Intramolecular dehydration of biomass-derived sugar alcohols in high-temperature water

Article abstract of DOI:10.1039/c6cp06831f

The intramolecular dehydration of biomass-derived sugar alcohols d-sorbitol, d-mannitol, galactitol, xylitol, ribitol, l-arabitol, erythritol, l-threitol, and dl-threitol was investigated in high-temperature water at 523-573 K without the addition of any acid catalysts. d-Sorbitol and d-mannitol were dehydrated into isosorbide and isomannide, respectively, as dianhydrohexitol products. Galactitol was dehydrated into anhydrogalactitols; however, the anhydrogalactitols could not be dehydrated into dianhydrogalactitol products because of the orientation of the hydroxyl groups at the C-3 and C-6 positions. Pentitols such as xylitol, ribitol, and l-arabitol were dehydrated into anhydropentitols. The dehydration rates of the pentitols containing hydroxyl groups in the trans form, which remained as hydroxyl groups in the product tetrahydrofuran, were larger than those containing hydroxyl groups in the cis form because of the structural hindrance caused by the hydroxyl groups in the cis form during the dehydration process. In the case of the tetritols, the dehydration of erythritol was slower than that of threitol, which could also be explained by the structural hindrance of the hydroxyl groups. The dehydration of l-threitol was faster than that of dl-threitol, which implies that molecular clusters were formed by hydrogen bonding between the sugar alcohols in water, which could be an important factor that affects the dehydration process.

Full text of DOI:10.1039/c6cp06831f

Showcasing research from the Group of Dr Aritomo Yamaguchi
at the National Institute of Advanced Industrial Science and
Technology (AIST), Japan
As featured in:
Intramolecular dehydration of biomass-derived sugar alcohols in
high-temperature water
This paper reports the intramolecular dehydration of sugar
alcohols D-sorbitol, D-mannitol, galactitol, xylitol, ribitol, L-arabitol,
erythritol, L-threitol, and DL-threitol in high-temperature water
without adding any hazardous acid catalysts. Kinetic parameters
of the dehydration reactions were determined and the factors
that contribute to the selectivity of the dehydration products
were discussed. This work provides fundamental insight into the
dehydration mechanism in high-temperature water and will be
meaningful for green chemical processes.
See Aritomo Yamaguchi et al.,
Phys. Chem. Chem. Phys.,
2017, 19, 2714.
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Intramolecular dehydration of biomass-derived
sugar alcohols in high-temperature water†
Cite this: Phys.Chem.Chem.Phys.,
2017, 19, 2714
Aritomo Yamaguchi,*ab Natsumi Muramatsu,a Naoki Mimura,a Masayuki Shiraiac
and Osamu Satoa
The intramolecular dehydration of biomass-derived sugar alcohols D-sorbitol, D-mannitol, galactitol,
xylitol, ribitol, L-arabitol, erythritol, L-threitol, and DL-threitol was investigated in high-temperature water
at 523–573 K without the addition of any acid catalysts. D-Sorbitol and D-mannitol were dehydrated into
isosorbide and isomannide, respectively, as dianhydrohexitol products. Galactitol was dehydrated into
anhydrogalactitols; however, the anhydrogalactitols could not be dehydrated into dianhydrogalactitol
products because of the orientation of the hydroxyl groups at the C-3 and C-6 positions. Pentitols such
as xylitol, ribitol, and L-arabitol were dehydrated into anhydropentitols. The dehydration rates of the
pentitols containing hydroxyl groups in the trans form, which remained as hydroxyl groups in the product
tetrahydrofuran, were larger than those containing hydroxyl groups in the cis form because of the
structural hindrance caused by the hydroxyl groups in the cis form during the dehydration process. In the
case of the tetritols, the dehydration of erythritol was slower than that of threitol, which could also be
explained by the structural hindrance of the hydroxyl groups. The dehydration of L-threitol was faster than
that of DL-threitol, which implies that molecular clusters were formed by hydrogen bonding between the
sugar alcohols in water, which could be an important factor that affects the dehydration process.
Received 6th October 2016,
Accepted 8th November 2016
DOI: 10.1039/c6cp06831f
www.rsc.org/pccp
lignocellulosic biomass could be directly converted to sugar
alcohols such as sorbitol, mannitol, and xylitol without deligni-
1. Introduction
The production of chemicals from renewable lignocellulosic fication of the biomass.15,16
biomass has attracted much attention because of its potential
The dehydration of hexitols has attracted attention as a
for creating a sustainable society.1–3 Cellulose and hemicellulose means to provide valuable chemicals from biomass-derived
are polysaccharides that are present in lignocellulosic biomass. sugar alcohols.17–20 Isosorbide and isomannide, which can be
They can be hydrolyzed into sugars such as glucose and xylose obtained from the dehydration of sorbitol and mannitol, have
and then hydrogenated into sugar alcohols. Therefore, sugar a rigid structure with two hydroxyl groups; thus, they are
alcohols such as sorbitol, xylitol, and erythritol are chemicals promising monomers for the preparation and enhancement
that can be derived from lignocellulosic biomass.4–6 Sorbitol and of the heat resistance of polyesters and polycarbonates.21
xylitol are on the DOE list of top 10 platform chemicals5 and on Anhydrohexitol is also a key material for the production of fatty
the list of new top 10 chemicals derived from biomass.2 Fukuoka acid esters, which are used as naturally derived surfactants and
et al. reported that cellulose could be directly converted into nontoxic food additives.22,23 Some researchers have reported
sorbitol by using supported metal catalysts and hydrogen.7 The the dehydration of sorbitol with inorganic acid catalysts such as
conversion of cellulose into sorbitol has been further developed sulfuric acid and hydrochloric acid at 377–408 K24,25 or with
by other research groups.8–13 Hemicellulose has also been zeolite catalysts.20,26 High-temperature water has attracted
directly converted into sugar alcohols by using supported metal much attention as a promising reaction medium for acid-
catalysts and hydrogen through hydrolysis and hydrogenation catalyzed reactions.27–29 The self-ionization of water at a high
reactions.14 We reported that cellulose and hemicellulose in temperature (ca. 523–573 K) is enhanced more than that at
ambient temperature, leading to high concentrations of protons
a Research Institute for Chemical Process Technology, National Institute of
Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino,
Sendai 983-8551, Japan. E-mail: a.yamaguchi@aist.go.jp; Fax: +81-22-237-5224
b JST, PRESTO, 4-2-1 Nigatake, Miyagino, Sendai 983-8551, Japan
and hydroxide ions in high-temperature water.30 We have
reported improved green chemistry methods for the dehydration
of sorbitol and mannitol in high-temperature water without the
addition of any acid catalysts.17,18 It is also important to under-
stand the chemistry of the intramolecular dehydration of other
c Department of Chemistry and Biological Sciences, Faculty of Science and
Engineering, Iwate University, Ueda 4-3-5, Morioka, Iwate 020-8551, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp06831f biomass-derived sugar alcohols such as pentitol and tetritol
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because the resulting products, anhydropentitol and anhydro- products was taken out of the reactor with distilled water, and
tetritol, are important chemicals31 for use as surfactants, cosmetics, the solids were separated by filtration.
plastic monomers,32 and raw materials for medicines.33
The quantitative analysis of the unreacted reactant and the
In this manuscript, we report the intramolecular dehydra- liquid products was conducted using high-performance liquid
tion of biomass-derived sugar alcohols D-sorbitol, D-mannitol, chromatography (HPLC, Shimadzu) equipped with a SUGAR
galactitol, xylitol, ribitol, L-arabitol, erythritol, L-threitol, and SC1211 column (Shodex) with a refractive index detector (Shimadzu,
DL-threitol in high-temperature water without the addition of RID-10A) and a UV-Vis detector (Shimadzu, SPD-20AV). The
any hazardous acid catalysts; furthermore, we investigate the products were identified by comparing the retention times with
chemistry of this process. We estimate the kinetic parameters those of standard materials: 1,4-anhydro-D-sorbitol (Toronto
of the dehydration reactions and discuss the factors that Research Chemicals, Inc.), 1,5-anhydro-D-sorbitol (Carbosynth,
contribute to the selectivity of the dehydration products. We Ltd), 2,5-anhydro-D-sorbitol (Toronto Research Chemicals, Inc.),
propose that the conformation of the hydroxyl groups in the isosorbide (Alfa Aesar), 1,4-anhydro-D-mannitol (Carbosynth, Ltd),
tetrahydrofuran ring of the product affects the dehydration 1,5-anhydro-D-mannitol (Carbosynth Ltd), 2,5-anhydro-D-mannitol
rates. The results obtained from the dehydration of L-threitol (Carbosynth, Ltd), isomannide (Tokyo Chemical Industries Co.,
and DL-threitol indicate that the dehydration rates are also Ltd), 1,4-anhydro-D-xylitol (Tokyo Chemical Industries Co., Ltd),
influenced by the formation of clusters of sugar alcohols in and 1,4-anhydro-D-erythritol (Carbosynth, Ltd). The HPLC peaks
water via hydrogen bonding.
that could not be identified by comparison with the standard
materials were identified by collecting the appropriate fractions
using a fraction collector, removing the solvent using a rotary
evaporator, and performing 1H and 13C nuclear magnetic
resonance (NMR) spectroscopy (Bruker AVANCE III HD 400 MHz)
(Fig. S1, ESI†). The products and rate constant for each step are
shown in Schemes 1–3 and Schemes S1, S2 in the ESI.† The
characteristic properties of the dehydrated product data were
previously reported in the literature.24,34–39
2. Experimental
D-Sorbitol, D-mannitol, galactitol, xylitol, ribitol, and erythritol
were purchased from Wako Pure Chemical Industries, Ltd;
L-arabitol from Tokyo Chemical Industries Co., Ltd; L-threitol
from Sigma-Aldrich Co., LLC.; and DL-threitol from Santa Cruz
Biotechnology, Inc. All sugar alcohols were used without any
further purification.
The dehydration of sugar alcohols was performed in a batch
reactor (inner volume: 6 cm3) made of a stainless steel 316 tube.17,18
3. Results and discussion
3.1. Dehydration of six-carbon sugar alcohols (hexitols)
An aqueous solution of the sugar alcohol (3 cm3, 0.5 mol dmꢀ3
)
was loaded into the reactor and then purged with argon gas to The dehydration reactions of D-sorbitol and D-mannitol in high-
remove the air. For the dehydration of galactitol, 3 cm3 of temperature water have been described in detail in previous
0.1 mol dmꢀ3 was used because of its low solubility in water. papers.17,18 The reaction schemes are shown in Schemes S1 and S2
The reactor was submerged into a molten-salt bath at the (ESI†), and the kinetic parameters for the dehydration reactions
desired reaction temperature for a given reaction time and are summarized in Tables S1 and S2 (ESI†). Briefly, 1-4-AHSO
then submerged into a water bath to cool quickly to ambient and 2-5-AHSO were produced by monomolecular dehydration of
temperature after the reaction. A mixture of the reactant and D-sorbitol in high-temperature water at 523–573 K without any
Scheme 1 The reaction pathways for the dehydration of galactitol.
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Scheme 2 The reaction pathways for the dehydration of (a) xylitol, (b) ribitol, and (c) L-arabitol.
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Scheme 3 The reaction pathways for the dehydration of (a) erythritol and (b) threitol.
catalysts; the formation rate of 1-4-AHSO was faster than that of Further dehydration of 1-4-AHGL did not proceed in high-
the other products formed by monomolecular dehydration. temperature water at 523–573 K because the hydroxyl groups at
Isosorbide was produced by dehydration of 1-4-AHSO. The major the C-3 and C-6 positions were oriented in opposite directions
products from the monomolecular dehydration of D-mannitol across the tetrahydrofuran ring in the molecular structure of
were 2-5-AHMA and 1-4-AHMA, in contrast to the one major 1-4-AHGL (Fig. S3, ESI†).35,38 In the cases of 1-4-AHSO and
product, 1-4-AHSO, obtained from the monomolecular dehydra- 1-4-AHMA, the hydroxyl groups at the C-3 and C-6 positions
tion of D-sorbitol. Isomannide was produced by the dehydration are oriented in the same direction (Fig. S3, ESI†), thus 1-4-AHSO
of 1-4-AHMA; however, the yield of isomannide was lower than and 1-4-AHMA could be converted into diols by dehydration
that of isosorbide.
between these hydroxyl groups. To understand the kinetics of
We have previously reported the intramolecular dehydration the galactitol dehydration, the rate constants (k1G, k2G, k3G, k5G
,
of (2R,5R)-(ꢀ)-2,5-hexanediol (2R,5R-HDO) into 2,5-dimethyltetra- and k6G in Scheme 1) were estimated (Table 1) using linear
hydrofuran (2,5-DMTHF) in high-temperature water to under- regression analyses by minimizing the residuals of the data
stand the reaction mechanism of polyalcohol dehydration.29 (eqn (S5)–(S8) in the ESI†). The activation energies for the rate
The cis selectivity of 2,5-DMTHF in the products was high, constants (k1G, k2G, and k6G) were evaluated from Arrhenius plots
indicating that the polyalcohol dehydration proceeded mainly (Fig. S4, ESI†) and are shown in Table 1. The rate constant k1G
via an SN2 substitution process. First, the hydroxyl group at the (galactitol to 1-4-AHGL) was the largest, indicating that the
C-x position is protonated, and then the oxygen atom at the C-y dehydration step of galactitol to 1-4-AHGL proceeded faster than
position attacks the C-x carbon and a water molecule is elimi- the other steps.
nated simultaneously, resulting in the production of a cyclic ether
The selectivities of the 1-4-form and the 2-5-form from the
(represented as x-y-AHzz, where zz represents the following two monomolecular dehydration of six-carbon sugar alcohols are
k1i
k2i
letter codes SO = sorbitol, GL = galactitol, etc.).
represented as
and
The dehydration of galactitol (dulcitol) was performed
at 523–573 K in high-temperature water without any acid catalysts.
The dehydration of galactitol proceeded in high-temperature water
at 523 K to yield 1-4-AHGL and 2-5-AHGL (Scheme 1 and Fig. S2a,
ESI†). The possible product 1-5-AHGL was not detected. The
formation rates of 1-4-AHGL and 2-5-AHGL increased with
increasing reaction temperature (Fig. 1a, b and Fig. S2a, b, ESI†).
k1i þ k2i þ k3i þ k6i
k1i þ k2i þ k3i þ k6i
(i = S, M, and G) (Table S3, ESI†), respectively. Table S3 (ESI†)
clearly shows that the selectivities of the 1-4-form were much
higher than those of the 2-5-form in the case of sorbitol and
galactitol dehydration. Conversely, the selectivity of the
2-5-form was higher than that of the 1-4-form in the case of
mannitol dehydration. The probable dehydration mechanism
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galactitol; thus, the major products from monomolecular
dehydration were 1-4-AHSO and 1-4-AHGL, indicating that the
1–4-form is the favorable product because of the absence of
structural hindrance from the hydroxyl groups at the C-2 and
C-3 positions. The selectivities of 2-5-AHSO (0.091–0.19) were
slightly higher than those of 2-5-AHGL (0.077–0.11) (Table S3,
ESI†). The hydroxyl groups at the C-3 and C-4 positions in
galactitol, which remain as hydroxyl groups in the tetrahydro-
furan ring of the product, are in the cis form during the
conversion process from galactitol to 2-5-AHGL; thus, structural
hindrance obstructs the reaction pathway to 2-5-AHGL, and the
selectivity of 2-5-AHGL is lower than that of 2-5-AHSO.
3.2. Dehydration of five-carbon sugar alcohols (pentitols)
The dehydration of xylitol, ribitol, and L-arabitol was investi-
gated in high-temperature water at 523–573 K without any
catalysts (Scheme 2). In the case of xylitol dehydration,
1-4-AHXY was the only product (Fig. 2a and Fig. S5, ESI†),
indicating that the oxygen atom at the C-4 position attacks the
carbon atom (C-1) that has a protonated hydroxyl group. Xylitol
is a meso compound (Scheme 2a), therefore 5-2-AHXY can be
obtained via an equivalent reaction to form 1-4-AHXY. The
measured amount of 1-4-AHXY reported in this manuscript
includes 5-2-AHXY, which is an enantiomer of 1-4-AHXY and
could not be separated using HPLC. Xylitol was not dehydrated
into 4-1-AHXY (or 2-5-AHXY) (Fig. 2a and Fig. S5, ESI†), indicat-
ing that the oxygen atom at the terminal carbons (C-1 and C-5)
did not attack the carbon atom (C-4 and C-2).
Ribitol was dehydrated into 1-4-AHRB (or 5-2-AHRB) and
4-1-AHRB (or 2-5-AHRB) in high-temperature water at 523–573 K
without any catalysts (Scheme 2b, Fig. 2b, and Fig. S6, ESI†).
Ribitol is also a meso compound and the main dehydration
product was 1-4-AHRB, indicating that the oxygen atom at the
C-4 or C-2 position attacked the terminal carbon atom (C-1 or
C-5, respectively) that has a protonated hydroxyl group. The rate
constants for the dehydration of xylitol and ribitol were esti-
mated using the rate constants (Scheme 2a and b) and the rate
equations (eqn (S13)–(S16) and (S21)–(S24)) and are summarized
in Tables 2 and 3, respectively. The rate constant for the
conversion of xylitol into 1-4-AHXY, k1X, was slightly larger than
that for the conversion of ribitol into 1-4-AHRB, k1R. The reason
for the difference in the rate constants is similar to that for the
dehydration of six-carbon sugar alcohols: the hydroxyl groups at
the C-2 and C-3 positions in xylitol, which remain as hydroxyl
groups in the product tetrahydrofuran, are in the trans form
during the conversion process from xylitol to 1-4-AHXY, and the
hydroxyl groups at the C-2 and C-3 positions in ribitol are in the
cis form during the conversion process from ribitol to 1-4-AHRB;
Fig. 1 Yields of 1-4-AHGL (’) and 2-5-AHGL (m), and unreacted galactitol
(.), as a function of elapsed time for the galactitol dehydration reactions at
(a) 548 and (b) 573 K in water (initial galactitol concentration: 0.1 mol dmꢀ3).
The lines show the best fits of the obtained data to the equations in the ESI†
with the kinetic parameters in Table 1.
Table 1 Kinetic parameters for the dehydration reactions of galactitol
(initial galactitol concentration: 0.1 mol dmꢀ3
)
Reaction
temperature (K) 523
Activation energy
548
560
573
(kJ molꢀ1
)
k1G (mol hꢀ1
k2G (mol hꢀ1
k5G (mol hꢀ1
k6G (mol hꢀ1
)
)
)
)
0.049
0.23
0.50
0.84
144
133
—
0.0056 0.034 0.075 0.14
—
0.017
—
0.022 0.062
0.075 0.13 0.24
163
k3G = 0.
is as follows. The hydroxyl groups at the C-2 and C-3 positions therefore, the structural hindrance during the dehydration of
in mannitol, which remain as hydroxyl groups in the product ribitol is larger than that during the dehydration of xylitol.
tetrahydrofuran ring, are in the cis form during the conversion
In the case of the dehydration of L-arabitol, 1-4-AHAR,
process from mannitol to 1-4-AHMA; thus, structural hindrance 4-1-AHAR, 5-2-AHAR, and 2-5-AHAR were produced (Scheme 2c,
obstructs the reaction pathway to 1-4-AHMA, and the selectivity Fig. 2c, and Fig. S7, ESI†). The molecules of 1-4-AHAR, 4-1-AHAR,
of 2-5-AHMA was relatively high. Conversely, the hydroxyl 5-2-AHAR, and 2-5-AHAR were the same as 4-1-AHXY, 1-4-AHXY,
groups at the C-2 and C-3 positions in sorbitol and galactitol 4-1-AHRB, and 1-4-AHRB, respectively (Scheme 2). The yields of
are in the trans form during the dehydration of sorbitol and 1-4-AHAR, 4-1-AHAR, 5-2-AHAR, and 2-5-AHAR from the dehydration
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Fig. 2 (a) Yield of 1-4-AHXY (’) and unreacted xylitol (.), (b) yields of 1-4-AHRB (’) and 4-1-AHRB (m), and unreacted ribitol (.), and (c) yields of the
sum of 1-4-AHAR and 4-1-AHAR (’) and the sum of 2-5-AHAR and 5-2-AHAR (m), and unreacted L-arabitol (.) as a function of elapsed time for the
xylitol, ribitol, and L-arabitol dehydration reactions at 548 K in water (initial concentration: 0.5 mol dmꢀ3). The lines show the best fits of the obtained data
to the equations in the ESI† with the kinetic parameters in Tables 2–4.
Table 2 Kinetic parameters for the dehydration reactions of xylitol (initial
Table 3 Kinetic parameters for the dehydration reactions of ribitol (initial
ribitol concentration: 0.5 mol dmꢀ3
xylitol concentration: 0.5 mol dmꢀ3
)
)
Reaction
temperature (K) 523
Activation energy Reaction
Activation energy
548
560
573
2.8
(kJ molꢀ1
)
temperature (K) 523
548
560
573
(kJ molꢀ1
)
k1X (mol hꢀ1
k4X (mol hꢀ1
k5X (mol hꢀ1
)
)
)
0.12
0.51
0.64
145
k1R (mol hꢀ1
k2R (mol hꢀ1
k4R (mol hꢀ1
k5R (mol hꢀ1
)
)
)
)
0.12
0.010
0.61
0.065
1.3
0.15
1.9
0.20
142
154
0.0035 0.0041 0.008 0.077 170
0.015 0.077 0.12 0.56 134
0.00059 0.0020 0.0073 0.022 180
0.017 0.096 0.24 0.25 144
k2X = 0, k3X = 0.
k3R = 0.
of L-arabitol at 548 K are plotted in Fig. S8 (ESI†); 1-4-AHAR was the
main product. From Fig. 2c, the sum of the yields of 1-4-AHAR and the rate equations (eqn (S29)–(S32)) and are summarized in Table 4.
4-1-AHAR, in which the hydroxyl groups in the tetrahydrofuran ring The rate constant for the conversion of L-arabitol into 1-4-AHAR and
of the product were in the trans form, was higher than that of 4-1-AHAR, k1A, was much lower than those for the dehydration of
5-2-AHAR and 2-5-AHAR, in which they were in the cis form. This can xylitol and ribitol, k1X and k1R (Tables 2 and 3), which is consistent
be explained by the structural hindrance of the hydroxyl groups and with the result that the consumption of L-arabitol (Fig. 2c) was much
is consistent with the selectivity observed during the dehydration of slower than that of xylitol and ribitol (Fig. 2a and b) at 548 K in
six-carbon sugar alcohols. The rate constants for the dehydration of water. The reason for the difference in the rate constants between
L-arabitol were estimated using the rate constants in Scheme 2c and L-arabitol, xylitol, and ribitol is still unclear. One possible reason is
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Table 4 Kinetic parameters for the dehydration reactions of L-arabitol
3.3. Dehydration of four-carbon sugar alcohols (tetritols)
(initial L-arabitol concentration: 0.5 mol dmꢀ3
)
The dehydration of four-carbon sugar alcohols, such as erythritol,
L-threitol, and DL-threitol, was investigated in high-temperature
water at 523–573 K without any catalysts (Scheme 3). Erythritol was
dehydrated into 1-4-AHER, which is cis-3,4-tetrahydrofurandiol
(Scheme 3a, Fig. 3a, and Fig. S9, ESI†). Both L-threitol and
DL-threitol were dehydrated into 1-4-AHTH, which is trans-3,4-
tetrahydrofurandiol (Scheme 3b, Fig. 3b, c and Fig. S10, S11,
ESI†). The rate constants for the dehydration of erythritol,
L-threitol, and DL-threitol were estimated using the rate con-
Reaction
temperature (K) 523
Activation energy
548
560
573
(kJ molꢀ1
)
k1A (mol hꢀ1
k2A (mol hꢀ1
k5A (mol hꢀ1
)
)
)
0.0084 0.047
0.0039 0.023
0.0024 0.0039 0.0059 0.0083
0.078
0.041
0.15
0.077
144
149
61
k3A = 0, k4A = 0.
that molecular clusters of sugar alcohols are formed in water via stants in Scheme 3 and the rate equations (eqn (S35), (S36),
hydrogen bonding40,41 and affect the dehydration rate. If the hydro- (S39), (S40), (S43), and (S44)) and are summarized in Tables 5
gen bonds in the molecular clusters are strong, the dehydration rate and 6, respectively. The order of the rate constants for the
is slow because of the energy required to cleave the hydrogen bonds. dehydration was L-threitol (k1TL) 4 DL-threitol (k1TDL) 4 erythritol
The formation of clusters with strong hydrogen bonding could (k1E). The erythritol dehydration is slower than the threitol
explain the lower rate constant of L-arabitol. We discuss the for- dehydration, which can be explained by the structural hindrance
mation of molecular clusters in water in Section 3.3.
of the hydroxyl groups at the C-2 and C-3 positions. This is
Fig. 3 (a) Yield of 1-4-AHER (’) and unreacted erythritol (.), (b) yield of 1-4-AHTH (’) and unreacted L-threitol (.), and (c) yield of 1-4-AHTH (’)
and unreacted DL-threitol (.) as a function of elapsed time for the erythritol, L-threitol, and DL-threitol dehydration reactions at 548 K in water
(initial concentration: 0.5 mol dmꢀ3). The lines show the best fits of the obtained data to the equations in the ESI† with the kinetic parameters in
Tables 5 and 6.
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Table 5 Kinetic parameters for the dehydration reactions of erythritol
not be dehydrated into dianhydrogalactitol products because
the hydroxyl groups at the C-3 and C-6 positions were oriented
in the opposite directions across the tetrahydrofuran ring.
Pentitols such as xylitol, ribitol, and L-arabitol were dehydrated
into several types of anhydropentitols. The dehydration rates of
the compounds containing hydroxyl groups in the trans form,
which remained as hydroxyl groups in the product tetrahydro-
furan, were larger than those containing hydroxyl groups in
the cis form because of the structural hindrance during the
dehydration process. In the case of tetritol dehydration, the
order of the rate constants for the dehydration was L-threitol
(k1TL) 4 DL-threitol (k1TDL) 4 erythritol (k1E). The dehydration
of erythritol was slower than the dehydration of threitol, which
could be explained by the structural hindrance of the hydroxyl
groups. The rate constant for the dehydration of L-threitol is
larger than that of DL-threitol, indicating that the dehydration
reaction of L-threitol or D-threitol proceeds with an interaction
with the other chiral molecule. We propose that molecular
clusters formed between the sugar alcohols in water via hydro-
gen bonding affect the effect the dehydration.
(initial erythritol concentration: 0.5 mol dmꢀ3
)
Reaction
temperature (K) 523
Activation energy
548
560
573
(kJ molꢀ1
)
k1E (mol hꢀ1
k2E (mol hꢀ1
k3E (mol hꢀ1
)
)
)
0.027
—
0.12
—
0.24
0.014 0.023
0.44
141
—
0.0045 0.020 0.043 0.097 152
Table 6 Kinetic parameters for the dehydration reactions of L-threitol and
DL-threitol (initial L-threitol or DL-threitol concentration: 0.5 mol dmꢀ3
)
Reaction
temperature (K) 523
Activation energy
548
560
573
(kJ molꢀ1
)
k1TL (mol hꢀ1
k2TL (mol hꢀ1
k3TL (mol hꢀ1
)
)
)
0.073
—
0.018
0.42
—
0.085 0.20
0.91
—
4.1
0.025
0.94
193
—
190
k1TDL (mol hꢀ1
k2TDL (mol hꢀ1
k3TDL (mol hꢀ1
)
)
)
0.036
—
0.16
—
0.29
—
0.80
0.011
150
—
165
0.0064 0.033 0.060 0.19
consistent with the results of the dehydration of six-carbon and
five-carbon sugar alcohols. The L-threitol dehydration is faster
than the DL-threitol dehydration. The physicochemical pro-
perties of enantiomers such as L-threitol and D-threitol should
be the same. If the dehydration reaction of L-threitol or
D-threitol proceeds without any interaction with the other
chiral molecules, the rate constants of L-threitol and DL-threitol
should be the same. However, the rate constant of L-threitol was
larger than that of DL-threitol, indicating that the dehydration
reaction of L-threitol or D-threitol proceeds with an interaction
with the other chiral molecule. We can explain this result by
considering the formation of molecular clusters between the
sugar alcohols in water via hydrogen bonding, which we have
discussed in Section 3.2. The DL-threitol dehydration is slower
than the L-threitol dehydration, which indicates that D-threitol
and L-threitol form clusters with each other with stronger
hydrogen bonds than the clusters formed by only D-threitol. In
other words, the interaction of the hydrogen bonds between a
‘‘right-hand’’ molecule and a ‘‘left-hand’’ molecule is larger than
that between ‘‘right-hand’’ molecules. We do not have any direct
evidence for the formation of these molecular clusters by hydro-
gen bonding; thus, computational simulation of this system
should be carried out to evaluate this possibility.
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