Preparation of the pyridinium salts differing in the length of the N-alkyl substituent

Article abstract of DOI:10.3390/molecules15031967

Quaternary pyridinium salts with chains ranging from C₈ to C₂₀ belong in the large group of cationic surfactants. In this paper, the preparation of such cationic surface active agents based on the pyridinium moiety and differing in the length of the N-alkyl chain is described. Additionally, HPLC technique was established to distinguish each prepared pyridinium analogue. This study represents universal method for preparation and identification of quaternary pyridinium detergents.

Full text of DOI:10.3390/molecules15031967

Molecules 2010, 15, 1967-1972; doi:10.3390/molecules15031967
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Preparation of the Pyridinium Salts Differing in the Length of
the N-Alkyl Substituent
Jan Marek 1,2, Petr Stodulka 1, Jiri Cabal 1, Ondrej Soukup 1, Miroslav Pohanka 1,
Jan Korabecny 3, Kamil Musilek 1 and Kamil Kuca 1,*
1
Faculty of Military Health Science, University of Defence, Hradec Kralove, 50001, Czech Republic
2
Vakos XT, Prague, 18600, Czech Republic
3
Faculty of Pharmacy in Hradec Kralove, Charles University in Prague, Hradec Kralove 50001,
Czech Republic
* Author to whom correspondence should be addressed; E-Mail: kucakam@pmfhk.cz.
Received: 14 December 2009; in revised form: 4 March 2010 / Accepted: 10 March 2010 /
Published: 19 March 2010
Abstract: Quaternary pyridinium salts with chains ranging from C8 to C20 belong in the
large group of cationic surfactants. In this paper, the preparation of such cationic surface
active agents based on the pyridinium moiety and differing in the length of the N-alkyl
chain is described. Additionally, HPLC technique was established to distinguish each
prepared pyridinium analogue. This study represents universal method for preparation and
identification of quaternary pyridinium detergents.
Keywords: pyridinium salt; synthesis; micellar catalysts; HPLC; TLC
1. Introduction
Cationic surfactants offer a huge amount of applications for industry. They are frequently used in
analytical and physical chemistry. Besides, some pyridinium salts are used as disinfectants in various
medical applications (e.g., eye drops, solutions, foams). In this field, quaternary pyridinium salts are
compounds of high interest, on account of their similarity to benzalkonium salts [1]. Their surfactant
characteristics (e.g., critical micellar concentration, surface tension) have already been determined and
published everal times [2]. Some pyridinium salts (C12 and C16) were also used as solubilizers for
water insoluble compounds in Analytical Chemistry. Differently, such surfactants were employed in

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the complexes with dyes to determine the type and concentration of electrolytes by a
spectrophotometric method [3]. A method describing spray application of a compound with a C16 side
chain on the pyridinium ring (cetylpyridinium, CP) for protection of the poultry against bacterial
contamination has also been published [4]. Another application of CP consisted in its addition to
chewing gums as an anti-plaque agent [5]. The same application may be possible in the dentistry [6].
The surface activity of CP predicted its ability to form the micelles (Figure 1). This property was used
by method called micellar-enhanced ultrafiltration that was applied for the removal of the pollutants
(e.g., arsenic, perchlorate) from ground waters and also for the removal of the heavy metals from
solutions [7–8]. The usage of pyridinium detergents as carriers improving adsorption of the
oligonucleotides at the phospholipid membrane was also described [9–10]. The pyridinium analogues
with the alkyl chains longer than C10 are known as nicotinic receptor antagonists [11]. Some
pyridinium salts bearing shorter alkyl chains were prepared and tested as cholinesterase inhibitors [12].
Figure 1. Forming of the micelles.
Preparation of the different pyridinium salts was already described before [13–14]. However, there
has been no description of the synthesis of the whole series of such salts differing in the alkyl chains
(C8 to C20). Formerly, a similar method for preparation of the benzalkonium salts was reported [15].
Consequently, a universal method for preparation of monoquaternary pyridinium salts with chain
substituents was developed. In this study, this method was successfully applied for the preparation of
the described pyridinium salts.
2. Results and Discussion
The results regarding the synthesis and analysis of the newly prepared compounds are summarized
in Table 1. Yield, melting point and HPLC retention time were determined. The preparation of salts
with C12-C20 alkyl chains was found to be easier than in the case of C8-C10 ones and the final yields
were higher. White crystaline products were obtained. Satisfactory purity was obtained after one
(C12-C16) or several crystallizations (C8-C10, C18-C20) from ether suspensions. The compounds with
C18-C20 alkyl chains were contaminated with starting material that was detected by TLC. Hence, they
were crystallized from acetone. There were also some difficulties in the preparation of C8-C10

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pyridinium salts. These compounds were crystallized several times from ether before the crystals were
formed. After all these crystallizations, the yield was dramatically decreased.
Table 1. Yield, melting point and retention time of prepared pyridinium salts.
Compound Side alkylating chain (R)
Yield (%)
m.p. (°C)
28–30
30–32
73–75
58–60
63
HPLC Rt (min)
3
4
5
6
7
8
9
C8
13
43
82
76
99
95
79
3.98
4.58
5.27
6.06
6.98
8.05
9.31
C10
C12
C14
C16
C18
C20
65–67
73–75
Additionally, the HPLC analysis was carried out for individual compounds. A newly developed
HPLC method was able to distinguish among all prepared quaternary pyridinium salts (Figure 2). The
shortest retention time was found for the C8 pyridinium salt. This novel HPLC method could be easily
used for characterization of mixtures of pyridinium detergents.
Figure 2. HPLC chromatogram of the prepared pyridinium series mixture.
3. Experimental
3.1. General
The reactions were monitored by TLC (Kieselgel Merck; mobile phase chloroform/methanol 100/1;
detection UV 254, Dragendorff reagent). The yields (%) and melting points (Boetius, m.p. were
1
uncorrected) are summarized in Table 1. H-NMR spectra characterizing individual compounds were
recorded in DMSO-d6 on a Varian Gemini 300 instrument.

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3.2. Synthesis
A universal method for the preparation of monoquaternary pyridinium salts 3-9 was developed
(Scheme 1): pure pyridine (1; 1 eq) in dry ethanol was mixed with 1-bromoalkane (2; 1.4 eq). The
mixture was refluxed for 40 hours. The solution was evaporated under reduced pressure and the crude
oily product was crystallized from ether, filtered under reduced pressure, washed with ether and
allowed to dry at r.t..
Scheme 1. Preparation of pyridinium salts.
N-Octylpyridinium bromide (3): 1H-NMR ppm 9.12 (d, J = 5.7 Hz, 2H), 8.61 (t, J = 7.6 Hz, 1H), 8.22-
8.11 (m, 2H), 4.58 (t, J = 7.5 Hz, 2H), 1.98-1.80 (m, 2H), 1.22 (m, 12H), 0.82 (t, J = 6.6 Hz, 3H).
1
N-Decylpyridinium bromide (4): H-NMR ppm 9.14 (d, J = 5.63 Hz. 2H), 8.61 (t, J = 7.58 Hz, 1H),
8.22-8.11 (m, 2H), 4.60 (t, J = 7.46 Hz, 2H), 2.01-1.80 (m, 2H), 1.16 (m, 16H), 0.83 (t, J = 6.64 Hz,
3H).
1
N-Dodecylpyridinium bromide (5): H-NMR ppm 9.14 (d, J = 5.6 Hz. 2H), 8.61 (t, J = 7.8 Hz, 1H),
8.22-8.11 (m, 2H), 4.61 (t, J = 7.5 Hz, 2H), 1.98-1.80 (m, 2H), 1.23 (m, 20H), 0.79 (t, J = 6.3 Hz, 3H).
N-Tetradecylpyridinium bromide (6): 1H-NMR ppm 9.14 (d, J = 5.6 Hz, 2H), 8.60 (t, J = 7.8 Hz, 1H),
8.22-8.11 (m, 2H), 4.61 (t, J = 7. 5 Hz, 2H), 1.98-1.80 (m, 2H), 1.23 (m, 24H), 0.83 (t, J = 6.6 Hz, 3H).
1
N-Hexadecylpyridinium bromide (7): H-NMR ppm 9.13 (d, J = 5.7 Hz, 2H), 8.61 (s, 1H), 8.22-8.11
(m, 2H), 4.61 (t, J = 7.4 Hz, 2H), 2.03-1.80 (m, 2H), 1.21 (m, 28H), 0.83 (t, J = 6.5 Hz, 3H).
1
N-Octadecylpyridinium bromide (8): H-NMR ppm 9.12 (d, J = 5.4 Hz, 2H), 8.61 (s, 1H), 8.22-8.11
(m,, 2H), 4.60 (t, J = 7.3 Hz, 2H) 1.89-1.80 (m, 2H), 1.22 (m, 32H) 0.82 (d, J = 6.3 Hz. 3H).
1
N-Eicosylpyridinium bromide (9): H-NMR ppm 9.14 (d, J = 5.63 Hz, 2H), 8.60 (t, J = 7.8 Hz, 1H),
8.23-8.11 (m, 2H), 4.61 (t, J = 7.5 Hz, 2H), 1.98-1.80 (m, 2H), 1.23 (m. 36H), 0.83 (t. J = 6.6 Hz, 3H).
3.3. HPLC Analysis
A suitable method for the characterization of the newly prepared compounds in the mixture was
developed using HPLC. The HPLC system consisted of a P200 gradient pump (Spectra-Physics
Analytical, Fremont, CA, USA), a 7125 injection valve – 10 µL loop (Rheodyne, Cotati, CA, USA), an
UV1000 detector (Spectra-Physics Analytical) and a CSW Chromatography Station 1.5 software
(DataApex, Praha, Czech Republic). A 250 × 4.6 mm I.D. Waters Spherisorb cyano (5 µm) column

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was used for analysis (Supelco Inc., Bellefonte, WA, USA). The mobile phase consists of 45%
acetonitrile and 55% water and was prepared as 0.1 M sodium acetate solution. Finally the pH was
adjusted via acetic acid to 5.0. The column was rinsed isocratically with a flow-rate of 1 mL/min. The
absorbance was measured at 257 nm.
4. Conclusions
The preparation of pyridinium salts with C8-C20 N-alkyl chains was described. Such preparation
might be used not only under laboratory conditions, but also for semi-industrial purposes. It is
anticipated that the synthesis wil prove very useful for the next effort of preparing micelle forming
compounds. The developed HPLC experimental protocol was found valuable for the estimation of the
purity of the newly prepared compounds.
Acknowledgements
This work was supported by the project of Ministry of Defence of the Czech Republic No.
OVUOFVZ200803.
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Sample Availability: Samples of all prepared compounds are available from the authors.
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