Synthesis and kinetic study of a series of chloro- and m-carboxypyridium triazinyl reactive dyes

Article abstract of DOI:10.1016/j.dyepig.2021.109147

Five monochloro-s-triazinyl reactive dyes (MCT) and five m-carboxypyridium-s-triazinyl reactive dyes (NTR) were synthesised with the same red chromophore bearing an –NHCN, –OCH₃, –CH₃NSO₂CH₃, –N-methyl phenyl or –OH group as ‘second-leg’ substituents. A kinetic study of the hydrolysis of these dyes was conducted, and the rate constant (k_obs) and half-life time values were determined. The k_obs of the MCT and NTR dyes was found to follow the order –OCH₃ > –CH₃NSO₂CH₃ > –N-methyl phenyl > –NHCN > –OH, which was approximately in agreement with the values obtained for the Hammett substituent constants. Overall, the higher the electron-donating property of the substituent on meta-position to the leaving group in the triazine ring, the lower the hydrolysis rate constant.

Full text of DOI:10.1016/j.dyepig.2021.109147

Dyes and Pigments 188 (2021) 109147
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis and kinetic study of a series of chloro- and m-carboxypyridium
triazinyl reactive dyes
,b,
Huei-Chin Huanga *, Chun-Guey Wua
a Department of Chemistry, National Central University, No. 300, Zhongda Rd., Zhongli City, Taoyuan, 320, Taiwan
b Product Stewardship Division, Everlight Chemical Industrial Corporation, No. 12, Industrial 3rd Rd, Kuanyin Industrial Park, Taoyuan, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Five monochloro-s-triazinyl reactive dyes (MCT) and five m-carboxypyridium-s-triazinyl reactive dyes (NTR)
were synthesised with the same red chromophore bearing an –NHCN, –OCH3, –CH3NSO2CH3, –N-methyl phenyl
or –OH group as ‘second-leg’ substituents. A kinetic study of the hydrolysis of these dyes was conducted, and the
rate constant (kobs) and half-life time values were determined. The kobs of the MCT and NTR dyes was found to
follow the order –OCH3 > –CH3NSO2CH3 > –N-methyl phenyl > –NHCN > –OH, which was approximately in
agreement with the values obtained for the Hammett substituent constants. Overall, the higher the electron-
donating property of the substituent on meta-position to the leaving group in the triazine ring, the lower the
hydrolysis rate constant.
M-carboxypyridium triazinyl
Kinetic study
Rate constant
Kobs
Cyanamide
N-Methyl methylsulfonyl amine
Hammett substituent constant
σmeta
1. Introduction
applied to cellulosic substrates under disperse dyeing conditions. These
dyes can be applied under mildly acidic conditions at high temperature
The use of triazine derivatives as functional groups for the attach-
ment of dyes to cellulosic substrates is well established. Indeed, the first
widely available reactive dyes for cotton were Procion, which are
dichlorotriazine dyes developed by Imperial Chemical Industries in the
1950s. In particular, dichlorotriazines were highly reactive and suitable
for cold pad-batch dyeing and exhaust dyeing at 40 ◦C; however, the
resulting dyeing is unstable toward mild acids. This prompted the
introduction of monochlorotriazines, which solved the issue of acid
stability, although requiring more energetic application conditions.
Procion dyes were followed by masked vinyl sulphones, such as CI
Reactive Black 5 by Hoechst AG and fluorotriazines such as Cibacron F
dyes by Ciba Geigy. Both classes of dyes are more reactive than mono-
chlorotriazines and, as such, can be applied at lower temperature, which
enables energy saving. The difficult handling of fluorotriazines has
driven the replication of their reactivity level using other leaving groups
that were more suitable for commercial-scale manufacture. Among the
leaving groups known to increase the reactivity of mono-
halogenotriazine reactive dyes, such as pyridine derivatives, trimethyl-
amine and nicotinic acid [1–3], only 3-carboxypyridine achieved
(ca. 130 ◦C), alongside disperse dyes obviating the requirement for
lengthy two-stage dyeing processes [4,5]. The reactivity of reactive dyes
is important for cotton dyeing. Although the major determinant of the
reactivity is the nature of the leaving group, the changes in the nature of
the 4-substituents in the triazine ring also influence the reactivity.
Several studies have demonstrated that the change in the structure of the
‘second leg’ in monochloro-s-triazine or m-carboxypyridinium-s-triazine
reactive dyes can influence the reactivity of dye molecules for exhaust
cotton dyeing. V. Kampyli et al. [6] investigated dyes containing a
chlorine leaving group and an oxido group, which exhibited dyeing
performance that is significantly inferior to that of dyes with nicotinic
acid/oxido, chlorine/o-toluidine and nicotinic acid/o-toluidine combi-
nations. E. Karapinar et al. [7] used methanol as a model for cellulose to
investigate the reactivity of different substituents at the 4-position of
2-chloro-s-triazinyl reactive dyes. They found that the order of reactivity
was 4-alkylthio > 4-methoxy > 4-dimethylamino. D. L. Liu et al. [8]
studied the hydrolysis kinetics and dyeing properties of mono-
chloro-s-triazinyl and m-carboxypyridinium-s-triazinyl reactive dyes
containing aniline as a ‘second-leg’ substituent. They found that the pH
value was a key factor at low temperature to achieve dyeing efficiency,
whereas the hydrolysis of the dye molecule is important at high tem-
perature. Although numerous ‘second-leg’ substituents have been
commercial success as
a
component of bis-3-carboxypyr-
idinium-s-triazines, the so-called “nicotinic quats”. Nippon Kayaku used
bis(3-carboxypyridinium) to develop Kayacelon Reacts, which are
* Corresponding author. Department of chemistry, National Central University, No. 300, Zhongda Rd., Zhongli City, Taoyuan, 320, Taiwan.
E-mail addresses: amyhuang@ecic.com.tw (H.-C. Huang), t610002@cc.ncu.edu.tw (C.-G. Wu).
https://doi.org/10.1016/j.dyepig.2021.109147
Received 16 July 2020; Received in revised form 17 December 2020; Accepted 8 January 2021
Available online 18 January 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
methanesulfonamide was purchased from TCI. All reagents were used as
received unless otherwise specified.
2.2. Instruments and analysis methods
Ultraviolet/visible (UV/Vis) absorption spectroscopy was performed
using a Shimadzu UV-2700 UV/Vis spectrophotometer. Thin layer
chromatography (TLC) was employed to monitor the reactions using
aluminium plates coated with silica gel (60 F254, Merck) as a stationary
phase and a mixture of iso-butanol/n-propanol/ethyl acetate/water at a
volume ratio of 2:4:1:3 as a mobile phase. The developed plates were
visualised under short and long UV light (254 and 365 nm, respectively).
High-performance liquid chromatography (HPLC) was performed using
the following conditions: (1) For the organic purity analysis, a WATERS
e2695-2998 separation module with a 10-cm Purospher RP-18 column
with 5 μm packing and Inertsil ODS 4.6 × 250-mm cartridges were used.
The mobile phase was 0.005 M tetrabutylammonium bromide aqueous
solution as solvent A and methanol (HPLC grade) as solvent B. These
solvents were passed through the column at A/B gradient ratios from
50:50 to 75:25 at a gradient rate of 1 mL/min within 40 min at ambient
temperature. The samples were analysed using a diode array detector at
a wavelength ranging from 200 to 700 nm. (2) For the kinetic study, the
hydrolysis product of the model dye was analysed via HPLC by detecting
the product absorbance at a wavelength of 254 nm using a WATERS PDA
u0416-2998 separation module with a 10-cm Mightysil RP-18 4.6 × 250
Fig. 1. General structure of the model dyes with different X and Y substituents.
Table 1
Model dyes and their substituents.
Name of Dye
X
Y
mm (5 μm) column. The mobile phase for the analysis of 1A, 2A, 3A, 5A,
1A
Cl
NHCN
NHCN
2B, 3B, 4B and 5B was 0.005 M tetrabutylammonium bromide aqueous
solution as solvent A and methanol (HPLC grade) as solvent B. These
solvents were passed through the column at A/B gradient ratios from
50:50 to 34:66 at a gradient rate of 1 mL/min within 16 min at ambient
temperature. For the analysis of 4A, the same mobile phase and column
were used, but the A/B gradient was changed from 45:55 to 30:70 in 15
min. The mobile phase used for the analysis of 1B was 0.005 M tetra-
butylammonium bromide aqueous solution with 0.01 M (NH4)2HPO4
aqueous solution as solvent A and methanol (HPLC grade) as solvent B.
These solvents were passed through the same column at A/B gradient
ratios from 50:50 to 38:62 at a gradient rate of 1 mL/min within 12 min
at ambient temperature. Analysis of all the samples was conducted using
a diode array detector at a wavelength ranging from 200 to 700 nm. A
German Elementar Vario EL cube analyser was utilised to conduct
elemental analysis to determine N, C, S and H by digesting and burning
4–5 mg of the dye sample in a furnace at a temperature of up to 1200 ◦C
to achieve 100% recovery even for samples that were difficult to burn.
To ensure that the sample was completely burned, oxygen was directly
injected into the combustion point of the sample. Mass spectra were
recorded using a Q Exactive™ Plus Hybrid Quadrupole-Orbitrap™ mass
spectrometer fitted with a thermo scientific ion max API HESI source
operating with an advanced quadrupole technology and an Orbitrap
mass analyser. Data were obtained by ultrafast real-time data acquisi-
tion using an instrument control system. Nuclear magnetic resonance
(NMR) spectroscopy (1H and 13C) was performed using a Bruker-
AVANCE III 400 MHz spectrometer at an operating temperature of
297.3 K using deuterated dimethyl sulfoxide (DMSO‑d6) and deuterated
water (D2O) as solvents and tetramethylsilane as the internal reference.
Fourier transform infrared (FTIR) spectra were obtained using a Perki-
nElmer 100 FTIR spectrometer fitted with a universal attenuated total
reflection (ATR) accessory for ATR-FTIR measurement. The working
range was 7800–350 cmꢀ 1, and the resolution was 0.5 cmꢀ 1. The
effective agent (e.a.) content of each dye was calculated using the
following formula: (found% of carbon obtained by elemental analysis/
theoretical% of carbon in sodium form molecular weight of dyestuff) ×
100%. The electron density of the dyes was simulated by HyperChem
Release 7.52 using two models, i.e. MM + molecular mechanics for the
optimisation of the molecular geometric configuration and PM3 semi-
empirical quantum mechanics for the calculation of the electron density
of the dye molecule [11].
1Ba
2A
2B
Cl
Cl
Cl
Cl
OCH3
OCH3
3A
3B
N(CH3)–SO2CH3
N(CH3)–SO2CH3
4A
4B
N(CH3)-phenyl
N(CH3)-phenyl
5A
a
OH
The numbering of 1’ to 4’ is represented the proton of Nicotinic acid.
investigated, they are mainly simple amines, including aliphatic, aro-
matic and mixed aliphatic/aromatic amines. Consequently, we were
interested in evaluating the effect of the different 4-substituents on the
reactivity of a series of 2-chlorotriazines and 2-(3-carboxypyridinium)
triazines. In this paper, various substituents were selected and syn-
thesised to represent a range of reactivity, including cyanoamino
(–NHCN) [9], N-methyl methylsulfonylamino (–NCH3SO2CH3) [10],
methoxyl (–OCH3), N-methylphenyl and hydroxyl (–OH) substituents.
2. Experimental section
2.1. Chemicals
1-Amino-8-hydroxynaphthalene-3,6-disulphonic acid (H-acid), ace-
tic anhydride, cyanuric chloride (CC), 2-amino-1-naphthalenesulfonic
acid (Tobias acid), N-methyl aniline, nicotinic acid, 32% hydrogen
chloride solution, 45% sodium hydroxide solution, sodium carbonate
(anhydrous), sodium bicarbonate (anhydrous) and toluene were ob-
tained from Everlight Chemical Industrial Corp. (Taoyuan, Taiwan R.
O⋅C.). Cyanamide was obtained from Acros Organics, and N-methyl
2

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
2.3. Preparation of the dye molecules
2.3.4. 1-[4-(Cyanoamino)-6-[[8-hydroxy-3,6-disulfo-7-[(1-sulfo-2-
naphthyl)azo]-1-naphthyl]amino]-1,3,5-triazin-2-yl]pyridin-1-ium-3-
carboxylate (Dye 1B)
A total of 10 model dyes were studied. Their general structure is
presented in Fig. 1. The detailed names and components are listed in
Table 1, and their synthesis was conducted as described below.
Dye 1B was made from Dye 1A. Dye 1A (0.097 mol) was dissolved in
water (1000 mL), and nicotinic acid (59.97 g, >99%, 0.482 mol) was
added. The pH was adjusted to 5.5–6.0 by adding 100 mL of a 15 wt%
Na2CO3 aqueous solution. The resultant solution was heated to 60 ◦C-
65 ◦C for 4 h. When TLC monitoring (Dye 1B, Rf = 0.38) indicated that
the reaction had completed, the mixture was salted out with 15% w/v of
sodium chloride (210 g) to give a red solid, which was isolated by
filtration and oven-dried at 50 ◦C. Dye 1B was obtained as a red crude
product (120 g, 91% conversion). The organic purity of Dye 1B was
determined to be 93.51% via HPLC (RT = 18.498 min). The e. a. content
of dye was calculated to be 63.44%.
2.3.1. 5-amino-4-hydroxy-3-[(1-sulfo-2-naphthyl)azo]naphthalene-2,7-
disulfonic acid (Compound 3, Red base)
Compound 3, Red base (0.117 mol), was prepared as described by J A
Bone et al. in Colouration Technology [12]. The reaction solution (560
mL) was monitored via TLC (Compound 3, Rf = 0.50). When the reaction
was completed, sodium chloride (84 g, 15% w/v) was slowly added over
30 min to the cooled solution (40 ◦C-45 ◦C) under stirring. The resulting
precipitate was collected and oven-dried at 50 ◦C, affording 103 g of the
product (82% conversion). The organic purity of the product was
determined to be 100% via HPLC (Retention Time = 14.997 min). The e.
a. content of dye was calculated to be 69.96%.
IR (KBr): –NH, –OH, –COOH stretching 3550–3200 cmꢀ 1 (broad, s);
nitrile stretching 2172 cmꢀ 1 (m), aromatic –C C– bending 1628 cm
,
(s), sp3-C–O–
7.480–7.566
ꢀ 1
–
–
2
ꢀ 1
1564 cmꢀ 1 (m, m); sp -C O stretching 1214 cm
–
–
IR (KBr): –NH, –OH stretching 3550–3200 cmꢀ 1 (broad, s); aromatic
stretching 1048 cmꢀ 1 (s); 1H NMR (DMSO‑d6):
δ
ꢀ 1
2
–
–
–
–C C– bending 1625, 1583 cm (m, m); sp -C O stretching 1210
(C2,3,7,8,16-4H, cm), 7.897 (C6–1H, d, J = 8.0 Hz), 7.967 (C5–1H, d, J
= 9.2 Hz), 8.269 (C3′, t, J = 6.8, 7.2 Hz), 8.735 (C4–1H, J = 9.2 Hz),
9.062 (C9–1H, d, J = 8.4 Hz), 9.177 (C2′-1H, d, J = 7.6 Hz), 9.315
–
cmꢀ 1 (s), sp3-C–O– stretching 1048 cmꢀ 1 (s); 1H NMR (DMSO‑d6): δ
6.935 (C2–1H, d, J = 1.2 Hz), 7.068 (C1–1H, d, J = 1.2 Hz), 7.337
(C3–1H, s), 7.402 (C7–1H, t, J = 7.6 Hz), 7.483 (C8–1H, t, J = 7.6 Hz),
(C1–1H, broad), 9.970 (C4′-1H, d, J = 6.4 Hz), 10.202 (C1′-1H, s),
–
–
7.842 (C6–1H, d, J = 7.6 Hz), 7.890 (C5–1H, d, J = 9.2 Hz), 8.0–8.2
12.813 (broad, azo hydrazone –NH–N C–), 16.436 (sharp, intra-
–
–
–
(broad, azo hydrazone –HN–N C–), 8.689 (C4–1H, d, J = 9.2 Hz),
molecular H-bonding –C O⋯H–N–); Elemental analysis: Found: C
–
9.011 (C9–1H, d, J = 8.4 Hz), 16.075 (sharp, intramolecular H-bonding
26.59%, N 9.23%. Calc. for C30H16N9Na3O12S3: C 41.92%, N 14.66%;
HR mass: Found m/z 792.02374 (Mꢀ H)ꢀ . Calc. for C30H19N9O12S3
792.0242 (Mꢀ H)ꢀ .
–
–
C
C
O⋯H–N); Elemental analysis: Found: C 27.13%, N 4.64%. Calc. for
20H12N3Na3O10S3: C 38.78%, N 6.78%; HR mass: Found m/z 551.9852
(Mꢀ H)ꢀ . Calc. for C20H15N3O10S3 551.9842 (M ꢀ H)ꢀ .
2.3.5. 5-[(4-chloro-6-methoxy-1, 3, 5-triazin-2-yl)amino]-4-hydroxy-3-
[(1-sulfo-2-naphthyl)azo]naph-thalene- 2,7-disulfonic acid (Dye 2A)
NaHCO3 (15.14 g) was added to cold methanol (150 mL) at 10 ◦C,
followed by cyanuric chloride (27.67 g, >99%; 0.15 mol) within 10 min
while stirring. The mixture was maintained at 20 ◦C-25 ◦C for 1 h. Water
(150 mL) was added, and the resulting mixture was stirred for further 2
h. The white dichloro methoxyl triazine (DCT-OCH3) product was iso-
lated by filtration and directly used for the following reaction [7].
DCT-OCH3 was added to an aqueous solution of Compound 3 (66.36 g,
0.12 mol), and the pH was adjusted to 6.5–7.0 by adding 40 mL of a 15
wt% Na2CO3 aqueous solution. The mixture was heated to 40 ◦C-45 ◦C
for 3 h at pH 6.5–7.0 with monitoring via TLC (Dye 2A, Rf = 0.55).
Salting out with 10% w/v sodium chloride (120 g) afforded a dark red
solid, which was isolated by filtration and oven-dried at 50 ◦C to give the
product (121 g, 86% conversion). The organic purity of Dye 2A was
determined via HPLC to be 90.95% (RT = 18.436 min). The e. a. content
of dye was calculated to be 64.66%.
2.3.2. 5-[(4,6-dichloro-1,3,5-triazin-2-yl)amino]-4-hydroxy-3-[(1-sulfo-
2-naphthyl)azo]naphthalene-2,7-disulfonic acid (Compound 4, DCT dye)
Cyanuric chloride (22.00 g, 99%, 0.119 mol) was added to an ice-
cold solution of Compound 3 (0.117 mol, pH 8) in 900 mL water
under stirring. The reaction temperature was maintained at 15 ◦C-18 ◦C
and the pH at 5.5–6.0 over 2–3 h; then, 45 mL of a 15% w/v sodium
carbonate solution was added. The reaction was monitored via TLC until
Compound 3 completely disappeared and Compound 4 was obtained.
Due to its high instability, Compound 4 was used immediately for the
next reaction without isolation.
2.3.3. 5-[[4-chloro-6-(cyanoamino)-1,3,5-triazin-2-yl]amino]-4-hydroxy-
3-[(1-sulfo -2-naphthyl)azo]naphthalene-2,7-disulfonic acid (Dye 1A)
Cyanamide (5.57 g, 95%, 0.126 mol) was added to a solution of
Compound 4 (0.117 mol in 850 mL water). The mixture was heated to
50 ◦C-55 ◦C for 4 h under stirring while maintaining the pH at 8.5–9.0 by
adding 80 mL of a 15 wt% Na2CO3 aqueous solution. When TLC moni-
toring (Dye 1A, Rf = 0.43) demonstrated that the reaction had
completed, the reaction solution was cooled down to room temperature,
and 15% w/v of sodium chloride (140 g) was slowly added. The
resulting precipitate was collected by filtration to give a red solid, which
was dried at 50 ◦C to give Dye 1A as a red crude product (114 g, 83%
conversion). The organic purity of Dye 1A was determined to be 96.02%
via HPLC (RT = 21.296 min). The e. a. content of dye was calculated to
be 65.92%.
IR (KBr): –NH, –OH stretching 3550–3200 cmꢀ 1 (broad, s); aromatic
2
–C C– bending 1632 cmꢀ 1, 1550 cmꢀ 1 (m, m); sp -C O stretching
–
–
–
–
1211 cmꢀ 1 (s), sp3-C–O– stretching 1051 cmꢀ 1 (s); 1H NMR (DMSO‑d6):
δ 4.133 (–OCH3, 3H, s), 7.491–7.594 (C3,7,8-3H, cm), 7.674 (C2–1H, s),
7.915 (C6–1H, d, J = 7.8 Hz), 7.991 (C5–1H, d, J = 9.2 Hz), 8.734
(C4–1H, d, J = 9.2 Hz), 9.085 (C9–1H, d, J = 8.6 Hz), 9.281 (C1–1H,
–
broad), 13.363 (s, azo hydrazone –NH–N C–), 16.533 (sharp, intra-
–
–
molecular H-bonding –C O⋯H–N–); Elemental analysis: Found: C
–
24.43%, N 7.04%. Calc. for C24H14ClN6Na3O11S3: C 37.78%, N 11.01%;
IR (KBr): –NH, –OH stretching 3550–3200 cmꢀ 1 (broad, s); nitrile
HR mass: Found m/z 230.98740 (Mꢀ 3H)3ꢀ . Calc. for C24H17ClN6O10S3
stretching 2179 cmꢀ 1 (m), aromatic –C C– bending 1617 cmꢀ 1, 1566
230.98626 (Mꢀ 3H)3ꢀ
.
–
–
2
ꢀ 1
cmꢀ 1 (m, m); sp -C O stretching 1210 cm (s), sp3-C–O– stretching
–
–
1052 cmꢀ 1 (s); 1H NMR (DMSO‑d6): δ 7.478–7.570 (C3,7,8-4H, complex
multiplets), 7.570 (C2–1H, s), 7.903 (C6–1H, d, J = 7.8 Hz), 7.972
(C5–1H, d, J = 9.2 Hz), 8.748 (C4–1H, d, J = 9.1 Hz), 9.075 (C9–1H, d, J
= 8.7 Hz), 9.339 (broad, C1–1H), 12.719 (broad, azo hydrazone
2.3.6. 1-[4-[[8-hydroxy-3, 6-disulfo-7-[(1-sulfo-2-naphthyl) azo]-1-
naphthyl]amino]-6-methoxy-1,3,5- triazin-2-yl]pyridin-1-ium-3-
carboxylate (Dye 2B)
Dye 2B was prepared from Dye 2A (0.103 mol) as follows: Dye 2A
was dissolved in 1400 mL water, followed by the addition of nicotinic
acid (73.60 g, >99%, 0.60 mol). The pH was adjusted to 6.0 by adding
80 mL of a 15 wt% Na2CO3 aqueous solution. The resultant solution was
heated to 60 ◦C-65 ◦C for 7 h. When TLC monitoring (Dye 2B, Rf = 0.33)
demonstrated that the reaction had completed, the mixture was salted
–
–
–NH–N C–), 16.425 (sharp, intramolecular H-bonding –C O⋯H–N–);
–
–
Elemental analysis: Found:
C
24.58%,
N
9.75%. Calc. for
C
24H12ClN8Na3O10S3: C 37.29%, N 14.5%; HR mass: Found m/z
234.31660 (Mꢀ 3H)3ꢀ
.
Calc. for C24H15ClN8O10S3 234.31812
(Mꢀ 3H)3ꢀ
.
3

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
out with 10% w/v sodium chloride (160 g) to give a red solid, which was
isolated by filtration and oven-dried at 50 ◦C to produce Dye 2B as a red
crude product (105 g, 95% conversion). The organic purity of the
product was determined to be 84.26% via HPLC (RT = 14.124 min). The
e. a. content of dye was calculated to be 79.25%.
(35 g), and the resulting precipitate was isolated by filtration to give a
red solid, which was oven-dried at 50 ◦C to give the product (17.52 g,
95% conversion). The organic purity of the product was determined to
be 94.71% via HPLC (RT = 12.803 min). The e. a. content of dye was
calculated to be 75.34%.
IR (KBr): –NH, –OH, –COOH stretching 3550–3200 cmꢀ 1 (broad, s);
IR (KBr): –NH, –OH, –COOH stretching 3650–3200 cmꢀ 1 (broad, s);
2
2
aromatic –C C– bending 1630 cmꢀ 1, 1557 cmꢀ 1 (m, m); sp -C
O
aromatic –C C– bending 1631 cmꢀ 1, 1557 cmꢀ 1 (m, m); sp -C
O
–
–
–
–
–
–
–
–
stretching 1214 cmꢀ 1 (s), sp3-C–O– stretch 1053 cmꢀ 1 (s); 1H NMR
(D2O): δ 4.332 (-OCH3, 3H, s), 7.311 (C3–1H, s), 7.427 (C2–1H, s),
7.464–7.607 (C8, C7–2H, cm), 7.633 (C6–1H, d, J = 8 Hz), 7.694
(C5–1H, d, J = 9.2 Hz), 7.777 (C3′-1H, t, J = 6 Hz), 7.946 (C4–1H, d, J =
6.4 Hz), 8.032 (C2′-1H, d, J = 8.8 Hz), 8.460 (C9–1H, d, J = 8.4 Hz),
stretching 1208 cmꢀ 1 (s), sp3-C–O– stretching 1047 cmꢀ 1 (s); 1H NMR
(D2O): δ 3.783 (–SO2–CH3, 3H, s), 3.790 (–N–CH3, 3H, s), 7.453 (C3–1H,
s), 7.476–7.645 (C7,8-2H, cm), 7.508 (C2–1H, s), 7.676 (C6–1H, d, J =
7.6 Hz), 7.766 (C5–1H, d, J = 9.2 Hz), 7.903 (C3′-1H, t, J = 6.8 Hz),
8.126 (C4–1H, d, J = 9.2 Hz), 8.169 (C2′-1H, d, overlapped with C4),
8.510 (C9–1H, d, J = 8.8 Hz), 9.126 (C1–1H, s), 9.700 (C1′-1H, s),
9.193 (C1–1H, s), 9.518 (C1′-1H, s), 9.997 (C4′-1H, d, J = 4.4 Hz),
′
–
–
–
12.799 (s, azo hydrazone –NH–N N–), 15.887 (s, intramolecular H-
10.072 (C4 -1H, d, J = 6 Hz) 12.998 (s, azo hydrazone –NH–N N–),
–
–
–
–
–
bonding –C O⋯H–N–); Elemental analysis: Found: C 33.61%, N
16.016 (s, intramolecular H-bonding –C O⋯H–N–); Elemental anal-
9.17%. Calc. for C30H18N7Na3O13S3: C 42.41%, N 11.54%; HR mass:
Found m/z 782.02686 (Mꢀ H)ꢀ . Calc. for C30H21ClN7O13S3 782.02867
(Mꢀ H)ꢀ .
ysis: Found: C 30.27%, N 9.16%. Calc. for C31H21N8Na3O14S4: C 40.18%,
N 12.09%; HR mass: Found m/z 859.02167 (Mꢀ H)ꢀ . Calc. for
C
31H24N8O14S4 859.02220 (Mꢀ H)ꢀ .
2.3.7. 5-[[4-chloro-6-[methyl(methylsulfonyl)amino]-1,3,5-triazin-2-yl]
amino]-4-hydroxy-3-[(1-sulfo-2-naphthyl)azo]naphthalene-2,7-disulfonic
acid (Dye 3A)
2.3.9. 5-[[4-chloro-6-(N-methylanilino)-1,3,5-triazin-2-yl]amino]-4-
hydroxy-3-[(1-sulfo-2-naphthyl) azo]naphthalene-2,7-disulfonic acid (dye
4A)
3A was prepared according to the procedure reported by B. Stephen
et al. [10]. A solution of N-methyl methanesulfonamide (6.35 g, >99%,
0.058 mol) in water (8 mL) was added to a sodium hydroxide (2.33 g,
>99%, 0.058 mol) solution in water (3 mL). Toluene (20 mL) was added,
and water was azeotropically removed. Then, cyanuric chloride (10.8 g,
>99%, 0.058 mol) was added, and the mixture was stirred below 3 ◦C for
8 h, followed by stirring at room temperature for 12 h to give a sus-
pension. A solid was filtered out from the suspension, and toluene was
removed in vacuum to give 2,4-dichloro-6-(N-methanesulphonyl)
methylamino s-triazine as a white crude solid (13.45 g). Subsequently,
an aqueous solution of Compound 3 (0.029 mol) was added to a solution
of 2,4-dichloro-6-(N-methanesulphonyl)methylamino s-triazine (9.70 g,
0.042 mol) in acetone (25 mL) under stirring. The pH was adjusted at
7.0–8.0 by adding 15 mL of a 15% w/v Na2CO3 aqueous solution, and
the mixture was heated to 50 ◦C for 6 h until TLC monitoring (3A, Rf =
0.56) demonstrated complete consumption of Compound 3. The reac-
tion solution was then filtered for the removal of insoluble materials, the
filtrate was allowed to cool to room temperature, and 15% w/v sodium
chloride (75 g) was added. The resulting precipitate was collected and
oven-dried to give the title product as a red solid (19.14 g, 52% con-
version). The organic purity of the product was determined to be
96.24% via HPLC (RT = 18.09 min). The e. a. content of dye was
calculated to be 66.48%.
N-Methyl aniline (8.66 g, 99%, 0.080 mol) was slowly added to an
aqueous solution of Compound 4 (0.078 mol). The pH of the reaction
mixture was adjusted to 7.5–8.0 by sodium ash powder. The resultant
solution was heated to 45 ◦C-50 ◦C for 4 h while being monitored via TLC
(Dye 4A, Rf = 0.58). By salting out with 15% w/v sodium chloride (135
g), a dark red solid was produced, which was isolated by filtration and
oven-dried at 50 ◦C to give Dye 4A as a red solid (79 g, 74% conversion).
The organic purity of the product was determined to be 98.81% via
HPLC (RT = 25.02 min). The e. a. content of dye was calculated to be
61.39%.
IR (KBr): –NH, –OH stretching 3500–3200 cmꢀ 1 (broad, s); aromatic
2
–C C– bending 1616 cmꢀ 1, 1562 cmꢀ 1 (m, m); sp -C O stretching
–
–
–
–
1234, 1209 cmꢀ 1 (s), sp3-C–O– stretching 1052 cmꢀ 1 (s); 1H NMR
(DMSO‑d6): δ 3.624 (N-methyl,-3H, s), 7.343–7.572 (N-phenyl-5H,
C7–1H, C8–1H, total 7H, cm), 7.535 (C3–1H, s), 7.592 (C2–1H, s), 7.985
(C6–1H, d, J = 7.8 Hz), 7.963 (C5–1H, d, J = 9.2 Hz), 8.752 (C4–1H, d, J
= 9.2 Hz), 9.076 (C9–1H, d, J = 8.7 Hz), 9.456 (C1–H1, broad), 13.000
–
–
(azo hydrazone –NH–N C–, broad), 16.470 (NH, sharp, intramolecular
–
–
H-bonding –C O⋯H–N–); Elemental analysis: Found: C 26.39%, N
7.17%. Calc. for C30H19ClN7Na3O10S3: C 42.99%, N 11.70%; HR mass:
Found m/z 256.00357 (Mꢀ 3H)3ꢀ
.
Calc. for C30H22ClN7O10S3
256.00202 (Mꢀ 3H)3ꢀ
.
IR (KBr): –NH, –OH, –COOH stretching 3650–3200 cmꢀ 1 (broad, s);
2.3.10. 1-[4-[[8-hydroxy-3,6-disulfo-7-[(E)-(1-sulfo-2-naphthyl)azo]-1-
naphthyl]amino]-6-(N-methyl-anilino)-1,3,5-triazin-2-yl]pyridin-1-ium-3-
carboxylate (Dye 4B)
2
aromatic –C C– bending 1635 cmꢀ 1, 1559 cmꢀ 1 (m, m); sp -C
O
–
–
–
–
stretching 1228 cmꢀ 1 (s), sp3-C–O– stretching 1046 cmꢀ 1 (s); 1H NMR
(DMSO‑d6): δ 3.512 (–SO2–CH3, 3H, s), 3.658 (–N–CH3, 3H, s),
7.469–7.571 (C3,7,8-3H, cm), 7.631 (C2–1H, s), 7.892 (C6–1H, d, J =
7.6 Hz), 7.962 (C5–1H, d, J = 9.2 Hz), 8.736 (C4–1H, d, J = 9.2 Hz),
9.076 (C9–1H, d, J = 8.4 Hz), 9.127 (C1–1H, broad), 13.410 (broad, azo
Dye 4B was prepared from Dye 4A (0.058 mol) as follows: Dye 4A
was dissolved in water (1000 mL), followed by the addition of nicotinic
acid (57.68 g, >99%, 0.460 mol). The pH was adjusted to 5.5–6.0 by
adding 160 mL of a 15 wt% Na2CO3 aqueous solution. The resultant
solution was heated to 60 ◦C-65 ◦C for 40 h while being monitored via
TLC (Dye 4B, Rf = 0.5). The mixture was then salted out with 5% w/v
sodium chloride (60 g) to give a red solid, which was isolated by
filtration and oven-dried at 50 ◦C to give Dye 4B as a red product (41 g,
62% conversion). The organic purity of the product was determined to
be 100% via HPLC (RT = 17.110 min). The e. a. content of dye was
calculated to be 81.35%.
–
hydrazone –NH–N N–), 16.537 (sharp, intramolecular H-bonding
–
–
–C O⋯H–N–); Elemental analysis: Found: C 23.76%, N 8.11%. Calc.
–
for C25H17ClN7Na3O12S4: C 35.74%, N 11.67%; HR mass: Found m/z
256.6527 (Mꢀ 3H)3ꢀ . Calc. for C25H20ClN7O12S4 256.6508 (Mꢀ 3H)3ꢀ
.
2.3.8. 1-[4-[[8-hydroxy-3,6-disulfo-7-[(1-sulfo-2-naphthyl)azo]-1-
naphthyl]amino]-6-[methyl(methyl-sulfonyl) amino]-1,3,5-triazin-2-yl]
pyridin-1-ium-3-carboxylate (Dye 3B)
IR (KBr): –NH, –OH stretching 3500–3200 cmꢀ 1 (broad, s); aromatic
2
–C C– bending 1634 cmꢀ 1, 1544 cmꢀ 1 (m, m); sp -C O stretching
–
–
–
–
3A (0.015 mol) was dissolved in water (300 mL), nicotinic acid (7.67
g, 99%, 0.062 mol) was slowly added, and the pH was adjusted to
5.5–6.0 by adding 20 mL of a 15 wt% Na2CO3 aqueous solution. The
resultant solution was heated to 50 ◦C-55 ◦C for 7–8 h. When TLC
monitoring (Dye 3B, Rf = 0.47) indicated that the reaction had
completed, the mixture was salted out with 10% w/v sodium chloride
1238 cmꢀ 1 (s); 1H NMR (D2O): δ 3.846 (C12–3H, s), 7.531–7.726
(C5–1H,C6–1H,C7–1H,C8–1H, N-phenyl-5H, in total 9H, cm), 7.442
(C3–1H, s), 7.609 (C2–1H, s), 7.986 (C3′-1H, t, J = 6 Hz), 8.260 (C4–1H,
d, J = 8.8 Hz), 8.301 (C2′-1H, d, J = 7.2 Hz), 8.488 (C9–1H, d, J = 8.8
Hz), 9.480 (C1–1H, s), 9.318 (C1′-1H, s), 10.036 (C4′-1H, d, J = 5.2 Hz),
4

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
7.877 (C6–1H, d, J = 8.0 Hz), 7.897 (C5–1H, d, J = 9.2 Hz), 8.723
(C4–1H, d, J = 9.2 Hz), 9.042 (C9–1H, d, J = 8.8 Hz), 9.358 (C1–1H,
Table 2
λmax, ε max and full width at half maximum (FWHM) of 10 model dyes.
–
sharp), 12.261 (NH, sharp, azo hydrazone hydrogen –NH–N C–),
–
Dye
λmax nm (in
water)
εmax
Organic purity
(HPLC %)
FWHM
(nm)
(dm3molꢀ 1cmꢀ 1
)
–
–
16.338 (NH, sharp, intramolecular H-bonding –C O⋯H–N–);
Elemental analysis: Found:
C
24.60%,
N
6.95%. Calc. for
1A
1B
2A
2B
3A
3B
4A
4B
5A
5B
547
546
545
546
546
546
547
548
546
546
40011
38595
39594
38192
38612
41119
40901
37995
40760
40653
96.02
93.51
90.95
84.26
96.24
94.71
98.81
100.00
99.45
97.14
86.08
87.93
87.61
92.11
89.19
92.06
93.48
89.72
87.66
87.40
C23H12ClN6Na3O11S3: C 36.88%, N 11.22%; HR mass: Found m/z
226.31537 (Mꢀ 3H)3ꢀ
.
Calc. for C23H15ClN6O11S3 226.31437
(Mꢀ 3H)3ꢀ
.
2.3.12. 1-[4-hydroxy-6-[[8-hydroxy-3,6-disulfo-7-[(E)-(1-sulfo-2-
naphthyl)azo]-1-naphthyl]amino]-1,3,5-triazin-2-yl]pyridin-1-ium-3-
carboxylate (Dye 5B)
An aqueous solution of Dye 5A (0.021 mol, 300 mL) was prepared by
dissolving the dye in water under stirring, followed by the addition of
nicotinic acid (9.78 g, 99%, 0.079 mol). The resultant solution was
heated to 60 ◦C-65 ◦C for 8 h while being monitored via TLC (Dye 5B, Rf
= 0.42), and the pH was adjusted to 5.5–6.0 by adding 20 mL of a 15 wt
% Na2CO3 aqueous solution. The temperature was then decreased to
40 ◦C, and 10% w/v sodium chloride (39 g) was added. The resulting red
solid was collected via filtration and re-dissolved in water (300 mL) at
room temperature. Sodium chloride (5% w/v, 15 g) was then added,
affording a red solid that was isolated by filtration and oven-dried at
50 ◦C to give Dye 5B (19.28 g, 83% conversion). The organic purity of
the product was determined to be 97.14% via HPLC (RT = 15.62 min).
The e. a. content of dye was calculated to be 75.38%.
–
12.832 (–NH, weak, –NH–N N–), 16.103 (–NH, weak, intramolecular
–
–
–
H-bonding of –C O⋯H–N–); Elemental analysis: Found: C 38.04%, N
9.94%. Calc. for C36H23N8Na3O12S3: C 46.76%, N 12.12%; HR mass:
Found m/z 857.07519 (Mꢀ H)ꢀ . Calc. for C36H26N8O12S3 857.075409
(Mꢀ H)ꢀ .
2.3.11. 5-[(4-chloro-6-hydroxy-1,3,5-triazin-2-yl)amino]-4-hydroxy-3-
[-(1-sulfo-2-naphthyl)azo] naphthalene-2,7-disulfonic acid (Dye 5A)
Na2CO3 (15 g, >99%, 0.118 mol) was added to 1000 mL aqueous
solution of Compound 4 (0.053 mol), and the mixture was heated to
80 ◦C under stirring for 2 h. When TLC monitoring (Dye 5A, Rf = 0.50)
demonstrated that the reaction had completed, the mixture was allowed
to cool down to room temperature, and 16% w/v sodium chloride (176
g) was slowly added. The resulting red solid was collected by filtration
and oven-dried at 50 ◦C to give Dye 5A as a red product (44.82 g, 75%
conversion). The organic purity of the product was determined to be
99.45% via HPLC (RT = 17.92 min). The e. a. content of dye was
calculated to be 66.70%.
IR (KBr): –NH, –OH stretching 3600–3200 cmꢀ 1 (broad, s); aromatic
ꢀ 1
2
–C C– bending 1630 cm (m); sp -C O stretching 1214 cmꢀ 1 (s), sp3-
–
–
–
–
C–O– stretching 1049 cmꢀ 1 (s); 1H NMR (DMSO‑d6): δ 7.489 (C3–1H, s),
7.494 (C2–1H, s), 7.468–7.576(C7 and C8–2H, cm), 7.895 (C6–1H, d, J
= 7.6 Hz), 7.964 (C5–1H, d, J = 9.2 Hz), 8.258 (C3′-1H, t, J = 7.2 Hz),
8.739 (C4–1H, d, J = 9.2 Hz), 9.062 (C9–1H, d, J = 8.8 Hz), 9.138 (C2′-
1H, d, J = 7.6 Hz), 9.404 (C1–1H, sharp), 9.988 (C4′-1H, d, J = 6.4 Hz),
′
–
–
IR (KBr): –NH, –OH stretching 3600–3200 cmꢀ 1 (broad, s); aromatic
10.228 (C1 -1H, s), 12.542 (NH, sharp, azo hydrazone –NH–N C–),
–
–
–C C– bending 1634 cmꢀ 1, 1617 cmꢀ 1 (m, m); sp -C O stretching
2
16.412 (NH, sharp, intramolecular H-bonding –C O⋯H–N–);
–
–
–
–
1214 cmꢀ 1 (s), sp3-C–O– stretching 1049 cmꢀ 1 (s); 1H NMR (DMSO‑d6):
δ 7.419 (C3–1H, s), 7.449 (C2–1H, s), 7.469–7.557 (C7 and C8–2H, cm),
Elemental analysis: Found:
C
31.42%,
N
8.83%. Calc. for
C29H16N7Na3O13S3: C 41.68%, N 11.73%; HR mass: Found m/z
Table 3
Chemical shifts (ppm) of aromatic protons for 1A–5A (X = Cl) in DMSO‑d6.
H1
H2
H3
H4
H5
H6
H7
H8
H9
–
–
1A (Y NHCN)
9.339
9.281
9.127
9.456
9.358
7.570
7.674
7.631
7.592
7.449
7.542
7.571
7.524
7.535
7.419
8.748
8.734
8.736
8.752
8.723
7.972
7.991
7.962
7.963
7.897
7.903
7.915
7.892
7.985
7.877
7.478–7.524
7.491–7.594
7.469–7.571
a7.343–7.572
7.469–7.557
9.075
9.085
9.076
9.076
9.042
–
–
2A (Y OCH3)
–
–
3A (Y NCH3SO2CH3)
–
–
4A (Y N-methyl phenyl)
–
–
OH)
5A (Y
a
Five aromatic protons of the N-methyl phenyl moiety are also in the same area.
Table 4
Chemical shifts (δ ppm) of the aromatic protons of 1B–5B (X =
).
–
–
(in D2O)
–
–
–
–
–
–
–
–
1B(Y NHCN)
2B(Y OCH3)
3B(Y NCH3SO2CH3) (in D2O)
4B(Y NCH3phenyl) (in D2O)
5B(Y OH)
(in DMSO‑d6)
(in DMSO‑d6)
H1
H2
H3
H4
H5
H6
H7
H8
H9
H1’
H2’
H3’
H4’
9.315
7.566
9.193
7.427
9.126
7.508
9.480
7.609
7.442
8.260
9.404
7.494
7.513
7.311
7.453
7.489
8.753
7.946
8.126
8.739
7.967
7.694
7.766
7.531–7.726 (including five protons of N-phenyl)
7.964
7.897
7.633
7.676
7.895
7.480–7.539
7.464–7.607
7.476–7.645
7.468–7.576
9.062
10.202
9.177
8.269
9.970
8.460
9.518
8.032
7.777
9.997
8.510
9.700
8.169
7.903
10.072
8.488
9.318
8.301
7.986
10.036
9.062
10.228
9.138
8.258
9.988
5

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
Fig. 2. Plots of the logarithms of the relative concentration (A/A0) against time at 40 ◦C for 1A, 2A, 3A, 4A and 5A.
768.01210 (Mꢀ H)ꢀ . Calc. for C29H19N7O13S3 768.012474 (Mꢀ H)ꢀ .
triazinyl dyes (NTR dyes) have a difference of only ±1 nm (close to the
resolution of the spectrometer). Therefore, the absence of colour change
when the chloro group was replaced with nicotinic acid on the triazinyl
dye can be considered. The molar extinction coefficient (ϵmax) of the
model dyes were between 38000 and 41119, which are typical values for
red chromophores based on naphthyl-azo-naphthalene sulfonic acid
dyestuff. Meanwhile, the full width at half maximum (FWHM) indicates
the shade brightness. The data demonstrate that the dyes with higher
organic purity (by HPLC) afforded narrower FWHM values.
2.4. Kinetic study
The kinetic study was conducted by modifying the OECD Guideline
111 (2004) ‘Hydrolysis as a function of pH’ [13]. Hydrolysis of the
model dyes (Table 1) was performed by mixing approximately 5 × 10ꢀ 5
mol dye with 5 × 10ꢀ 3 mol reagent-grade sodium carbonate in water
(100 mL, pH = 10.5) at 40 ◦C [14,15]. Samples (0.5 mL) were taken at
suitable intervals and quenched with a pre-cooled 0.1 N HCL aqueous
solution (0.5 mL), giving a pH of 6.0–7.0. The samples (1 mL) were kept
in an ice bath before HPLC analysis.
3.2. Synthesis and 1H NMR spectroscopic characterisation of the dye
molecules
3. Results and discussion
Monochlorotriazines containing cyanoamino, N-methyl phenyl and
hydroxy substituents in the ‘second leg’ were prepared from the reaction
of an aqueous solution of dichlorotriazine (Compound 4) with cyana-
mide, N-methyl aniline and hydroxide, respectively, at 45 ◦C-55 ◦C in
the presence of 15 w/w% sodium carbonate (aq.) as acid scavenger. N-
methylphenylchlorotriazine (4A) and hydroxychlorotriazine (5A) were
prepared under a normal pH condition, whereas the reaction of
3.1. Optical properties of model dyes
The optical properties of the 10 model dyes are presented in Table 2.
The values of the wavelength of the maximum absorbance (λmax) of the
monochloro-s-triazinyl dyes (MCT dyes) and the m-carboxypyridium-s-
6

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
Fig. 3. Plots of the logarithms of the relative concentration (A/A0) against time at 40 ◦C for 1B, 2B, 3B, 4B and 5B.
water. The reaction route to methoxychlorotriazine dye is well known
and consists of a condensation of cyanuric chloride with aqueous
methanol in the presence of sodium hydrogen carbonate to form
methoxydichloro-s-triazine, followed by a second condensation reaction
with Red base (Compound 3) to obtain methoxychlorotriazine (2A) with
high yield and purity [7]. Likewise, sulfonamides are insufficiently
nucleophilic to react cleanly with dichlorotriazine, and competitive
hydrolysis to hydroxychlorotriazine occurs. This undesired side product
cannot be easily removed when mixed with the desired product. To
confirm this speculation, dichlorotriazine (Compound 4) was reacted
with N-methylmethanesulfonamide at pH 7.5–9.0 and temperature of
40 ◦C-50 ◦C. The product was analysed via HPLC, and it was found that
the hydrolysis compound (Dye 5A, 83.36% area by HPLC) was the major
product, and a small amount of the desired Compound 3A (10.23% area
by HPLC) was obtained. Therefore, the alternative synthetic route (see
2.3.7) was adopted via a condensation of cyanuric chloride with
Table 5
Pseudo-first-order hydrolysis rate constants of Dyes 1A, 2A, 3A, 4A and 5A.
MCT Dye
Overall reactivity
Half-life time (t )
R2 value
½
Rate constants (kobs
)
[hours]
[hoursꢀ 1
]
1A
2A
3A
4A
5A
4.15 × 10ꢀ 3
1.01 × 100
3.38 × 10ꢀ 1
4.38 × 10ꢀ 3
2.30 × 10ꢀ 4
168
0.68
2.00
168
0.9895
0.9945
0.9981
0.9607
0.9437
3000
Order of Reactivity: 2A > 3A > 4A > 1A > 5A
cyanamide with dichlorotriazine to produce cyanoaminochlorotriazine
(1A) was performed at a fairly high pH to partly ionise the cyanamide,
since the ionised form is the reactive nucleophile. Under these condi-
tions, the desired product is relatively stable as the chlorotriazine is
deactivated due to the ionisation of triazine–NH–CN. However, these
conditions were not suitable for the reaction of methanol and N-meth-
ylmethanesulfonamide HN(CH3)SO2CH3, probably due to the low
nucleophilicity of methanol and sulfonamides compared to that of
N-methylmethanesulfonamide sodium salt to form
a
dichlor-
o-6-(N-methanesulphonyl)methylamino s-triazine, followed by a second
condensation reaction with Red base (Compound 3) to obtain methyl
(methylsulfonyl)aminochlorotriazinyl Compound 3A with high purity
7

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
Scheme 1. Hydrolysis of Dyes 1A and 4A
compared with protons H2 and H3 due to the
effect of the triazinyl ring [16].
π-electron de-shielding
When D2O was used as a solvent for 2B, 3B and 4B, downfield pro-
tons H10 and H11 were replaced with deuterium, and the corresponding
resonances exhibited very low intensity in the 1H NMR spectra (see
Table 4).
3.3. Kinetic study
The hydrolysis reaction of the reactive dyes was considered to be a
pseudo-first-order reaction in the presence of a large excess of hydroxide
ions (pH 10.5). To quench the reaction, 0.1 N HCl solution was utilised
to adjust the pH to 6.0–7.0. The hydrolysis products were then deter-
mined via HPLC.
Scheme 2. Hydrolysis of Dye 2A
For the kinetic study, the following pseudo-first-order equation was
used:
log(At/A0 × 100%) = ꢀ a × t + b,
where t denotes the time [hour]; a, the slope [hꢀ 1]; b, the intercept; , A0,
the initial concentration of the dye; and At, the concentration of the dye
after t hour of the reaction. The rate constant (kobs) can be determined
using the following equation:
kobs = ꢀ a × 2.303 [hꢀ 1],
where 2.303 is the conversion factor between natural and base 10
logarithms.
The half-life time (t ), which is the time required for the concen-
½
tration of the reactive dye in the test substance to decrease to 50% from
its initial concentration for the first-order reaction, was calculated using
Scheme 3. Hydrolysis of Dye 3A
the equation t = ln (2/kobs). The concentration change of the dye was
½
calculated from the peak area percentage of the corresponding HPLC
chromatogram, and the logarithmic plot of log (A/A0) against time at
40 ◦C for the 10 model dyes (presented in Figs. 2 and 3) was used to
(93.64% area by HPLC).
For 1A, 2A, 3A, 4A, 5A, 1B and 5B, nice and clean 1H and 13C NMR
spectra were obtained in DMSO‑d6, whereas 2B, 3B and 4B afforded
messy spectra in the same solvent. Contrarily, clean NMR spectra were
obtained for 2B, 3B and 4B when D2O was used. The chemical shifts of
the aromatic protons of dyes 1A–5A are presented in Table 3. Azo dyes
are known to be in keto–enol tautomeric equilibrium, with the keto-
hydrazone form being highly predominant in DMSO‑d6. For example,
the 1H NMR spectrum of 1A exhibits one downfield proton at 16.410
ppm, which is attributed to the nitrogen proton in the H-acid ring, thus
calculate kobs and t . The R2 values obtained for the curves of the 10
½
model dyes confirmed the pseudo-first-order kinetic for this reaction.
The hydrolysis rate constants and t of the MCT dyes are presented in
½
Table 5. The methoxychlorotriazinyl compound (2A, OCH3/Cl) exhibi-
ted the highest rate constant for the disappearance of the starting ma-
terial (kobs), followed by the methyl (methylsulfonyl)amino
chlorotriazinyl compound (3A, N(CH3)SO2CH3/Cl). The cyanoamino-
chlorotriazinyl (1A, NHCN/Cl) and N-methylphenylchlorotriazinyl (4A,
–
forming a hydrogen bond with the –C O group, affording a six-
–
N(CH3)phenyl/Cl) compounds exhibited similar reactivity and t for the
½
membered ring, and another at 12.704 ppm due to the nitrogen pro-
ton of the hydrazone. In the same spectrum, three singlet peaks can be
attributed to the protons in the H-acid ring. Significant downfield shifts
are observed for the H1 proton in the ortho position to the amino group
Table 7
Pseudo-first-order hydrolysis rate constants of NTR Dyes (1B–5B) at 40 ◦C.
NTR Dye
Overall reactivity
Half-life time (t )
R2 value
½
Rate constants (kobs
)
[hours]
[hoursꢀ 1
]
Table 6
Rate constants of 2A and 3A.
1B
2B
3B
4B
5B
3.42 × 10ꢀ 1
2
0.52
0.52
1
0.9983
0.9930
0.9910
0.9945
0.9967
1.34 x 100
MCT Dye
Rate constants [hoursꢀ 1
]
Ratio of rate constants
1.32 x 100
kobs
KCl
KOCH3
KNMMS
KCl/KOCH3
KCl/KNMMS
4.70 × 10ꢀ 1
7.65 × 10ꢀ 2
9
2A
3A
1.01 × 100
0.245
0.218
0.765
–
0.12
0.32:1
–
3.38 × 10ꢀ 1
–
–
1.82:1
Order of reactivity: 2B > 3B > 4B > 1B > 5B
8

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
Scheme 4. Hydrolysis of NTR dyes 1B, 2B, 3B, 4B and 5B
similar; however, the displacement of methoxide was about six times
faster than that of methyl (methylsulfonyl)amino. This may be due to the
steric hindrance of the methyl (methylsulfonyl)amino group, which
renders it less prone to undergo nucleophilic attack by the hydroxide
group.
Table 8
Reactivity enhancement from MCT (A series) to NTR (B series)
dyes.
k obs of Dye B / k obs of Dye A
Ratio
83
kobs 1 B/kobs 1A
The rate constants (kobs) of the NTR dyes (1B, 2B, 3B, 4B and 5B) are
presented in Table 7. The HPLC data confirmed that the hydrolysis re-
action of the NTR dyes mainly proceeded by the displacement of nico-
tinic acid by the hydroxide group (see Scheme 4). From Tables 5 and 7, it
can be seen that the reactivity of the NTR dyes was faster than that of the
MCT dyes, except for Dye 2B. However, the degree of rate enhancement
when comparing chloride- and nicotinic acid-substituted dyes dramati-
cally varies. For example, the reactivity of Dye 3B was four times faster
than that of Dye 3A. Nevertheless, Dyes 1B, 4B and 5B were 83, 107 and
kobs 2 B/kobs 2A
1.32
4
k
obs 3 B/kobs 3A
kobs 4 B/kobs 4A
obs 5 B/kobs 5A
107
333
k
Table 9
Hammett substituent constants (σ meta) of the ‘second-leg’ substituents.
2nd leg substituent
‘
σ
’
meta
-NHCN/–(NCN)-
0.21/expected negative value
333 times faster than Dyes 1A, 4A and 5A, respectively ([NHCN] knic
/
-OCH3
0.11
kCl ~ 83, [OCH3] knic/kCl = 1.32, [NMMS] knic/kCl = 3.9, [N(CH3)
Phenyl] knic/kCl = 107, [OH] knic/kCl = 333; see Table 8). The order of
the overall reactivity (kobs) of the MCT and NTR dyes followed the same
sequence: 2A > 3A > 4A > 1A > 5A and 2B > 3B > 4B > 1B > 5B. These
results may be partly due to the relative efficiency of the substituents as
leaving groups. The pka values of the cyanoamino and hydroxy protons
in 1A, 1B, 5A and 5B were simulated using the Instant Cheminformatics
Solution-Chemicalise tool [17], affording the values 10.93, 10.62, 7.29
and 1.55, respectively. It can be deduced that both the hydroxyl and
cyanoamino groups would be deprotonated in alkali aqueous solution.
The resulting negatively charged substituents, i.e. Oꢀ and –(NCN)ꢀ ,
would disfavour the nucleophile attack of the hydroxyl group to the
reactive chloro or nicotinic acid group as the triazine ring would be more
electron-rich. This explains why Dyes 5A and 5B have the lowest reac-
tivity, followed by Dyes 1A and 1B, probably because of the partial
ionisation of the cyanoamino group at pH 10.5, which would render the
triazine ring more electron-rich, thus deactivating the reactive chloro or
nicotinic acid group towards nucleophile attack. Contrarily, the most
reactive dyes were 2A, 2B, 3A and 3B, which have electron-withdrawing
groups that render the triazine ring more electron-poor, therefore
favouring the nucleophile attack by OHꢀ .
-N(CH3)SO2CH3
-NHC6H5/-N(C6H5)2/-N(CH3)2
-OH/-O-
0.29
ꢀ 0.12/ꢀ 0.07/ꢀ 0.15
0.13/ꢀ 0.47
hydrolysis reaction. As expected, a very small amount of the hydrolysis
product of the hydroxylchlorotriazinyl compound (5A, OH/Cl) was ob-
tained due to the deactivation of the chloride reactive group by the
strong electron-donating substituent Oꢀ in the triazine ring.
The displacement of chloride in the hydrolysis reaction was detected
for Dyes 1A and 4A (Scheme 1), as demonstrated by HPLC and HR mass
spectrometry. According to a previous study [15], the hydrolysis of Dye
2A proceeds via the displacement of chloride (KCl) and displacement of
methoxide (KOCH3) in an alkali aqueous solution (see Scheme 2). In this
study, it was found that the hydrolysis of Dye 3A also involved two
competing reaction steps, i.e. displacement of chloride (KCl) and
displacement of N-methyl (methylsulfonyl)amino (KNMMS) (see Scheme
3).
The individual rate constants KCl, KOCH3 and KNMMS were calculated
from a statistical average of the ratio between the amount of Dyes 2A or
3A that underwent chloride displacement and that with methoxide
(–OCH3) or methyl (methylsulfonyl)amino (–N(CH3)SO2CH3) displace-
ment obtained via HPLC. The rate constant ratios KCl/KOCH3 and KCl/
To confirm the above hypothesis, the Hammett substituent constants
[18],
σ meta, of the ‘second-leg’ substituents were compared (see
K
NMMS are presented in Table 6. The results indicate that the displace-
ment of methoxide was faster than that of chloride in Dye 2A, whereas
the displacement of chloride was faster than that of methyl (methyl-
sulfonyl)amino in Dye 3A. The KCl values of Dyes 2A and 3A were
Table 10
Calculated electron density of the triazine carbon atom of Dyes 1A, 2A, 3A, 4A
and 5A.
MCT Dye
Electron density
X
C2
Y
C3
1A
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ꢀ 0.001
ꢀ 0.093
ꢀ 0.006
ꢀ 0.002
ꢀ 0.036
0.000
-NHCN
= N–CN
-OCH3
0.014
0.121
0.195
0.136
0.051
0.219
0.251
1Aa(resonance)
2A
3A
-NCH3SO2CH3
4A
-N-methyl phenyl
5A
-OH
5Aa(resonance)
ꢀ 0.142
= O
a
Fig. 4. Triazine carbon labels of the 10 model dyes.
Deprotonation structure of NHCN and OH in alkali solution.
9

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
Fig. 5. The Structures and resonance structures of 1A and 5A
Table 9). High values of
σ
indicate strong electron-withdrawing
meta
Table 11
property. It was found that the –OCH3 and –N(CH3)SO2CH3 sub-
stituents exhibited stronger electron-withdrawing ability than the
others, which can explain the higher reactivity of 2A, 2B, 3A and 3B. It
was assumed that the –NHCN group of 1A and 1B would be partially
ionised in alkali solution, as in the case of the –OH group. Thus, the
value of σmeta of –(NCN)ꢀ would be expected to be negative, which in-
dicates electron-donating property. The σmeta values of the phenyl-
–N–Me group of 4A and 4B would be in-between those of –NHC6H5, –N
(C6H5)2 and –N(CH3)2. The least-reactive group would be the –OH
substituent of 5A and 5B, which has the strongest electron-donating
group (–Oꢀ ). As a result, the σmeta values follow the order –N(CH3)
SO2CH3 > –OCH3 > –N-methyl aniline > –(NCN)ꢀ > –Oꢀ , which
Electron density of triazine carbon atom of Dyes 1B, 2B, 3B, 4B and 5B.
NTR Dye
Electron density
X
C2
Y
C3
1B
Nic
Nic
Nic
Nic
Nic
Nic
Nic
ꢀ 0.068
ꢀ 0.273
ꢀ 0.082
0.026
-NHCN
= N–CN
-OCH3
0.034
0.245
0.210
0.258
0.057
0.217
0.253
1Ba(resonance)
2B
3B
-NCH3SO2CH3
4B
5B
ꢀ 0.095
ꢀ 0.142
ꢀ 0.210
-N-methyl phenyl
-OH
5Ba(resonance)
= O
a
Deprotonation structure of NHCN and OH in alkali solution.
Fig. 6. The structures and resonance structures of 1B and 5B
10

H.-C. Huang and C.-G. Wu
Dyes and Pigments 188 (2021) 109147
approximately matches the trend observed for the hydrolysis constant
(Kobs): Dye (OCH3) > Dye (N(CH3)SO2CH3) > Dye (N-methyl aniline) >
Dye (NHCN) > Dye (OH).
Acknowledgements
The Authors wish to express our gratitude to the Minister of Science
and Technology of R.O.C (Taiwan), National Central University, Taiwan
and Everlight Chemical Industrial Corporation for chemicals and
financial support. The authors thank to Dr. John A. Taylor for his
continuing encouragement and advice in the kinetic study of this paper.
The electronic effect of the substituents on the rate constant was also
investigated by quantum mechanics calculation of the model dyes (see
Fig. 4). The modelling data presented in Table 10 demonstrates the
electron density of the carbon atom bonded to the chloro atom in Dyes
1A, 2A, 3A, 4A and 5A. The electron density was found to increase in the
order 3A < 2A < 4A < 1A* < 5A* (1A* and 5A* are the resonance
structures of 1A and 5A in alkali solution, respectively, as presented in
Fig. 5). This sequence is nearly inverse to that of the Kobs value of the rate
constant (2A > 3A > 4A > 1A > 5A). The higher Kobs of 2A compared
with that of 3A is most likely due to the steric effects; –OCH3 is smaller
than –N(CH3)SO2CH3, and therefore, Dye 2A is more reactive than Dye
3A.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109147.
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
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11