Humic substances as catalysts m condensation reactions

Article abstract of DOI:10.1016/S0045-6535(00)00294-0

Humic substances (HS) demonstrate appreciable impact on the rate of the condensation reactions as shown in the example of the reaction between hydrazine and 4-(dimethylamino)-benzaldhyde in an aquatic environment. The catalytic activity of HS has also been demonstrated in Knoevenagel and Claisen-Schmidt reactions for condensation of carbonyl compounds with CH acids. The aquatic fulvic acids are the most active in these reactions. The velocity of the studied reactions also depends on pH, temperature, the concentration and origin of HS used. A possible micellar and acid-base catalysis mechanism in aquatic media has been suggested.

Full text of DOI:10.1016/S0045-6535(00)00294-0

Chemosphere 44 "2001) 737±742
www.elsevier.com/locate/chemosphere
Humic substances as catalysts in condensation reactions
ꢀ
ꢁ *
ꢀ
ꢀ
ꢀ
Maris Kßlavinßs , Judõte Dipane, Kristõne Babre
Department of Environmental Science, University of Latvia, Rainßa blvd. 19, LV 1586, Rꢀõga, Latvia
Received 12 April 2000;accepted 30 June 2000
Abstract
Humic substances "HS) demonstrate appreciable impact on the rate of the condensation reactions as shown in the
example of the reaction between hydrazine and 4-"dimethylamino)-benzaldhyde in an aquatic environment. The
catalytic activity of HS has also been demonstrated in Knoevenagel and Claisen±Schmidt reactions for condensation
of carbonyl compounds with CH acids. The aquatic fulvic acids are the most active in these reactions. The velocity of
the studied reactions also depends on pH, temperature, the concentration and origin of HS used. A possible micellar
and acid±base catalysis mechanism in aquatic media has been suggested. Ó 2001 Elsevier Science Ltd. All rights
reserved.
Keywords: Humic substances;Catalysts;Condensation
1. Introduction
though HS contain many groups and structures which
in similar macromolecules are responsible for their
catalytic activity. This question is of special importance
because micro-organisms are regarded as the main
factor responsible for removal of organic xenobiotica
"pesticides, PCB's, PHA's and many other substances),
neglecting the role of HS. The catalytic activity of
other biomolecules "proteins, nucleic acids) has been
widely studied, but there are only a few publications in
which catalytic activity of HS has been described
"Dunnivant et al., 1992;Klavins, 1993;Moza et al.,
1995;Kieber et al., 1999). The study of the catalytic
activity of HS may be very important, considering their
possible role in the fate of di€erent xenobiotics and
high concentrations in natural environments. At the
same time, if HS exist as micelles, then they may also
possess several structural features "for example, cata-
lytic activity) common to micelles and widely studied
for di€erent reactions. Of di€erent reactions, the velocity
of which may be in¯uenced by the presence of HS,
condensation reactions should be mentioned ®rst, they
have been much studied to evaluate the acid±base and
micellar catalysis. The aim of the present investigation
Humic substances "HS) greatly in¯uence soil fertil-
ity, and they play a principal role in the turnover of
organic carbon "Peuravuori, 1992). HS form the major
part of the organic component of soil, peat and natural
waters, and they in¯uence groundwater properties and
the formation of fossil fuels "Klavins, 1998). HS are
known to complex heavy metals and persistent organic
xenobiotics "Stewart, 1984). The interaction of HS with
xenobiotics may modify the uptake and toxicity of
these compounds by aquatic organisms, and a€ect the
fate of pollutants in the environment. Studies of humic
matter have a history of more than 200 years, but
many of their properties are not yet clear. Only re-
cently has it been suggested that HS in natural envi-
ronments exist in the form of micelles "Guetzlo€ and
Rice, 1994). One of the humic properties which has
been analysed somewhat is their catalytic activity,
* Corresponding author. Fax: +371-733-2704.
ꢁ
E-mail address: mklavins@lanet.lv "M. Kßlavinßs).
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 " 0 0 ) 0 0 2 9 4 - 0

738
M. Kßlavinßꢁs et al. / Chemosphere 44 )2001) 737±742
was to study the catalytic activity of HS of di€erent
origin in the reactions of condensation. These reactions
can be of importance in the fate of environmental
pollutants.
2. Materials and Methods
Humic "HA) and fulvic "FA) acids were isolated
from soil and peat by extraction with 0.1 M NaOH
"Methods of Soil Analysis, 1999), but from water by
the Malcom and Thurman method "Thurman and
Malcolm, 1981). Concentrations of functional groups
in HS were determined by standard methods NaOH
"Methods of Soil Analysis, 1999). Commercial humic
acid "CHA) was used for comparison "Aldrich).
Properties of HS used are summarized in Table 1. UV
spectra were obtained on Hitachi 850 spectrometer,
but the velocity of the reaction has been studied using
spectrometers HACH DR/2000, SF-46.
Fig. 1. Condensation of 4-"dimethylamino)-benzaldehyde with
hydrazine: without catalyst and in the presence of humic acid
and surfactants "r ± without catalyst; ꢁ ± catalyst ± Triton
X-100; Â ± catalyst commercial humic acid; Ã ± catalyst ± soil
humic acid; Ã ± catalyst ± sodium dodecylsulphate).
2.1. Condensation kinetics)4-)dimethylamino)-benzalde-
hyde with hydrazine)
on a linear scale. The plots were straight lines for 70%
conversion or more. The slope was taken as Kobsd
.
To the solution "100 ml) of 4-"dimethylamino)-
benzaldehyde "5 Â 10À4 M, pH 2 adjusted with 6N HCl)
in water, either "10 ml, 0.1 M) solution of catalyst "so-
dium dodecylsulphate or Triton X-100) or changing
amounts of humic matter solutions "Figs. 1±4) were
added "pH 2). In a quartz cell initial adsorption ꢀA0† and
after addition of 1 ml of hydrazine sulphate "1 Â 10À3 M,
pH 2) absorption ꢀAt† of the reaction product "II) "468
nm) as a function of time was measured. After at least 10
half-lives, the absorption was measured for complete
2.2. Condensation reactions between carbonyl compounds
and CH-acids
20 mmol of active methylene or methyl compound
were dissolved in 50 ml of solvent "Table 2), containing
‡
100 mg of HS or their NH4 salt, and 20 mmol of car-
bonyl compound were added. After stirring at heating
"Table 2) the solvent had evaporated, and the residue
was washed with 0.1 N NaHCO3, and the obtained
product was recrystallized or distilled in vacuo. The
yields and properties of substances isolated are sum-
marized in Table 3.
reaction ꢀA †. Catalysis by bu€er only "blank) was
1
measured in the same fashion.
The measured data were treated as ®rst-order kinetics
by plotting ꢀA1 À A0†=ꢀA1 À At† on a log scale vs time
Table 1
Properties of humic substances from water, soil and peat of Latvia
Elemental composition "%)
Humic substances
±COOH "mmol/g)
ArOH "mmol/g)
C
H
N
Humic acids
River Salaca
AHA-S
SHA
54.60
48.89
48.11
52.47
52.34
3.70
4.76
5.60
4.18
4.28
0.93
3.91
1.95
2.15
3.86
3.83
2.45
±
2.86
1.86
±
Soil "Sod-podzolic)
Peat "Sece bog)
Peat "Spigu bog)
Peat "Olaine bog)
PSHA
PSpHA
POHA
±
2.26
±
1.48
Fulvic acids
River Salaca
River Daugava
Lake Islienas
AFA-S
AFA-D
AFA-I
49.68
51.42
56.41
4.25
4.48
3.85
0.40
0.97
0.87
4.32
5.07
4.17
3.12
±
1.43

M. Kßlavinßꢁs et al. / Chemosphere 44 )2001) 737±742
739
2.3. Condensation kinetics )4-)dimethylamino)-benzalde-
hyde with nitromethane)
Solution of 4-"dimethylamino)-benzaldehyde "25 ml,
0.2 M), nitromethane "25 ml, 0.25 M) and HS "Fig. 5)
was stirred at heating. For 5 h at certain time intervals, 5
ml aliquots were taken o€ and the sorption of reaction
product "4-dimethylamino-x-nitrostyrene) at 435 nm
was determined. The results of the condensation reac-
tions are summarized in the Fig. 5.
3. Results and Discussion
Fig. 2. Dependence of the humic acid "CHA) catalyzed con-
densation of 4-"dimethyl-amino)-benzaldehyde with hydrazine
on pH.
HS were selected for catalytic activity studies to
represent the common types of HS and they are com-
parable with reference samples and commercially avail-
able ones. The HS were isolated from soil, peat and
water in Latvia and their properties were within the
ranges, common for HS "Stevenson, 1982;Orlov, 1990).
The dominant structures of HS are benzene and phe-
nolecarboxylic acids "aromatic structural units), and
residues of carbohydrates and mono- and dicarboxylic
acid "aliphatic structural units) "Klavins et al., 1999).
The molecular mass of the studied HS varied from 850
"aquatic fulvic acids) to ꢂ10 000 "peat humic acids)
Klavins et al., 1999).
We studied well-known "Roska et al., 1988) reac-
tions of 4-"dimethylamino)-benzaldehyde condensation
with hydrazine in the presence and absence of cata-
lyst in aquatic media. The condensation of the 4-
"dimethylamino)-benzaldehyde with hydrazine in the
excess of the aldehyde goes with the formation of
azine "II).
Fig. 3. Dependence of the humic acid "CHA) catalyzed con-
densation of 4-"dimethyl amino)-benzaldehyde with hydrazine
on humic acid concentration.
In acidic media, the azine passes into the colored
protonated form. Because of the strong reversibility, the
reaction was conducted in aqueous organic media in
excess of the aldehyde, but as found previously in the
presence of anionic surface-active substances, containing
acidic groups, practically, a complete shift in the equi-
librium toward formation of the azine takes place. The
condensation was studied as a pseudo-®rst-order reac-
tion. Investigations of condensation reaction kinetics
"Fig. 1) show that HS act as catalysts. HS "soil humic
acids) are poorer catalysts than sodium dodecylsulphate
Fig. 4. Catalytic activity of vartious humic acids on conden-
sation of 4-"dimethyl-amino)-benzaldehyde with hydrazine.

740
M. Kßlavinßꢁs et al. / Chemosphere 44 )2001) 737±742
Table 2
Typical experimental conditions in condensation reactions catalyzed by humic substances and their ammonia salta
Active methylene "methyl)
compound
R
Catalyst
Solvent
T ꢀ°C†
Time
"h)
Product
NCCH2COOC2H5
Dimedone
PhCH2CN
Hydantoin
MeCOPh
H
NO2
AHA-S
SHA
SHA
DMF
DMF
Bz
75
80
75
75
75
65
20
8
IIIa
IVb
Vc
H
15
20
25
10
2,4-ꢀMeO†
MeO
Me2N
SHAANH4 salt
SHAANH4 salt
SHAANH4 salt
MeCN
Bz
EtOH
VIa
VIIf
VIId
2
MeNO2
a See general procedure.
Table 3
Properties and yields of condensation products
Product
Yield in
presence of
catalyst "%)
Yield of
uncatalyzed
reaction "%)
Melting point ꢀ°C† found
1H-NMRꢀd ppm†
"from the literature)
IIIa
IVa
IVb
58
64
38
51
28
16
49 "49 [Thurman and Malcolm, 1981])
1.42 "t, 3H), 4.23 "q, 2H),
7.33-7.39 "m, 5H), 8.13 "s, 1H)
±
194 "193 [Thurman and Malcolm,
1981])
230 "230 [Thurman and Malcolm,
1981])
1.12 "s, 6H), 1.29 "s, 6H), 2.40
"s, 8H), 5.51 "s,1H)
IVc
Vc
43
32
32
18
143 "144 [Zabicky, 1961])
95 "95-96 [Thurman and Malcolm,
1981])
±
3.08 "s, 3H), 5.82 "s, 1H), 6.64
"s, 1H),7.68 "m, 5H)
VIa
VIIf
43
75
31
43
222 "222 [Stevenson, 1982])
86 "85-87 [Stewart, 1984])
±
3.45 "s, 1H), 6.56 "s, 1H),
7.79±8.12 "m, 8H),
±
VIIIa
VIIId
83
34
64
21
58 "58 [Stevenson, 1982])
179 "179 [Stevenson, 1982])
2.25 "s, 6H), 5.06 "s, 1H), 5.45
"s, 1H)
and their catalytic activity di€ers from each other "if
compared with commercial HA). But they are true cat-
alysts, and better than nonionic surfactants and cationic
surfactants, thus behaving as substances with acidic
functional groups and existing in the form of micelles in
aquatic media. The velocity of reactions also depends on
the pH "Fig. 3) and the concentration of HS "Fig. 2) and
is highest at low pH values.
Increase of HS concentration increased the velocity
of the reaction. The velocity of the condensation reac-
tion also depends on the humic matter origin and
highest catalytic activity in the studied reaction dem-
onstrates aquatic fulvic acids, but the lowest humic acids
were isolated from peat "Fig. 4).
Fig. 5. Condensation of 4-"dimethylamino)-benzaldehyde with
nitromethane: uncatalyzed and in presence of humic acid. A ±
without catalyst, solvent ± acetonitrile;B ± catalyst ± soil humic
acid, solvent ± acetonitrile;C ± catalyst ± soil humic acid am-
monia salt, solvent ± benzene;D ± catalyst ± soil humic acid
ammonia salt, solvent ± acetonitrile, E ± catalyst ± soil humic
acid ammonia salt, solvent ± ethanol.
For a more detailed study of the condensation re-
actions, the Knoevenagel reaction has been selected.
In the course of the reaction, an aromatic aldehyde
reacts with methylene compounds activated by elec-
tron acceptor groups. The facility of the performed
reaction is determined by the type and number of

M. Kßlavinßꢁs et al. / Chemosphere 44 )2001) 737±742
741
substituents in aromatic aldehydes as well as by the
type of catalysts required "Jones, 1967), but commonly
it is catalyzed by acids. These reactions can be realized
in nonaquatic media isolating the condensation prod-
ucts preparatively. HA and FA are able to catalyze
the condensation of ethyl cyanoacetate or of cyclic
1,3-diketones with aromatic aldehydes, and the prod-
ucts are formed with comparatively high yields,
whereas reaction product yields for condensations
between aldehyde and diethyl malonate or benzyl cy-
anide "requiring stronger acid for ionization of meth-
ylene group) are much lower. Condensations of
heterocycles are also catalyzed by HS. Thus, benzal-
dehyde and hydantoin give a preparative yield of 5-
benzylidenehydantoine "VIa), the yield being poor in
the absence of HS "Table 3).
In the condensation of 4-"dimethylamino)-benzalde-
hyde with nitromethane, the reaction velocity increases
with the increase of the humic acid ammonia salt con-
centration in the reaction mixture "Fig. 5).
4. Conclusion
The results showed that HS from water, soil and peat
exhibit considerable catalytic activity in di€erent con-
densation reactions and may in¯uence the fate of pol-
lutants in the environment. The rate of condensation
reactions depends on the nature and concentration of
humic substance, pH and temperature. The maximum
catalytic activity is characteristic for aquatic fulvic acids,
but the reaction mechanism possibly involves micellar
catalysis "in aquatic media) or acid±base catalysis "in
nonaquatic media).
References
Dunnivant, F.M., Schwarzenbach, R.P., Macalady, D.L., 1992.
Reduction of substituted nitrobenzenes in aqueous solutions
containing natural organic matter. Environ. Sci. Technol.
26, 2133±2141.
Guetzlo€, T.A., Rice, J.A., 1994. Does humic acid form a
micelle? Sci. Total Environ. 152, 31±35.
Jones, G., 1967. The Knoevenagel condensation. Org. React.
15, 204±601.
Kieber, R.J., Li, A., Seaton, P.J., 1999. Production of nitrite
from the photodegradation of dissolved organic matter in
natural waters. Environ. Sci. Technol. 33, 993±998.
Klavins, M., 1993. Catalytic activity of humic substances in
condensation reactions. Latv. Chem. J. 3, 370±374.
Klavins, M., 1998. Aquatic humic substances: structure and
genesis. University of Latvia, Riga.
Klavins, M., Serzane, J., Supe, A., 1999. Properties of soil and
peat humic substances from Latvia. Proc. Latv. Acad. Sci
ser B 53 "5), 249±256.
Methods of Soil Analysis, 1999. American Society of Agron-
omy, Madison, WI.
Moza, P.N., Hustert, K., Feicht, E., Kettrup, A., 1995.
Comparative rates of photolysis of triadimefon in aqueous
solution in the presence of humic and fulvic acid. Chemo-
sphere 30, 605±610.
The study of Claisen±Schmidt condensation reac-
tion between aromatic aldehydes and methyl com-
pounds also demonstrates the catalytic activity of HS.
In their presence "Table 2), the respective yields of
chalcones "VII) and nitrostyrenes "VIII) obtained from
the condensation reactions of aromatic aldehydes with
acetophenone or nitromethane are higher by 5±15%
"Table 3). It should be noted that condensation with
nitromethane requires the presence of HS in the form
of ammonium salts, whereas HA and potassium or
trimethylammonium salts of HA fail to catalyze the
condensation.
Orlov, D.S., 1990. Soil humic acids and general humi®cation
theory, MGU, Moscow "in Russian).
Peuravuori, J., 1992. Isolation, fractionation and characterisa-
tion of aquatic humic substances. Does a distinct humic
molecule exist? Finnish Humus News 4, 1±334.
ꢂ
Roska, A., Kßlavinßs, M., Zicmanis, A., 1988. High molecular
catalysts in organic synthesis. XVI Catalysis of condensa-
tion reactions by polymer-supported crown ethers. Izv.AN
LatvSSR Ser. Khim. 1, 91±96.
Stevenson, F.J., 1982. Humus chemistry. Wiley, New York.
Stewart, A.J., 1984. Interaction between dissolved humic
materials and organic toxicants. In: Cowser, K.E. "Ed.),
Synthetic Fossil Fuel Technologies: Results on Health

742
M. Kßlavinßꢁs et al. / Chemosphere 44 )2001) 737±742
and Environmental Studies. Butterworth, Boston, pp.
505-521.
Zabicky, I.J., 1961. The kinetics and mechanism of carbonyl
methylene condensation reactions. IX. Stereochemistry of
products. J. Chem. Soc. 2, 683±692.
Thurman, E.M., Malcolm, R.L., 1981. Preparative isolation of
aquatic humic substances. Environ. Sci. Technol. 15, 463±
466.