Kinetic effects induced by cellulose on water-catalyzed reactions. Hydrolysis of 2,4-dinitrophenyl cellulose xanthate and some sugar xanthate ester analogues

Article abstract of DOI:10.1139/v98-046

The hydrolysis of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) was studied in 10% v/v aqueous ethanol at 25°C and μ = 0.1 (KCl). The water- catalyzed hydrolysis showed that, as for p-nitrobenzyl cellulose xanthate, it occurs through two parallel reactions with rate constants k'(H2O) = 4.40 x 10^-3 s^-1 for the fast hydrolysis, and k''(H2O) = 6.90 x 10^-5 s^-1 for the slow hydrolysis. The entropy of activation of the fast hydrolysis was 0.7 ± 1.8 cal K^-1 mol^-1. External nucleophiles such as hydroxide and simple amines show simple first-order kinetics. The spontaneous hydrolysis of CelXDNP in acetone-water mixtures indicates that the fast reaction does not occur through water polymers and that for water molarity higher than 30 M there are no acetone molecules (or very few) in the highly ordered cybotactic region of cellulose. The spontaneous hydrolysis of methyl 4,6-O-benzylidene- α-D-glucopyranoside 3-(S-p-nitrobenzyl-xanthate) although is faster than the 6-isomer, it is slower than the fast hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB). Also ΔS(+) is highly negative (-41.0 cal K^-1 mol^-1), as it is for alkyl and sugar analogues. Only for the fast hydrolyses of CelXDNP and CelXNB is the entropy of activation almost zero. It is concluded that there is no neighbouring OH effect on the fast hydrolysis of cellulose xanthate esters. The hydrolysis of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) was studied in 10% v/v aqueous ethanol at 25°C and μ = 0.1 (KCl). The water-catalyzed hydrolysis showed that, as for p-nitrobenzyl cellulose xanthate, it occurs through two parallel reactions with rate constants k′_H2O = 4.40 × 10^-3 s^-1 for the fast hydrolysis, and k″_H2O = 6.90 × 10^-5 s^-1 for the slow hydrolysis. The entropy of activation of the fast hydrolysis was 0.7 ± 1.8 cal K^-1 mol^-1. External nucleophiles such as hydroxide and simple amines show simple first-order kinetics. The spontaneous hydrolysis of CelXDNP in acetone-water mixtures indicates that the fast reaction does not occur through water polymers and that for water molarity higher than 30 M there are no acetone molecules (or very few) in the highly ordered cybotactic region of cellulose. The spontaneous hydrolysis of methyl 4,6-O-benzylidene-α-D-glucopyranoside 3-(S-p-nitrobenzyl-xanthate) although is faster than the 6-isomer, it is slower than the fast hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB). Also ΔS^≠ is highly negative (-41.0 cal K^-1 mol^-1), as it is for alkyl and sugar analogues. Only for the fast hydrolyses of CelXDNP and CelXNB is the entropy of activation almost zero. It is concluded that there is no neighbouring OH effect on the fast hydrolysis of cellulose xanthate esters.

Full text of DOI:10.1139/v98-046

960
Kinetic effects induced by cellulose on
water-catalyzed reactions. Hydrolysis of
2,4-dinitrophenyl cellulose xanthate and some
sugar xanthate ester analogues
Eduardo Humeres, Luiz Fernando Sequinel, Mauricéa Nunes, Célia M.S.
Oliveira, and Patrick J. Barrie
Abstract: The hydrolysis of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) was studied in 10% v/v aqueous ethanol at 25°C
and µ = 0.1 (KCl). The water-catalyzed hydrolysis showed that, as for p-nitrobenzyl cellulose xanthate, it occurs through two
parallel reactions with rate constants k′H2O = 4.40 × 10–3 s–1 for the fast hydrolysis, and k′′H2O = 6.90 × 10–5 s–1 for the slow
hydrolysis. The entropy of activation of the fast hydrolysis was 0.7 ± 1.8 cal K–1 mol–1. External nucleophiles such as
hydroxide and simple amines show simple first-order kinetics. The spontaneous hydrolysis of CelXDNP in acetone–water
mixtures indicates that the fast reaction does not occur through water polymers and that for water molarity higher than 30 M
there are no acetone molecules (or very few) in the highly ordered cybotactic region of cellulose. The spontaneous hydrolysis
of methyl 4,6-O-benzylidene-α-D-glucopyranoside 3-(S-p-nitrobenzyl-xanthate) although is faster than the 6-isomer, it is
slower than the fast hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB). Also ∆S≠ is highly negative (–41.0 cal K–1
mol–1), as it is for alkyl and sugar analogues. Only for the fast hydrolyses of CelXDNP and CelXNB is the entropy of
activation almost zero. It is concluded that there is no neighbouring OH effect on the fast hydrolysis of cellulose xanthate
esters.
Key words: hydrolysis, water catalysis, cellulose xanthate esters, methyl glucose, xanthate esters, neighbouring OH effect.
Résumé : On a étudié l’hydrolyse du xanthate de 2,4-dinitrophénylcellulose (CelXDNP) en opérant à 25°C, en solutions
aqueuses d’éthanol à 10% v/v et à µ = 0,1 (KCl). L’hydrolyse catalysée par l’eau montre que, comme dans le cas du xanthate
de p-nitrobenzylcellulose, celle-ci se produit par le biais de deux réactions parallèles avec des constantes de vitesse de k′H2O
4,40 × 10-3 s-1 pour l’hydrolyse rapide et de k′′H2O = 6,90 × 10-5 s-1 pour l’hydrolyse lente. L’entropie d’activation de
l’hydrolyse rapide est égale à 0,7 ± 1,8 cal K-1 M-1. Les nucléophiles externes, tels l’hydroxyde et les amines simples,
=
réagissent avec une cinétique du premier ordre simple. L’hydrolyse spontanée du CelXDNP dans des mélanges eau–acétone
indique que la réaction rapide ne se produit pas avec des polymères-eau et que, à des molarités d’eau supérieures à 30 M, il
n’y a pas (ou très peu) de molécules d’acétone dans la région cybotactique bien organisée de la cellulose. L’hydrolyse
spontanée du 3-(S-p-nitrobenzylxanthate) de 4,6-O-benzylidène-α-D-glucopyranoside de méthyle, même si elle est plus rapide
que celle de son isomère en position 6, est plus lente que l’hydrolyse rapide du xanthate de la p-nitrobenzylcellulose
(CelXNB). Les ∆S≠ est très négatif (–41,0 cal K-1 mol-1), comme c’est le cas avec les analogues alkylés ou avec les sucres. Ce
n’est que dans le cas des hydrolyses rapides des CelXDNP et CelXNB que l’entropie d’activation est pratiquement égale à
zéro. On en conclut qu’il n’y a pas d’effet avoisinant de OH sur l’hydrolyse rapide des esters xanthiques de la cellulose.
Mots clés : hydrolyse, catalyse par l’eau, esters xanthiques de la cellulose, esters xanthiques de glucosides de méthyle, effet
avoisinant de OH.
[Traduit par la rédaction]
Introduction
Received December 9, 1997.
This paper is dedicated to Professor Erwin Buncel in
recognition of his important contribution to physical organic
The nonbonding interactions of carbohydrates with water de-
pend on their stereochemistry and have been the subject of
numerous studies (1). Kinetic medium effects induced by car-
bohydrates give some insight into the specificity of the stereo-
chemical interaction with water (2) and are important to the
understanding of their role in the sugar–protein recognition
involved in carbohydrate transport (3).
chemistry.
E. Humeres,1 L.F. Sequinel, M. Nunes, and C.M.S. Oliveira.
Departamento de Química, Universidade Federal de Santa
Catarina, Florianópolis, SC 88040-900, Brazil.
P.J. Barrie. Department of Chemistry, University College
London, 20 Gordon Street, London WC1H OAJ, U.K.
The study of the reactivity of a moiety covalently bound to
a polysaccharide system showed that the reaction of water was
much faster compared to small analogue molecules (4). The
water-catalyzed hydrolysis of p-nitrobenzyl cellulose xanthate
1
Author to whom correspondence may be addressed.
Telephone: 55 48 331 9219. Fax: 55 48 331 9711. E-mail:
humeres@mbox1.ufsc.br; humeres@qmc.ufsc.br
Can. J. Chem. 76: 960–965 (1998)
© 1998 NRC Canada

961
Humeres et al.
(CelXNB) occurred through a fast reaction and a slower par-
allel reaction. The faster hydrolysis was ascribed to the reac-
tion of the C-2 + C-3 isomers and the slower hydrolysis to the
C-6 isomer. The acceleration observed in the fast hydrolysis
was attributed to an entropic effect due to a well-oriented water
molecule, catalyzed by a second water molecule that acts as a
general base as shown by proton inventory. However, it was
also recognized that instead of a second water molecule, a
neighbouring OH was able to act as a general base. External
nucleophiles such as hydroxide ions and amines do not dis-
criminate between the isomers and react like small analogue
molecules (5).
This paper deals with the hydrolysis of 2,4-dinitrophenyl
cellulose xanthate (CelXDNP), a cellulose ester with a good
leaving group, in order to characterize the spontaneous reac-
tion in comparison to CelXNB (eq. [1]).
[1]
The hydrolysis of the 3- and 2,3-p-nitrobenzyl methyl glucose
xanthate esters was studied to determine if there was a neigh-
bouring OH group involved in the rate acceleration observed
in the spontaneous hydrolysis of CelXNB.
Experimental
All chemicals and solvents were of analytical grade and were
used as received, except the amines, which were previously
distilled.
The solid-state 13C NMR measurements were obtained us-
ing cross polarization (CP), magic angle spinning (MAS), and
total sideband suppression (TOSS), and were performed using
a Bruker MSL-300 spectrometer. The NMR spectra in solution
were obtained with Bruker AC-200, WM-250, and AMX-400
equipment.
2,4-Dinitrophenyl cellulose xanthate (CelXDNP)
Commercial cotton tissue (2.2 g) was washed with 1 M HCl
for 1 h and then several times with water. The sample was
mechanically shaken in 100 mL of 2 M NaOH for 1 h. Half of
the basic solution was taken away, a solution of 30 mL of CS2
in 20 mL of p-dioxane was added, and the suspension was
shaken for 2 h. The cellulose xanthate was filtered and treated
with 0.1 M phosphate buffered solution at pH 8.0, then washed
with dried acetone, and allowed to react with a solution of 3 g
of 2,4-dinitro-1-fluorobenzene dissolved in 100 mL of dried
acetone for 72 h with mechanical shaking. The cellulose xan-
thate ester was filtered, washed rapidly with 0.1 M HCl, then
with acetone and finally with diethyl ether, and dried under
vacuum for 10 h at room temperature over P2O5. The degree of
substitution (DS, number of xanthate ester groups per 100 glu-
coanhydropyranose units) was 4.6 determined as described
previously (4), measuring the number of moles of 2,4-dini-
trothiophenol liberated in the ethylaminolysis, observed at
410 nm, using the extinction coefficient of 1.38 × 104 (6).
The CelXDNP was characterized from the 13C NMR spec-
tra with 12 h acquisition time, 2 ms contact time, 4 s recycle
delay between scans, using CP/MAS TOSS (Table 1).
The thiocarbonyl signal of CelXDNP appears at 195 ppm,
about 19 ppm at higher field than for the p-nitrobenzyl cellu-
lose xanthate (CelXNB) where there is a methylene bridge
between the xanthate group and the phenyl ring. All the signals
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Can. J. Chem. Vol. 76, 1998
Table 1. 13C chemical shifts of CelXDNP.a
Carbon Chemical shift (ppm)
Table 2. 13C chemical shifts (ppm) for some sugar xanthate esters.
Carbon
MGX-6a
MGX-3b
MGX-2,3b,c
1
2
3
4
5
6
7
a
b
c
d
e
f
99.9 (100.0)
72.4 (72.2)
74.7 (74.1)
70.6 (70.6)
73.5 (72.5)
69.6 (61.6)
100.2
72.0
99.9
(99.9)
(72.4)
(70.5)
(80.8)
(62.0)
(68.5)
(101.5)
(200)d
77.5
78.6
78.9
81.2
79.2
62.7
62.4
68.9
68.7
101.5
214.3
39.6
101.6
C-1
104.5b
89.2b
75.2b
65.4b
98.1b
213.6
211.7; 212.7
39.2; 39.4
143.5; 143.2
129.6; 129.9
123.7; 123.8
C-4
83.4c
73.0b
62.7c
40.0
C-2,-3,-5
C-6
71.8c
147.7
143.6
129.8
123.8
143.6
55.8
(144.4)e
(129.8)e
(125.5)e
130.4
124.1
147.7
147.27; 147.29 (146.8)e
g
h
i
55.7 (55.9)
55.4
136.5
129.3
128.3
126.0
(54.9)
136.8
129.2
128.3
126.1
(137.7)e
(129.2)e
(128.4)e
(125.5)e
j
C-a
C-b
C-c
C-d
C-e
C-f
195.0
k
140.6 (140)
129.0 (128)
129.0 (130)
155.7 (146)
122.4 (119)
155.7 (147)
a In CD2Cl2. Values in parentheses for methyl α-D-glucopyranoside (ref.
11).
b In CDCl3.
c Values in parentheses for methyl 4,6-O-benzylidene-α-D-glucopyranoside
unless indicated (ref. 10).
d For CS2 in solution.
C-g
e Calculated values (ref. 9).
a CP/MAS TOSS.
b From crystalline components.
c From noncrystalline components (refs. 7 and 8). Values in
25
λ
for C15H19NO8S2: C 44.4, N 3.4, H 4.7, S 15.8; found: C 43.1,
max 283 nm; [α]D +72.3 (c 0.41, chloroform). Anal. calcd.
parentheses are calculated (ref. 9).
N 3.4, H 4.8, S 15.5.
for the 2,4-dinitrophenyl ring in the 120–150 ppm region
agreed with the calculated values (9). Considering the area of
the signals due to the 2,4-dinitrophenyl ring with respect to
that of C-1, the DS was 5.1, reasonably close to that calculated
by ethylaminolysis.
After n-butylaminolysis of CelXDNP, the NMR spectrum
of n-butyl cellulose thioncarbamate showed that the signal at
195 ppm disappeared, producing a signal at 189.6 ppm as-
signed to the thioncarbamate group [O-C(S)-N] and the calcu-
lated signals for the n-butyl group at 45.1, 30.6, 20.6, and
14.5 ppm. This spectrum was identical to that obtained from
CelXNB after n-butylaminolysis.
13C NMR (Table 2); 1H NMR (400 MHz, CD2Cl2), δ: 8.13
(d, J = 6.7 Hz, 2H, Ce-H), 7.54 (d, J = 6.7 Hz, 2H, Cd-H), 5.08
(s, 1H, OH), 4.85 (dd, J = 11.7, 2.0 Hz, 1H, C6-H), 4.73 (d, J
= 3.6 Hz, 1H, C1-H, dd, J = 11.7, 6.0, 1H, C6-H′), 4.46 (s, 3H,
Cb-H2, H2O?), 4.1 (d, J = 7.2, 1H, OH), 3.89 (ddd, J = 9.7, 5.8,
1.9, 1H, C5-H), 3.73 (t, J = 9.2, 1H, C3-H), 3.52 (m, 1H, C2-H),
3.43 (ddd, J = 9.5, 9.5, 7.0, 1H, C4-H), 3.37 (s, 3H, OMe).
Couplings between C6-H and C5-H are not equal (2.0 and
6.0 Hz), showing that the side chain is in a preferred confor-
mation.
Methyl 4,6-O-benzylidene-α-D-glucopyranoside 3- and
2,3-(S-p-nitrobenzyl-xanthate) (MGX-3 and MGX-2,3)
Twenty five milliliters of CS2 and 10 mL of 5 N NaOH were
added to a solution of 10 g of methyl 4,6-O-benzylidene-α-D-
glucopyranoside in 25 mL of p-dioxane, and the reaction mix-
ture was stirred for 30 min at room temperature. Then it was
neutralized with acetic acid at 5°C. p-Nitrobenzyl bromide
(7.6 g) was slowly added, and the mixture was stirred for 5 h.
The layers were separated, and the organic layer was concen-
trated under vacuum to a solid that was extracted with diethyl
ether. After evaporation, the solid was recrystallized several
times with dried 2-propanol, giving white crystals of MGX-3:
Methyl-D-glucopyranoside 6-(S-p-nitrobenzyl xanthate)
(MGX-6)
In a solution of 20 g of methyl-α-D-glucopyranoside in 20 mL
of water, 4.5 g of sodium hydroxide was slowly added, and
when it was completely dissolved, 3.1 mL of carbon disulfide
was mixed dropwise with constant stirring for 3 h at room
temperature. The mixture was then neutralized with cooled
acetic acid. A solution of 11.02 g of p-nitrobenzyl bromide in
methylene chloride (20 mL) was added slowly while stirring,
and the mixture was allowed to react for 2 h. The solution was
concentrated under vacuum. The solid product was dissolved
in 90% v/v aqueous ethanol and allowed to crystallize. Upon
recrystallization in the same solvent, the white crystals were
dried under vacuum over phosphorus pentoxide. The product
gave positive test of carbohydrate, showing only one spot of
TLC on alumina with ethyl acetate (Rf 0.75); mp 70–71°C;
TLC, alumina, 2-propanol, Rf 0.74; mp 176°C; λmax 283;
22
[α]D
C22H23NO2S2: C 53.6, H 4.6, N 2.8, S 12.9; found: C 53.3, H
+ 51.9 (c 0.03, chloroform). Anal. calcd. for
4.6, N 2.9, S 13.0.
IR (KBr): 3448 cm–1 (sharp, C2-OH); 13C NMR (Table 2);
1H NMR (250 MHz, CDCl3), δ: 8.10 (d, J = 7.7 Hz, 2H, Ce-H),
© 1998 NRC Canada

963
Humeres et al.
Fig. 1. pH–rate profile of the hydrolysis of CelXDNP at 25°C, in
10% v/v aqueous ethanol, µ = 0.1 (KCl).
Table 3. Rate constant for the hydrolysis of some sugar xanthates
in 10% v/v aqueous ethanol at 25°C.a
Ester
kH2O, s–1
kOH, M–1 s–1b
9.25 × 10–2
CelXDNP
k′H2O
k′′H2O
4.40 × 10–3
6.90 × 10–5
3.85 × 10–6c
0.82 × 10–3e
1.4 × 10–5e
1.36 × 10–6c
1.01 × 10–7c
1.87 × 10–5b
3.27 × 10–4b
1.94 × 10–5b
EtXDNP
CelXNBd
5.22c
1.40 × 10–3b
k′H2O
k′′H2O
EXNB
4.16 × 10–2c
2.05c
43.10b
MGX-6
MGX-3
MGX-2,3
k1
k2
a µ = 0.1 (KCl).
b µ = 1.0 (KCl).
c 20% v/v aqueous methanol (ref. 11).
d Reference 5.
e µ = 0.6 (KCl).
methoxy or xanthate groups. The IR spectrum confirmed this,
showing a sharp OH band. The other proton couplings are
consistent with a rigid chair–chair conformation.
For the MGX-2,3, the assignment fits with the chair–chair
conformation.
Kinetics
The hydrolysis of CelXDNP was studied in a continous-flow
system, with mechanical stirring, under inert atmosphere, us-
ing deoxygenated water as described previously (5), following
the appearance of 2,4-dinitrothiophenol at 410 nm. The basic
hydrolysis was pseudo-first order. The spontaneous hydrolysis
of CelXDNP showed a biphasic plot of ln ∆A vs. time similar
to that observed for CelXNB, due to two parallel reactions with
rate constants k′H2O (fast) and k′′H2O (slow) (5).
In the range of pH 4–12 all the hydrolyses were carried out
at three different buffer concentrations, and the rate constants
were extrapolated to zero buffer concentration.
Hydrolyses of the methylglucose xanthates were followed
by the disappearance of the xanthate at 283 nm. The sponta-
neous hydrolysis of MGX-2,3 showed also a biphasic plot as
a consequence of the consecutive hydrolyses of the two xan-
thate groups with rate constants k1 and k2.
7.48 (d, J = 7.7 Hz, 2H, Cd-H), 7.35 (m, 5H, Ar-H), 6.31 (dd,
J = 9.7, 9.7 Hz, 1H, C3-H), 5.46 (s, 1H, C7-H), 4.86 (d, J = 3.9,
1H, C1-H), 4.47 (ab, J = 14.6 Hz, 2H, Cb-H), 4.32 (dd, J = 10.1,
4.5 Hz, 1H, C6-H), 3.91 (ddd, J = 9.9, 9.9, 4.3 Hz, 1H, C5-H),
3.85 (ddd, J = 11.6, 9.5, 3.7 Hz, 1H, C2-H), 3.76 (dd, J = 10.0,
10.0 Hz, 1H, C6-H′), 3.70 (dd, J = 9.5, 9.5 Hz, 1H, C4-H), 3.50
(s, 3H, OMe), 2.21 (d, J = 11.5 Hz, 1H, C2-OH).
The solid left after the extraction with diethyl ether was then
extracted with chloroform, concentrated, and crystallized in
chloroform–hexane, giving white crystals of MGX-2,3: TLC
on alumina producing one spot in chloroform:acetone (19:1),
Rf 0.76; mp 72°C; λmax 283 nm; [α]D25 +64.1 (c 0.07, CHCl3).
Anal. calcd. for C30H28N2O10S4: C 51.1, H 3.9, N 3.9, S 18.2;
found: C 51.0, H 3.9, N 3.7, S 18.5.
Activation parameters were calculated from the Eyring
equations at four temperatures.
13C NMR (Table 2); 1H NMR (400 MHz, CDCl3), δ: 8.18
(d, J = 8.8 Hz, 2H, Ce-H), 8.04 (d, J = 8.7 Hz, 2H, Ce′-H), 7.51
(d, J = 8.7 Hz, 2H, Cd-H), 7.46 (d, J = 8.7 Hz, 2H, Cd′-H), 7.38
(m, 5H, Ar-H), 6.70 (dd, J = 9.6, 9.6 Hz, 1H, C3-H), 5.90 (dd,
J = 9.8, 3.8 Hz, 1H, C2-H), 5.46 (s, 1H, C7-H), 5.10 (d, J =
3.7 Hz, 1H, C1-H), 4.46 (d, J = 14.6 Hz, 1H, Cb-H), 4.45 (d, J
= 15.0 Hz, 1H, Cb′-H), 4.37 (d, J = 15.0 Hz, 1H, Cb′-H′′), 4.34
(dd, J = 10.5, 5.0 Hz, 1H, C6-H), 4.17 (d, J = 14.6 Hz, 1H,
Cb-H′), 4.01 (dt, J = 9.9, 4.9 Hz, 1H, C5-H), 3.82 (dd, J = 10.4,
4.1 Hz, 1H, C4-H), 3.80 (dd, J = 10.2, 4.9 Hz, 1H, C6-H′), 3.38
(s, 3H, OMe).
Results
Hydrolysis of sugar xanthates
The spontaneous hydrolysis of CelXDNP occurs through two
parallel reactions with rate constants k′H2O for the fast hydroly-
sis and k′′H2O for the slow hydrolysis. At high pH where the
hydroxide catalyzed reaction predominates, kinetics become
simple first-order.
The pH–rate profile of the fast spontaneuos hydrolysis of
CelXDNP is shown in Fig. 1. It is similar to other xanthate
esters and can be represented by eq. [2], where kobs is the rate
constant extrapolated to zero buffer concentration; k′H2O and
For MGX-3 the C2-OH proton couples to the C2-H proton,
which is also coupled to the anomeric proton. This coupling
can only be due to a trans-CH-OH conformation. Thus, the
hydroxyl is not hydrogen-bonding to either of the neighbouring
k
OH are
[2]
kobs = k′H2O + kOH[OH–]
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Can. J. Chem. Vol. 76, 1998
Fig. 2. Hydrolysis of CelXDNP in aqueous acetone at 25°C;
(a) fast hydrolysis (k′H2O); (b) slow hydrolysis (k′′H2O).
Table 4. Activation parameters for the hydrolysis of sugar
xanthates in 10% v/v aqueous ethanol, µ = 1.0 (KCl).
∆G≠,
∆H≠,
∆S≠,
Ester
Nucleophile kcal mol–1 kcal mol–1 cal K–1 mol–1a
H2O (fast)b 20.68 ± 0.22 20.87 ± 0.43
0.7 ± 1.8
OH- 18.56 ± 0.18 7.50 ± 0.23 –37.1 ± 0.8
H2O (fast)c 21.67 ± 0.03 22.68 ± 0.20
3.3 ± 0.8
CelXDNP
CelXNB
MGX-3
OH-
H2O
OH-
H2O
H2O
21.17 ± 0.06 11.20 ± 0.30 –33.4 ± 0.8
24.13 ± 0.12 11.90 ± 0.01 –41.0 ± 0.4
18.15 ± 0.12 4.35 ± 0.15 –46.3 ± 0.6
22.23 ± 0.12 14.65 ± 1.06 –25.4 ± 3.7
23.52 ± 0.29 12.10 ± 0.97 –38.3 ± 1.0
MGX-2,3 (k1)
(k2)
a 1 M, 25°C.
b µ = 0.1 (KCl).
c µ = 0.6 (KCl), ref. 5.
for the spontaneous hydrolysis of the cellulose esters is the
entropy of activation near zero. For the other sugar xanthates,
MGX-3 and MGX-2,3 (k1 and k2), the values of ∆S≠ are highly
negative, as they also are for all the hydroxide ion-catalyzed
reactions.
Discussion
The spontaneous hydrolysis of cellulose esters is faster than it
is for ethyl xanthates. For CelXDNP, k′H2O is more than 1000
times the rate constant of 2,4-dinitrophenyl ethylxanthate
(EXDNP), and the ratio k′H2O/ k′′H2O is about 60. Therefore the
kinetic behaviour of the spontaneous hydrolysis CelXDNP is
similar to that of CelXNB. For CelXNB it was found that the
water molecule acting as a nucleophile is general-base cata-
lyzed by a second water molecule (5). The fast hydrolysis has
been attributed to the reaction of isomers C-2 + C-3. The reac-
tion of CelXDNP in aqueous acetone showed that k′H2O is
insensitive to the increase of the molarity of water higher than
30 M (Fig. 2), suggesting that the cybotactic region is tight
the rate constants for the water-catalyzed fast reaction and hy-
droxide-catalyzed reaction, respectively. In Table 3 are shown
k′H2O, k′′H2O, and kOH for CelXDNP and some sugar xanthates.
The rate constants for the ethylxanthate ester analogues are
also included for comparison. All sugar xanthates, except the
6-substituted methylglucose xanthates (MGXB and MG-6),
hydrolyze faster than the ethyl analogues. The rate constant k2
for MG-2,3 is the same as that for MG-3.
enough to expulse the cosolvent. The slow hydrolysis (k′′H2O
)
due to C-6 isomer is more sensitive to the composition of the
medium because C-6 might be in a looser region.
We then addressed the question as to whether the observed
acceleration of the fast hydrolysis of cellulose esters, such as
CelXNB, could be a consequence of the
Hydrolysis in aqueous acetone
To observe if the active species involved in the spontaneous
hydrolysis were water polymers, the reaction of CelXDNP in
acetone with increasing molarity of water was studied. The
rate constants showed a downward curve for both the fast and
slow spontaneous hydrolysis (Fig. 2), which is the reverse of
the type of the curve expected when water polymers are in-
volved in the transition state of the spontaneous hydrolysis
(12). Both rate constants k′H2O and k′′H2O increase with water
molarity, following the change of dielectric constant of the
medium, up to ca. 30 M water, whereas k′H2O remains approxi-
mately constant with further increase while k′′H2O decreases
rapidly in pure water.
neighbouring effect of the OH in the C-2 or C-3 positions.
The effect was expected to be the same for the 2- or 3-substi-
tuted ester. The spontaneous hydrolysis of the 3-isomer is in-
deed about 200 times faster than the 6-isomer (Table 3).
However, it is still slower than the fast hydrolysis of CelXNB.
The ratios of the rate constants for CelXNB (k′H2O):MGX-
3:EXNB are 603:14:1, and for CelXNB (k′′H2O):MGX-
6:EXNB they are 10:0.1:1. The fast hydrolysis of CelXNB is
much faster than expected from the 3-isomer. In addition, the
acceleration of the spontaneous hydrolysis is due not only to
the sugar moiety, because the 6-isomer is slower than EXNB.
The ratio k′H2O/k′′H2O for CelXNB is 64, also very different to
the ratio MGX-3:MG-6 of 185.
Activation parameters
The main argument against the anchimeric assistance of the
hydroxyl group comes from the activation parameters. In Ta-
ble 4 is shown that only for the fast hydrolyses of CelXDNP
and CelXNB is the entropy of activation almost zero. For the
other water- and hydroxide ion-catalyzed hydrolyses, ∆S≠ is
The activation parameters for the hydrolysis of sugar xanthates
are shown in Table 4. The average value of ∆G≠ was obtained
from four temperatures, and ∆H≠ was obtained from the least-
squares fit of ln(k/T) vs. 1/T. It is important to note that only
© 1998 NRC Canada

965
Humeres et al.
highly negative, supporting the theory that the rate accelera-
tion observed for the cellulose esters is due to the ordered water
around the cellulose matrix.
from the Chemistry Department of the University of Liver-
pool. We are grateful to Prof. Raymond J. Abraham for advice
in the assignments and comments.
The spontaneous hydrolysis of the first xanthate group of
MG-2,3 might be faster than the second due to steric compres-
sion that is relieved in that step. The hydrolysis of MGX-2,3
raises the possibility that k′H2O and k′′H2O correspond to the
rate constants of the hydrolysis of dixanthated anhydroglu-
copyranose rings in CelXNB. The ratios k′H2O/k1 and k′′H2O/k2
for the rate constants of CelXNB and the hydrolysis of MGX-
2,3 are 2,5 and 0.7, respectively. However, there are two pieces
of evidence against this alternative. In the first place, the C-6
isomer should predominate over the C-2 + C-3 isomers, be-
cause the xanthation of methylglucose in alkaline solution pro-
duces mainly the C-6 isomer (13, 14). Therefore, if k′H2O and
k′′H2O correspond to the hydrolysis of di- and monoxanthated
rings, the reactivity of the C-6 isomer should be the same as
the C-2 or C-3 isomer, when actually it is about 100 times
slower than MGX-3 (Table 3). The second piece of evidence
comes again from the negative entropies of activation of the
water catalyzed hydrolysis of MG-2,3 (Table 4).
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Acknowledgements
The solid-state NMR spectra were obtained through the
ULIRS Solid State NMR Service at University College Lon-
don. The NMR spectra of the sugar xanthates were obtained
© 1998 NRC Canada