Chemistry Reference
In-Depth Information
2.7.2
Steric Factors
The high rate of alcohol interchange has some interesting mechanistic implications
especially in view of the strong (100 - 110 kcal mol
1
) metal - oxygen bonds
306
,
307
in
titanium alkoxides. The presence of vacant d-orbitals in most of the metals offers a
facile initial step in a nucleophilic attack (S
N2
) of an alcohol molecule on the metal
alkoxide (Eq. 2.84), and thus a low activation energy for alcohol interchange involving
a four-membered cyclic transition state seems reasonable:
RO
OR
H
RO
OR
R
′
O
OR
R
′
and so on
O
M
O
M
M
R
′
H
RO
OR
R
′
O
R
R
′
O
R
(
+
ROH)
(
+
ROH)
2
.
84
Obviously, such reactions would be susceptible to steric factors. For example, in
an attempt to measure the kinetics of alcohol interchange in titanium and zirconium
alkoxides, Bradley
3
observed that an equilibrium was established between the reactants
when they were mixed at room temperature. This was later confirmed by
1
HNMR
spectra which indicated that a mixture of titanium tetraethoxide and ethanol gave only
one type of ethoxy signal, thus indicating the rapidity of the exchange of ethoxy groups.
A similar observation has been made by Mehrotra and Gaur
308
who found only one type
of isopropyl protons in the
1
H NMR spectrum of titanium dibromide diisopropoxide
in isopropyl alcohol solution indicating a dynamic equilibrium (Section 3.4). On the
other hand, a mixture of titanium
tert
-butoxide and
tert
-butyl alcohol showed different
types of methyl signals indicating that the rate of exchange is slow, which may be
ascribed to steric factors.
26
2.7.3
Fractionation of More Volatile Product
Even in cases where reactions are rather slow, the equilibrium can be pushed to
completion if the alcohol produced in the reaction is continuously fractionated out.
For example, alcoholysis of aluminium isopropoxide with primary as well as secondary
butyl alcohols can be completed by fractionating out the isopropyl alcohol produced:
293
Al
OPr
i
3
C 3Bu
n
OH ! Al
OBu
n
3
C 3Pr
i
OH "
2
.
85
However, in the case of
tert
-butyl alcohol even after careful fractionation, a maximum
of only two isopropoxy groups per aluminium atom appears to be replaced:
293
Al
OPr
i
3
C 2Bu
t
OH !
2
[Al
OBu
t
2
OPr
i
]
2
C 2Pr
i
OH "
2
.
86
As the product Al(OPr
i
)(OBu
t
)
2
is found to be dimeric it was suggested that alumi-
nium atoms on being surrounded by bulky isopropoxy and
tert
-butoxy groups in a
structure of the type (2-I), are shielded so effectively that the lone pair orbital of
the oxygen atom of another
tert
-butyl alcohol molecule cannot approach sufficiently
close to the 'd' orbitals of aluminium for the interaction to be initiated. A finer differ-
ence in susceptibilities to steric factors was further demonstrated by the fact that with
aluminium ethoxide, some further (albeit extremely slow) replacement was possible,
