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rates of 2,5-AHMA and 1,4-AHMA increased with an increase of dehydration pathway from mannitol to 1,4-AHMA was not
reaction temperature and the 2,5-AHMA yield was higher than dominant; thus, the yield of isomannide (ca. 20%) was not so
the 1,4-AHMA yield during the mannitol dehydration. The high as that of isosorbide.
1,4-AHMA yield had maximum and decreased with reaction
time; on the other hand, the 2,5-AHMA yield was almost during the mannitol dehydration in high-temperature liquid
constant or started to decrease slightly at 573 K. Isomannide water. The activation energies for rate constants (k1M, k2M, k3M
Fig. S1† shows the Arrhenius plots of the rate constants
,
was produced later than the formation of 1,4-AHMA because k4M, and k6M) were shown in Table 1. The activation energies of
isomannide “1,4-3,6-dianhydromannitol” was formed by the k1M and k2M (mannitol to 1,4-AHMA and 2,5-AHMA, respec-
stepwise dehydration of 1,4-AHMA (Scheme 1). We have suc- tively) were 148 and 153 kJ molÀ1, which were almost the same
ceeded in mannitol dehydration in high-temperature liquid to each other. On the other hand, the activation energy of k1S
water without adding any acid catalysts, producing 2,5-AHMA, (sorbitol to 1,4-AHSO) was 127 kJ molÀ1, which was much lower
1,4-AHMA and isomannide.
Dehydration reactions of mannitol and sorbitol in high- (Table S2†).
than that of k2S (166 kJ molÀ1) (sorbitol to 2,5-AHSO)
temperature liquid water proceeded via the sequence of steps
Sorbitol and mannitol are epimers, where their differences
in Scheme 1 and 2. The reverse reaction did not proceed as are only in the stereochemistry of hydroxyl group and hydrogen
conrmed by the reaction behavior of 1,4-AHSO, isosorbide, at the C-2 position; however, the major products of mono-
and isomannide in high-temperature liquid water. The amount molecular dehydration from sorbitol and mannitol were
of the formed isosorbide decreased in high-temperature liquid different from each other. The following reason can be given as
water (k5S step in Scheme 2), as shown in Fig. 2(b); on the other a probable dehydration mechanism. The hydroxyl groups at the
hand, the amount of the formed isomannide did not decrease C-2 and C-3 positions in sorbitol are trans-form during the
largely (k5M was treated as zero in Scheme 1). The concentra- process of intramolecular dehydration from sorbitol to
tions of the reactant and products in mannitol dehydration 1,4-AHSO and the major product of monomolecular dehydra-
were represented as eqn (6)–(10) (ESI†) using the rate constants tion was 1,4-AHSO, indicating that 1,4-AHSO are the favorable
(k1M, k2M, k3M, k4M, k5M, and k6M in Scheme 1). The rate product without a structural hindrance of hydroxyl groups at
constants (k1M, k2M, k3M, k4M, and k6M) were estimated (Table 1) the C-2 and C-3 positions. On the other hand, the hydroxyl
using linear regression analyses with minimization of residuals groups at the C-2 and C-3 positions in mannitol are cis-form
for the data of mannitol, 2,5-AHMA, 1,4-AHMA, 1,5-AHMA, and during the process from mannitol to 1,4-AHMA; thus, the
isomannide yields. The rate constant of k2M (mannitol to 2,5- structural hindrance causes a difficulty in reaction pathway to
AHMA) was the largest, compared with the other rate constants 1,4-AHMA and the 2,5-AHMA can be obtained as a major
(k1M, k3M, k4M, and k6M), indicating that the rst dehydration product. To conrm this dehydration mechanism, the arabitol
step of mannitol to 2,5-AHMA proceeded faster than the other dehydration (Scheme S1†) in high-temperature liquid water was
dehydration steps. The nal yields of 2,5-AHMA could be pre- carried out. The intramolecular dehydration of arabitol
dicted from the constant of (k2M/(k1M + k2M + k3M + k6M)) in Table provides
1,4-anhydroarabitol, 2,5-anhydroarabitol,
and
S1.† The obtained yields of 2,5-AHMA (Fig. 1) could be repro- 1,5-anhydroarabitol (less than 5%). The yield of 1,4-anhy-
duced by the calculated prediction (37–44%) at each tempera- droarabitol was always more than twice larger than that of
ture and that of 1,5-AHMA was also reproduced by the 2,5-anhydroarabitol (Fig. S2†). The difference between 1,4-
calculation (4.0–6.8%) of (k3M/(k1M + k2M + k3M + k6M)) (Table anhydroarabitol and 2,5-anhydroarabitol is only the conforma-
S1†). We reported the kinetic parameter of sorbitol dehydration tion of cis- and trans-form in the hydroxyl groups of cyclic
in high-temperature liquid water in Scheme 2 and Table S2.†10 ethers. The favorable product of 1,4-anhydroarabitol from the
In the case of sorbitol dehydration, the yield of 2,5-AHSO was arabitol dehydration has trans-form, indicating that the
less than 20% (Fig. 2), which was also consistent with the hydroxyl groups at the C-2 and C-3 positions in sugar alcohols
calculated 2,5-AHSO yield (k2S/(k1S + k2S + k3S + k6S)) (Table S1†). dehydration are important and trans-form provide a favorable
The rate constant of k1S (sorbitol to 1,4-AHSO) was the largest; product. The cis-form of hydroxyl groups at the C-2 and C-3
thus, high-yield isosorbide (yield ca. 60%) could be obtained by positions in mannitol to 1,4-AHMA causes
a difficulty
a stepwise dehydration of 1,4-AHSO. On the other hand, the in reaction pathway to 1,4-AHMA during the mannitol
dehydration.
In summary, we have succeeded in the intramolecular
dehydration of mannitol in high-temperature liquid water
without adding any hazardous acid catalysts and compared
mannitol dehydration behavior with that of sorbitol. We
found that 2,5-anhydromannitol and 1,4-anhydromannitol
were major products from the mannitol monomolecular
dehydration in contrast with the only major product,
1,4-anhydrosorbitol, from the sorbitol monomolecular
dehydration and that the yield of isomannide was lower than
that of isosorbide.
Table 1 Kinetic parameters for dehydration reactions of mannitol
(initial mannitol concentration: 0.5 mol dmÀ3
)
Ta (K)
523
548
560
573
Eab (kJ molÀ1
)
k1M (mol hÀ1
k2M (mol hÀ1
k3M (mol hÀ1
k4M (mol hÀ1
k6M (mol hÀ1
)
)
)
)
)
0.023
0.028
0.0044
0.0093
0.0097
0.12
0.15
0.020
0.053
0.077
0.22
0.31
0.038
0.12
0.18
0.44
0.60
0.065
0.17
0.51
148
153
135
151
196
a
Reaction temperature. b Activation energy.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 45575–45578 | 45577