Chemistry Reference
In-Depth Information
simple alcohols or glycerol, future relevant challenge is the use of this
clean reaction to the synthesis of ethers derived from higher polyols. A
first approach has for example involved the synthesis of ethers from
sugar alcohols (sorbitol, xylitol, erythritol...) and aldehydes or ketones.
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The etherification reaction is accompanied with a partial dehydration of
the polyol.
3.3 Acid catalyzed hydroalkoxylation reactions
Hydroalkoxylation reactions refer to the addition of an alcohol over an
insaturation.
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This highly atom economical process is potentially a
straightforward and clean access to ethers, and the reaction is success-
fully applied at the industrial level for the production of MTBE (methyl
tert-butyl ether) and ETBE (ethyl tert-butyl ether) from isobutylene and
methanol or ethanol.
63
If this transformation is well known with acti-
vated olefins (reaction referred to a Michael addition), a real challenge is
the synthesis of ethers from unactivated olefins.
64
Very few reactions
involving carbohydrates or polyols have been reported to date and most
of them involve isobutylene as this substituted olefin is prompt to
generate stabilized carbenium ion under acidic conditions. Dimerization
reactions of the alcohol or isobutylene are the main side reactions that
have to be avoided in order to reach high selectivities into the desired
ethers.
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Glycerol has been converted according to hydroalkoxylation type
reactions with isobutylene.
66
Various acid catalysts have been used such
as, for the most available, Amberlyst resins,
67
zeolites
68
and silicotungstic
acid.
69
A fine tuning of isobutene/glycerol ratio, reaction time,
temperature, stirring rate as well as acidic strength of the catalyst are
necessary to limit the side reactions, the dimerization or hydration of
isobutene as well as the dealkylation of the ethers. Isobutene/glycerol
ratios ranging from 1 to 5 have been commonly used and it is noteworthy
that even with more than 3 equivalents of isobutene, 1,2,3-tri-tert-
butoxyglycerol has never been obtained as sole product. Reaction tem-
peratures are commonly ranging between 60 and 80 1C. The use of higher
temperatures is limited by a fast oligomerization (including dimer-
ization) of isobutene. Furthermore isobutene is only slightly soluble in
glycerol thus limiting the contact between the two reactants. This mass
transfer limitation observed for low stirring rates results in lower yields in
ethers and larger proportions of butylene oligomers. High conversions of
glycerol are however obtained and converted in a mixture of glycerol
ethers (Scheme 23).
O
OH
O
O
O
O
OH
HO
OH
OH
+
+
O
Amberlyst-15
20 °C; DMC
OH
HO
OH
O
OH
O
O
Scheme 23 Synthesis of glycerol ethers from an hydroalkoxylation reaction.
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