Download Werner Mormann Mohamed Al

Werner Mormann Mohamed Al-Higari
Universitat Siegen, FB 8, Laboratorium fur Makromolekulare Chemie, Siegen, Germany
Acylation of Starch with Vinyl Acetate in Water
Acetylation of different types of starch with vinyl acetate in water in the presence of a base is
described. Maximum degree of substitution under these conditions is 1.1. Vinyl esters of higher
homologous acids (vinyl laurate) react very slowly giving very low degrees of substitution after
long reaction times. The reaction mixture is biphasic, because vinyl esters are not miscible with
water. Formation of two layers reduces loss of acylating agent due to reaction with water - which is
a major problem encountered for esterification with acid anhydrides or chlorides - to less than 5%.
Unlike acylation of starch in DMSO with vinyl esters, the acetates of starch obtained by the method
reported do not show regioselective distribution of acetyl groups within the anhydro-glucose units.
Keywords: Acetylation; Vinyl acetate; Base catalysis; Substituent distribution
1 Introduction
Esterification of starch to low degrees of substitution (DS) is mostly made to reduce the
gelatinization temperature or to improve solution stability by preventing retrograda-tion. Higher
degrees of substitution are necessary to achieve solubility in organic media. The α-D-glycosidic
bond between anhydroglucose units (AGU) in starch is very sensitive to acid catalyzed hydrolysis,
which causes degradation of the polymeric chain of starch. Because of the sensitivity to acid
hydrolysis esterification is usually made in the presence of a base such as sodium hydroxide [1] or
pyridine [2] using acid anhydrides or acid chlorides as acylating agents. These methods, however,
have not gained technical importance.
Acylation of starch with vinyl esters is an alternative approach that does not require molar amounts
of base as an acid scavenger. This method has been investigated by several research groups to
synthesize starch acetates with low DS. Acylation with vinyl acetate in aqueous medium was
studied by Tuschhoff [3], Joosten [4] and Mezynski [5] and reported mainly in patents. The degrees
of esterification obtained by these authors were between 0.1 and 0.35. Klemm and co-workers
studied acylation in organic solvents like dimethyl sulfoxide with enzymatic or base catalysis [6].
They established conditions for regioselective acylation of the 2-hydroxy group [7], [8].
Degrees of esterification in the order of 1 or higher have not been obtained in aqueous media.
Synthesis of highly esterified materials, especially if obtained with equivalent
Correspondence: Werner Mormann, Universitat Siegen, FB 8, Laboratorium fur Makromolekulare
Chemie, Adolf-Reichwein-Straße 2, 57068 Siegen, Germany. Phone: +49-271-740-4713, Fax: +490271-740-2226, e-mail: [email protected].
amounts or only slight excess of acylating agent, could be a significant progress in the manufacture
and availability of starch ester of organic acids.
In the present paper we describe an improved method for the synthesis of starch acetates by
acylation with vinyl acetate in water.
2 Materials and Methods
2.1 Materials
Potato starch, corn starch, wheat starch (Fluka, Buchs, Switzerland), white potato starch dextrin
Avedex W80, yellow potato starch dextrin Avedex LC36 (Avebe, Foxhol, The Netherlands) were
used as received.
2.2 Methods of characterization
IR-spectra were recorded on a Bruker Equinox 55 FTIR spectrometer as films on NaCI plates; 1Hand 13C-NMR spectra (Bruker AC-200 or WH-400 FT, Bruker, Karlsruhe, Germany) were obtained
in deuterochloroform or DMSO-d6 using the non-deuterated compounds as internal standard.
Thermal properties were determined on a Mett-ler TA 4000 System with DSC 30 and TG 50
equipment.
2.3 Acylation of starch: general procedure
A 500 mL flange flask with internal thermometer, mechanical stirrer and reflux condenser with
bubble counter was charged with 200 g (1.23 mol AGU) potato starch, 15.76 g (9 mol% with
respect to AGU) potassium carbonate in 100 mL water and 212.4 g (2.46 mol) vinyl acetate. The
mixture was stirred under gentle reflux while acetalde-
hyde was formed. After the reaction had finished, unreacted vinyl ester was distilled off, the
emulsion-like product was precipitated in isopropanol, isolated by filtration and dried at 60°C/20
Pa. Amounts, catalysts, reaction conditions, yields, and degree of esterification are given in the
respective tables.
2.4 Complete acetylation of partially acylated starch with acetic anhydride-d6[9]
Potato starch acetate (147 mg, 0.74 mmol) (DS 1.1) and 660 mg (6.1 mmol) acetic anhydride-d6 in
12.8 mL pyri-dine were stirred at 60°C until signals of hydroxy groups were no longer visible in the
IR-spectrum (48 h). Half of the solvent was distilled off and the starch was precipitated in 100 mL
methanol. The product was centrifuged and the supernatant liquid was removed, this procedure was
repeated twice, then the solid was filtered, dissolved in benzene and isolated by freeze drying and
drying at 20 Pa.
3 Results and Discussion
Acylation with vinyl esters proceeds via an addition-elimination mechanism with formation of ester
groups and acetaldehyde, which is gaseous under the reaction conditions and thus can be used to
follow the progress of the reaction. The reaction is represented in Equation 1.
Reactions were carried out with vinyl acetate and vinyl laurate as acylating agents. Acylation with
vinyl esters requires base catalysis. Potassium carbonate and diso-dium hydrogenphosphate were
used for this purpose; triethylamine as an organic base was also included in the present study. In a
series of experiments reaction conditions were varied to determine optimum conditions for
acylation of starch. Variables were the concentration of starch in water, amount and type of catalyst,
reaction time and temperature. Criterion of success was the degree of esterification (degree of
substitution: DS) achieved. The DS was determined by 1HNMR-spectros-copy. Details of
experimental conditions and results are given in Tab. 1.
Reaction temperatures studied were 80 and 100°C. The latter gave better results and was used in
further experiments. The concentration of starch in water obviously plays an important role. A 20%
concentration resulted in a DS 0.67 after 50 h, 30% concentration gave a DS of 1.0 after 24 h.
Further reduction of water, i.e. concentration of starch of 66% and reduction of the amount of vinyl
acetate with respect to AGU from 1 :4 to 1:2 gave a DS of 1 already after 5 h without significant
further increase
Tab. 1. Acylation of starch in water (100°C bath temperature).
after 24 h reaction time. Disodium hydrogenphosphate as catalyst is less efficient than potassium
carbonate, which has been reported to be a good catalyst in DMSO [7]. Use of triethylamine as cosolvent rather than as catalyst was the only experiment that gave a DS significantly higher (1.4)
than obtained in completely aqueous medium.
The type (provenience) of starch is not crucial for the reaction. Using the optimum reaction
conditions established for potato starch, corn starch, wheat starch and two potato starch dextrins
could be acetylated to a DS of approximately 1, with white potato starch dextrin even 1.26 was
achieved.
In an attempt to investigate the applicability of this method to other acids, vinyl laurate
(dodecanoate) was used under otherwise identical conditions. In this case a DS of only 0.2 was
achieved after 48 h. This may be attributed to the low solubility of vinyl laurate in water and of
starch in vinyl laurate.
To find out whether the limit of degree of esterification close to 1.0 is inherent to starch or caused
by the fact that some kind of heterogeneity is the limiting factor
DMSO was used as reaction medium. Dicke [7] reported regioselective acylation of the hydroxy
group in the 2-position of starch working in DMSO as solvent for starch when the reaction was
carried out at 40-70°C with 2% disodium hydrogenphosphate as catalyst. The results in Tab. 2 show
that a DS of 1.6 could be obtained if the temperature was increased to 110°C. In this series a 50%
solution of starch gave a lower DS (0.75) than a 10% solution. With vinyl laurate a DS of 2.3 was
obtained under these conditions.
Acylation of starch with anhydrides or acid chlorides is accomplished with high excess of acylating
agents, especially if carried out in water containing reaction media. To study occurrence and extent
of hydrolysis of vinyl acetate, the amounts of liquid components after reaction were determined by
gas chromatography in one experiment. Liquids were completely removed under normal pressure,
then in vacuum and collected in a flask cooled with liquid nitrogen to avoid losses of volatiles. Two
phases formed; tetradecane was used as internal standard for the organic phase and diethyleneglycol
mono-methylether for the aqueous phase. Amounts before and after reaction are given in Equation
2.
The organic phase contained 0.86 mol vinyl acetate and no acetic acid, while 9 mmol of acetic acid
were found in the aqueous phase, which means that less than 5% of the acylating agent was lost by
hydrolysis. By consequence and within experimental error acylation takes place with stoichiometric
amounts of vinyl acetate and the excess used can be recovered. These favorable
conditions for acylation are most likely due to the immis-cibility of vinyl esters and water. A
minimum of solubility, on the other hand, is required to enable reaction with the hydroxy groups of
starch. This explains why acylation with vinyl dodecanoate is too slow to be considered as an
alternative to acylation in organic solvents (cf. Tabs. 1 and 2).
Amylose containing starch types with DS from 0.9 to 1.2 are soluble (10%) in water and dipolar
aprotic solvents like DMSO, dimethyl formamide and dimethyl acetamide and insoluble in alcohols,
ketones, ethers, and aromatic hydrocarbons.
6-position [7], approximately 46% of C2-hydroxy groups, 29% of C3- and 28% of C6-hydroxy
groups have been esterified. This does not correspond to the expected reactivity of hydroxy groups
(2 > 6 > 3) but is rather a random distribution with some preference for the 2-position.
3.1 Regioselectivity of acetylation of potato starch
As acetylation of starch in DMSO under the conditions reported by Dicke [7] gave 2-O-acetyl
starch, we investigated the distribution of acetyl groups within the AGUs. Partially acetylated starch
contains non-, mono-, di-, and tri-substituted AGUs. NMR-methods were used to determine amount
and distribution of acetyl groups within the AGUs [9]. For better resolution of signals residual
hydroxy groups were completely acetylated with deuterated acetic anhydride (cf. Eq. 3). This
imparts solubility of the triacetates in deuterated chloroform which is the solvent of choice [9], and
allows to assign the signals of non-deut-erated acetyl groups to the 2-, 3-, and 6-positions of the
AGU.
In Fig. 1 the proton NMR-spectrum of acetylated potato starch (DS 1.1, entry 5b Tab. 1) is shown.
The resonances of the acetyl protons in the different positions of the AGU cannot be base-line
separated in a 400 MHz spectrum. It can be seen, however, that there is no regiospecificity of
acetylation, because significant signals for all three positions are present. Neglecting the signal at
2.01 ppm, which is not caused by acetyl groups in the 2-, 3-, or
6-position [7], approximately 46% of C2-hydroxy groups, 29% of C3- and 28% of C6-hydroxy
groups have been esterified. This does not correspond to the expected reactivity of hydroxy groups
(2 > 6 > 3) but is rather a random distribution with some preference for the 2-position.
4 Conclusions
Acetylation of starch with vinyl acetate using sodium carbonate as catalyst in an aqueous medium
has been investigated. Degree of acylation was between 1.0 and 1.1, whereas a DS of only 0.02 was
achieved for vinyl laurate (dodecanoate) under otherwise identical conditions. A reaction time of 5
h is sufficient for acetylation while 48 h were required to achieve DS 0.02 for the laurate. As vinyl
esters are not miscible with water, the organic phase acts as a reservoir of the reagent. This prevents
loss of vinyl esters by reaction with water so that equimolar amounts are consumed after
recuperation of excess vinyl acetate.
Acknowledgement
We are indebted to the Bundesministerium fur Ver-braucherschutz, Ernahrung und Landwirtschaft
(Fach-agentur Nachwachsende Rohstoffe, FKZ 99 NR 119) for financial support.
References
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(Received: July 26, 2003) (Revised: October 18, 2003) (Accepted: October 20, 2003)