Chemistry Reference
In-Depth Information
gallium isopropoxide and
tert
-butoxide to be dimeric in boiling benzene. The dimeric
behaviour of gallium isopropoxide was confirmed in a later study by Oliver and
Worrall
495
who found it to be dimeric cryoscopically in dilute benzene solution.
However, the cryoscopic molecular weight determination of a freshly prepared
concentrated benzene solution of the isopropoxide indicated it to be tetrameric.
Evidence in favour of the latter observations was obtained from
1
H NMR spectra, which
also showed the presence of tetrameric species in a fresh solution which dissociated
to the dimeric form on being aged.
3.2.12
Alkoxides of Group 14 Elements
Silicon and germanium alkoxides are highly volatile and most of the derivatives
of silicon
316
have been distilled unchanged at atmospheric pressure; for example,
silicon tetraethoxide,
316
,
496
and tetraphenoxide
316
,
497
distilled at 168
Ž
C and 417 - 420
Ž
C,
respectively. Similarly, germanium ethoxide
315
and phenoxide
293
could also be
volatilized at 85- 86
Ž
C/12.0 mm and 220 - 225
Ž
C/0.3 - 0.4 mm, respectively. The low
boiling points of these alkoxide derivatives of silicon and germanium are due to their
monomeric nature, irrespective of the chain length and branching of alkoxo groups.
Bradley
et al
.
113
,
273
,
274
,
432
measured the boiling points of a number of derivatives
under different pressures and observed that the results follow the equation, log
p
D
a
b
/
T
in the pressure range 2.0 - 10.0 mm. Although silicon alkoxides are monomeric,
the latent heat of vaporization, entropy of vaporization, and Trouton constants increase
with the length of the alkyl groups. This unexpected behaviour has been ascribed
to the molecular entanglement in the liquid state or to the restricted rotation of the
alkoxo group with increasing chain length. It is worth mentioning here that a molecule
in the liquid state will require special orientation in order to be disentangled before
vaporization, and this may cause an increase in the entropy of vaporization.
The thermal decomposition of silicon tetraethoxide yields silicon oxides at higher
temparatures.
475
,
498
By contrast, tin alkoxides appear to be thermally less stable than
the corresponding silicon and germanium analogues.
311
,
499 - 501
For example, all normal
alkoxides of tin undergo decomposition on being heated under reduced pressure.
311
The sterically hindered isopropoxide,
tert
-butoxide, and
tert
-amyloxide, however, were
distilled at 131
Ž
C/1.6, 99
Ž
C/4.0, and 102
Ž
C/0.2 mm, respectively.
311
The ebullioscopic
molecular weight measurements in boiling benzene indicate the following order of
complexities: methoxide (an insoluble derivative) appears to be highly associated; the
normal alkoxides are tetrameric, whereas isopropoxide and
tert
-alkoxides are trimeric
and monomeric, respectively.
311
The alkyl substituents at the tin atom, in view of its larger size have been found to
affect the molecular complexities as well as volatility of the derivatives considerably.
For example, Gaur
et al
.
210
,
211
observed that except for highly associated nonvolatile
methoxides, all secondary and tertiary trialkoxides of monoalkyltin trialkoxides are
highly volatile liquids which could be distilled in good yields under reduced pres-
sure. The complexities of ethoxides, isopropoxides, and
tert
-butoxides of monoalkyltin
fall in the order 3.0, 1.5, and 1.0, respectively in boiling benzene. These molecular
complexities are in agreement with the
119
Sn NMR spectra measured for a number of
monoalkyltin trialkoxides.
502
Similarly, dialkyltin dialkoxides and trialkyltin alkox-
ides also show high volatility but the molecular complexity is comparatively less