9)
(46.7)
Al(OPr
i
)
3
White solid
124.0
11.4
4240
19.4
48.9
3.0
(139.1)
(21.10)
(51.2)
Al(OBu
t
)
3
Colourless
172.0
11.3
4270
21.6
48.5
2.4
solid
(167.3)
(19.5)
(44.3)
Values in parentheses are reported by Wilhoit
488
and others by Mehrotra.
293
(39.5 and 42.0 cal deg
1
mol
1
, respectively),
274
but these are considerably lower
than those of polymerized zirconium primary and secondary alkoxides (the value of
entropy of vaporization for zirconium isopropoxide is reported as 66.1 - 72.1 cal deg
1
mol
1
).
113
,
137
In view of the comparatively lower values of entropies of vaporization
of aluminium alkoxides, it appears that aluminium alkoxides might be associated in
the vapour phase also. In fact Mehrotra
294
has shown that aluminium isopropoxide is
dimeric in the vapour phase, whereas it is trimeric in solution when freshly distilled.
293
This special behaviour of aluminium alkoxides and the stability of alkoxo bridges of
the type Al(
-OR)
2
Al, even in the vapour phase, is easily understandable on the basis
of the electron-deficient nature of tricovalently bonded aluminium atoms.
A number of investigators
484 - 487
have reported rather widely varying values of
the degree of association of aluminium alkoxides on the basis of molecular weight
determinations. These were, therefore, carefully reinvestigated by Mehrotra,
293
who
found that freshly distilled aluminium isopropoxide is trimeric in boiling benzene and it
changes to a tetrameric form on ageing. With increasing branching of alkoxo groups, the
complexity diminished and it was reduced to the dimeric state in the
tert
-butoxide.
293
These results were later confirmed by Shiner
et al
.
489
and Oliver
et al
.
490 - 492
who
made a careful NMR study of these aluminium alkoxides.
The variation in melting points of aluminium alkoxides was ascribed by Wilhoit
493
to various factors such as (i) sporadic hydrolysis, and (ii) the existence of allotropic
forms (
˛
and
ˇ
).
Wilson
494
measured the enthalpy of formation of solid aluminium isopropoxide
which was found to be
H
f
[Al
OPr
i
3
]
4
s
D 1230
.
8kcalmol
1
. On the basis of this
value, the enthalpy of formation of dimeric gaseous isopropoxide was calculated to be
H
f
[Al
OPr
i
3
]
2
g
D 584
.
8 kcal mol
1
. These two factors led to the minimum value
of Al - O bridge bond:
H
net
DC31
.
4kcalmol
1
.
Gallium alkoxides are also volatile compounds. For example, even the lowest
member of the series (the insoluble trimethoxide) has been sublimed at 275- 280
Ž
C
under 0.5 mm pressure.
184
The ethoxide and higher normal alkoxides are also volatile
and ebullioscopic molecular weight determinations in benzene indicated that these
normal alkoxides were tetrameric in character.
184
Mehrotra and co-workers
184
observed
