The in situ generation and reactive quench of diazonium compounds in the synthesis of azo compounds in microreactors

Article abstract of DOI:10.3762/bjoc.12.186

In this paper, a micro-fluidic optimized process for the continuous flow synthesis of azo compounds is presented. The continuous flow synthesis of Sudan II azo dye was used as a model reaction for the study. At found optimal azo coupling reaction temperature and pH an investigation of the optimum flow rates of the reactants for the diazotization and azo coupling reactions in Little Things Factory-MS microreactors was performed. A conversion of 98% was achieved in approximately 2.4 minutes and a small library of azo compounds was thus generated under these reaction conditions from couplers with aminated or hydroxylated aromatic systems. The scaled up synthesis of these compounds in PTFE tubing (i.d. 1.5 mm) was also investigated, where good reaction conversions ranging between 66-91% were attained.

Full text of DOI:10.3762/bjoc.12.186

The in situ generation and reactive quench of diazonium
compounds in the synthesis of azo compounds in
microreactors
Faith M. Akwi and Paul Watts*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Nelson Mandela Metropolitan University, University Way, Port
Elizabeth, 6031, South Africa
doi:10.3762/bjoc.12.186
Received: 09 June 2016
Email:
Accepted: 14 August 2016
Published: 06 September 2016
Paul Watts* - Paul.Watts@nmmu.ac.za
* Corresponding author
This article is part of the Thematic Series "Automated chemical
synthesis".
Keywords:
azo coupling; diazotization; microreactor; scale up
Guest Editor: I. R. Baxendale
© 2016 Akwi and Watts; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
In this paper, a micro-fluidic optimized process for the continuous flow synthesis of azo compounds is presented. The continuous
flow synthesis of Sudan II azo dye was used as a model reaction for the study. At found optimal azo coupling reaction temperature
and pH an investigation of the optimum flow rates of the reactants for the diazotization and azo coupling reactions in Little Things
Factory-MS microreactors was performed. A conversion of 98% was achieved in approximately 2.4 minutes and a small library of
azo compounds was thus generated under these reaction conditions from couplers with aminated or hydroxylated aromatic systems.
The scaled up synthesis of these compounds in PTFE tubing (i.d. 1.5 mm) was also investigated, where good reaction conversions
ranging between 66–91% were attained.
Introduction
Going green, a familiar catch phrase in the chemical industry, in production of these compounds is detrimental to the environ-
addition to environment protection laws have influenced and ment and human health.
also triggered the development of cleaner methods of produc-
tion. The production of azo compounds is one controversial Following the principles of green chemistry [1,2] such as less
sector of the fine chemical industry; color is highly desired and hazardous chemical synthesis, efficient atom economy, reduc-
used in almost everything, but the waste generated from the tion of waste produced, some alternative cleaner methods for
1987

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
the synthesis of azo dyes have been developed. These methods room temperature even in their dry state. In addition, the
are however only representative of particular coupling agents method also facilitated short reaction times and provided high
and diazotized amines. Nonetheless they highlight the green yields (Scheme 3).
benefits that they offer.
A number of other methods have been developed for the green
For example Noroozi-Pesyan et al. synthesized azo dyes by synthesis of azo compounds [6-16]. Thus far, the above exam-
grinding derivatives of aniline with solid sodium nitrite in the ples concentrate on modifying the procedure of diazotization
presence of p-toluenesulfonic acid [3]. It was found that the and azo coupling (using compounds that impart green benefits
yield of isolated azo dyes obtained increased with an increase in to the process or make it environment friendly, i.e., short reac-
electron donor strength of the coupling compound. This method tion times thus less energy is required for the process, high yield
eliminates the use of alkaline and acidic solutions (Scheme 1).
thus low amount of waste generated, etc). Equally, process
equipment can also be changed or modified to achieve the
In another method developed by Rahimizadeh et al., ferric above mentioned green benefits in addition to other advantages.
hydrogen sulfate was used as a catalyst to synthesize azo com-
pounds from aromatic amines and 2-naphthol. The method For example, isolated diazonium salts are known to be
boasts of shorter reaction times [4] with high yields (Scheme 2). hazardous due to their explosive and unstable nature. However,
microreactor technology makes it possible to safely perform
Mirjalili et al. also used silica supported boron trifluoride and reactions with unstable intermediates [17] such as these, as well
was able to carry out diazotization at room temperature [5] after as those that give rise to explosive [18] and hazardous products
they discovered that the diazonium salts obtained were stable at [19]. The small reagent volumes used in microreactors also
Scheme 1: PTSA-catalyzed diazotization and azo coupling reaction.
Scheme 2: Ferric hydrogen sulfate (FHS) catalyzed azo compound synthesis.
Scheme 3: Synthesis of azo compounds in the presence of silica supported boron trifluoride.
1988

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
reduce the amount of acidic and alkaline waste associated with solved in the organic phase, ethyl acetate (C4H8O2) and 11 in
the synthesis of azo compounds during research and develop- the aqueous phase (Scheme 4). The large specific interfacial
ment.
areas and reduced molecular diffusion distances were found to
have played a role in avoiding the undesirable side reaction,
In the conventional way of performing reactions, the amount of thus increasing the atom economy in the reaction. This in turn
waste generated is dealt with at the end of the reaction. On the reduces the amount of waste generated after the reaction.
contrary, microreactor technology enables the reduction of A reaction conversion of almost 100% was attained in
waste generated by increasing the atom efficiency of reactions 2.3 seconds. The same reaction performed at a macro scale and
and in so doing, the quantity of starting materials is reduced in at a strong stirring rate providing a calculated specific interfa-
turn minimizing the amount of waste generated. This aspect of cial area of 40 cm2, gave a comparable conversion to that
microreactor technology will definitely prove to be quite impor- attained at a micro scale (calculated specific interfacial area of
tant in the synthesis of these compounds more so that their 80 cm2).
production, however important they are, has adverse effects on
the environment, mammals [20,21] and aquatic life. Even with a In the synthesis of azo dyes and pigments, the cost of produc-
number of dye degradation techniques [22] currently being em- tion and quality of the product cannot be over looked. Wille et
ployed in waste water treatment, a certain percentage of the al., in their investigative research involving the synthesis of two
dyestuffs is still found in water bodies. This therefore is motiva- azo pigments (yellow and red pigments) in microreactors [24],
tion for developing better methods or techniques or processes demonstrated that scaling out in the microreactors provided
that can be used independently or in conjunction with the better and more consistent quality of the pigments as compared
existing techniques. Microreactor technology is one such tech- to scale up in the batch vessels.
nology that can be used in the manufacture of these dyes. If
used in conjunction with existing azo dye degradation tech- Similarly, yellow pigment 12 (15) was also synthesized by
niques the amount of waste generated can easily be managed. Pennemann et al. (Scheme 5) using a micro-mixer apparatus
[25]; the group’s comparison of the results with the batch syn-
Hisamoto et al., for example, used ‘phase transfer synthesis’ in thesis of the said pigment 15 affirmed the notion that mixing is
micro-chips for a diazo-coupling reaction [23]. The authors did an essential unit operation in the synthesis of azo pigments.
not however employ a phase transfer catalyst, but rather the
principle to increase the reaction selectivity in the diazo cou- The pigment synthesized in a micro-mixer (25 µm channel
pling of 5-methylresorcinol (10) to p-nitrobenzene diazonium width) at a flow rate of 30 mL/min had smaller pigment size
tetrafloroborate (11) in a biphasic laminar flow reaction system. distribution compared to the batch synthesized pigment. The
The bi-phasic reaction media consisted of compound 10 dis- fast mixing in the micro-mixer was noted to be responsible for
Scheme 4: Phase transfer catalyzed azo coupling of 5-methylresorcinol in microreactors.
Scheme 5: Synthesis of yellow pigment 12 in a micro-mixer apparatus.
1989

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
the improvement of glossiness (73%) and tinctorial strength syringes, two syringe pumps and an LTF-MS microreactor
(66%) of the yellow pigment thus yielding a good quality placed into a temperature control bath (as shown in Figure 6 in
product.
the experimental section). Following the experimental proce-
dures outlined, a range of reaction conditions were evaluated,
With all the various applications of azo compounds previously where the pH value is that of the stock solution (Table 1).
mentioned, it is therefore important to develop an optimized
process for their synthesis. This was achieved with the use of
Table 1: Data showing conversions attained at different reaction condi-
the microreactor technology. Since the benefits of microreactor
technology are well documented in literature [26-30],
the ease of reaction parameter optimization in the synthesis of
azo compounds in microreactors is highlighted.
tions.
Flow rate
(mL/min)
Temperature Conversion
Run
pH
(°C)
(%)
1
5.11
5.11
5.11
5.11
5.11
5.11
5.11
5.11
5.11
5.11
7.39
7.39
7.39
7.39
7.39
7.39
7.39
7.39
7.39
10.83
10.83
10.83
10.83
10.83
10.83
0.6
0.37
0.03
0.13
0.37
0.7
7
59
62
64
63
57
55
57
54
56
56
70
78
76
73
76
72
74
82
77
72
69
69
73
71
64
2
26
25
40
46
26
43
2
In this study, the continuous flow synthesis of Sudan II azo dye
(19, Scheme 6) constituting of the diazotization of 2,4-
dimethylaniline (16) and its in situ azo coupling to 2-naphthol
(18) within LTF-MS microreactors was investigated.
3
4
5
6
7
0.6
8
0.37
0.13
0.37
0.6
Although various groups have investigated similar reactions in
microreactors, to the best of our knowledge, there is no detailed
study combining the effect of pH, temperature and flow rate on
the azo coupling reaction. In addition, industry always ques-
tions why reactions are done in micro structured reactors rather
than simple tubular reactors. As such, we extended this investi-
gation to study the optimized conditions obtained within the
LTF microreactors within PTFE tubing (i.d. 1.5 mm) in order to
scale up the synthesis and to see if the size effect had any major
implication on the reaction performance.
9
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
7
0.37
0.13
0.37
0.7
26
41
49
25
43
2
0.6
0.37
0.13
0.37
0.37
0.13
0.37
0.7
8
26
26
40
50
26
43
26
Results and Discussion
Azo coupling reaction in the synthesis of
1-((2,4-dimethylphenyl)azo)naphthalen-2-ol
in LTF-MS microreactors
0.6
0.37
In an effort to exemplify the azo coupling of phenols as well as
naphthol derivatives in alkaline reaction conditions, the synthe-
sis of 1-((2,4-dimethylphenyl)azo)naphthalen-2-ol (19) also A quadratic model was fitted onto the resultant calculated
commonly known as Sudan II azo dye was used as a model conversions of 2-naphthol as obtained from reversed-phase
reaction. The synthesis involved diazotization of 2,4-dimethyl- HPLC analysis output. STATISCA 12 – Statsoft program was
aniline (16) to form diazonium salt intermediate 17, which is thereafter used to validate the model fitted. The findings of this
coupled with 2-naphthol (18) under alkaline conditions investigation are presented here in the form of simple profile
(Scheme 6). The experimental set up comprised of two plots.
Scheme 6: Continuous flow synthesis of Sudan II azo dye in LTF-MS microreactors.
1990

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Effect of reaction temperature, flow rate and
pH on the reaction conversion
reaction conversion in the experimental domain employed for
this investigation. In contrast, pH is shown to be of importance
The pH is of utmost importance in the azo coupling reaction, in this reaction considering that the p-value associated with its
the second step in the synthesis of azo compounds. This reac- estimated coefficient (b) is less than 0.005. Furthermore, evi-
tion parameter is also dependent on the kind of coupling com- dence of a quadratic relationship between pH and reaction
pound used [31]. For example, phenols are successfully coupled conversion was also seen (pH^2, p < 0.005). Similarly, there
in alkaline conditions in which the phenolate ion is formed. was also a slight indication of a quadratic relationship between
These conditions provide the desired electron-releasing group temperature and the response (Temperature^2, p = 0.076).
thus facilitating the electrophilic substitution reaction to afford There were no interactions observed between the reaction pa-
the azo compound. However, highly alkaline conditions are rameters investigated.
usually avoided as they lead to diazonium salt decomposition.
In the plot (Figure 1), it is seen that at a flow rate of
In the regression summary of the statistical data analysis 0.03 mL/min, the pH has a non-linear relationship with the pre-
(Table 2) of the observed data, it is seen from the p-values dicted conversion of the coupler. In addition, the predicted
(0.067 and 0.084) associated with the estimated coefficients (b) conversion increases with an increase in reaction pH in this ex-
of flow rate and temperature respectively, that these two reac- perimental domain. The optimum pH that would provide the
tion parameters seem not to have a significant effect on the best conversion was predicted to be approximately 8.5 and the
Table 2: Statistical multiple regression analysis output.
Regression summary for dependent variable: conversion %
N = 25
b*
Std. Err. (of b*)
b
Std. Err. (of b)
t(19)
p-value
Intercept
pH
−42.4512
27.9443
-6.5854
0.3187
10.97819
2.85784
3.38369
0.17495
0.17949
0.00325
−3.86687
9.77812
0.001038
0.000000
0.066572
0.084257
0.000000
0.076223
7.54901
−0.15896
0.57407
0.772030
0.081676
0.315099
0.774875
0.311906
Flow rate
Temperature
pH^2
−1.94621
1.82187
−7.02309
−0.58488
−1.6268
−0.0061
−9.06351
−1.87518
Temperature^2
Figure 1: pH profile plot at constant flow rate of 0.03 mL/min.
1991

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
most suitable reaction temperature was found to be 25 °C. acidic reaction conditions. The synthesis of 4-(2-(4-nitrophen-
Therefore, at a pH of about 8.5 and a reaction temperature of yl)diazenyl)-N-phenylbenzenamine was an interesting choice
25 °C, a reaction conversion of approximately 80% should be for demonstrating the synthesis of azo compounds involving
attained. As expected, at highly alkaline conditions (pH 9–11), couplers containing aminated aromatic systems.
the reaction conversion is predicted to gradually decrease. It is
also seen that at 0 °C and 50 °C, slightly lower conversion is It involves the diazotization of p-nitroaniline 2 to form 20
predicted (approximately 74%–75%). This could be attributed which is subsequently coupled to 21. Diphenylamine (21), the
to decomposition of the reaction intermediate at higher tempera- coupler used in this reaction is sparingly soluble in aqueous
tures (50 °C) and lowered rate of reaction at lower tempera- media thus rendering it quite difficult to use in this synthesis
tures (0 °C).
and as such, methanol was used as the azo coupling reaction
media (Scheme 7). It is due to this that the pH of the diazonium
At a higher flow rate of 0.7 mL/min (Figure 2), a similar trend compound solution was buffered to the preferred values for the
is observed with regard to the effect of pH on the reaction reaction investigation as opposed to the conventional buffering
conversion. Similarly, a reaction temperature of 25 °C was pre- of the coupler solution. Following the experimental procedures,
dicted to provide optimum conversion (75%). At this reaction a range of reaction conditions were evaluated (Table 3).
temperature (25 °C), the predicted reaction conversion at this
flow rate, is slightly lower (75%) compared to that predicted at A Logit model was then fitted onto the resultant calculated
a flow rate of 0.03 mL/min (80%). It was then concluded that conversions of diphenylamine as obtained from reversed phase
the flow rate of the reactants in this experimental domain had HPLC analysis output. STATISCA 12 – Statsoft program was
no significant effect on the reaction response. For information, thereafter used to validate the model fitted (see Supporting
this flow rate range covers a residence time of 0.37 to Information File 1). The result of this investigation is also
0.86 minutes.
presented here in form of simple profile plots.
Azo coupling reaction in the synthesis of
Effect of reaction pH, temperature and flow
4-(2-(4-nitrophenyl)diazenyl)-N-phenyl-
rate on the reaction conversion
benzenamine in LTF-MS microreactors
The synthesis of azo compounds involving couplers containing
Azo coupling reactions involving aromatic amines as coupling
agents are carried out in mildly acidic conditions such that a
water-soluble protonated version of the aromatic amine, which
aminated aromatic systems is usually carried out in slightly
Figure 2: pH profile plot at a constant flow rate of 0.7 mL/min.
1992

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Scheme 7: Azo coupling reaction under acidic conditions.
on this response (pH^2, p < 0.005). Conversely, within this ex-
perimental domain, there is evidence (Flow rate, p = 0.116) that
shows that the flow rate of the reactants has a negligible effect
on the reaction conversion. Looking at the p-value associated
with the estimated coefficient (b) of the reaction temperature
(temperature, p = 0.012), there is a slight indication of its signif-
icance on the reaction conversion.
Table 3: Data showing the conversion attained at various reaction
conditions.
Flow rate
(mL/min)
Temperature Conversion
Run
pH
(°C)
(%)
1
3.5
3.5
0.6
0.37
0.13
0.37
0.7
6
84
89
85
83
78
83
84
83
83
92
93
91
90
96
86
97
96
99
81
74
86
70
86
56
58
64
86
2
26
41
47
26
41
3
3
3.5
4
3.5
It is also demonstrated in Figure 3 that the effect of pH on the
azo coupling reaction is quite conspicuous. It is seen that the
reaction flourishes at a mildly acidic pH that being between 5.5
and 6.0 while temperature is predicted not to have a tremen-
dous effect on the reaction conversion (Figure 3). A variation in
reaction temperature between 0 °C and 50 °C provides a very
slight improvement in conversion (from 82% to 95%).
5
3.5
6
3.5
0.6
7
3.5
0.37
0.13
0.37
0.6
8
3.5
7
9
3.5
26
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
5.66
5.66
5.66
5.66
5.66
5.66
5.66
5.66
5.66
6.94
6.94
6.94
6.94
6.94
6.94
6.94
6.94
6.94
0.37
0.13
0.37
0.7
26
43
50
25
42
2
The flow rate of the reactants (diazotized primary amine and
coupler) was also investigated as mentioned earlier on. As is
shown in Figure 4, based on the predicted conversion, there is
no difference in carrying out the reaction at either 0.03 mL/min
or 0.7 mL/min.
0.6
0.37
0.13
0.37
0.6
7
26
7
Moving on to the reaction temperature, unlike pH, it has a more
or less linear relationship with predicted reaction conversion.
There is a slight drop in the predicted conversion as the reac-
tion temperature is increased from 0 °C to 50 °C. This is clearly
shown in Figure 5 below.
0.37
0.13
0.7
25
8
43
2
0.13
0.37
0.37
0.6
26
7
50
43
An increase in the flow rate from 0.03–0.7 mL/min at a con-
stant pH of 5.66 causes a 2% drop in predicted conversion
whereas an increase in the reaction temperature from 0–50 °C at
flow rates of 0.03, 0.33 and 0.7 mL/min at constant pH 5.66 is
0.37
is more reactive than its unprotonated version is availed; it is shown to lead to a less than 5% drop in the predicted conver-
also obvious that the protonation also renders the aromatic ring sion. It was thereafter concluded that in this experimental
less nucleophilic.
domain, the reaction temperature had no significant effect on
the reaction conversion. For both reactions investigated thus far,
In this reaction, like the one previously discussed, pH plays an it was found that the flow rate of the reactants in the chosen ex-
important role and significantly affects the reaction conversion perimental domain had little or no significant effect on the reac-
(p < 0.005) moreover (Table 4), it also has a non-linear effect tion conversion.
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Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Table 4: Statistical multiple regression analysis output.
Regression summary for dependent variable: logit
N = 24
b*
Std. Err. (of b*)
b
Std. Err. (of b)
t(19)
p-value
Intercept
pH
8.14853
−4.48258
0.01204
0.61041
0.43744
1.409724
0.583899
0.004340
0.370851
0.056793
5.78023
−7.67697
2.77534
1.64596
7.70238
0.000014
0.000000
0.012053
0.116214
0.000000
−9.27407
0.29741
0.17639
9.30475
1.208038
0.107162
0.107167
1.208036
Temperature
Flow rate
pH^2
Figure 3: pH profile plot at a constant flow rate of 0.03 mL/min.
The effect of reaction parameters i.e. flow rate, temperature and attempt to perform both reaction steps involved in this synthe-
pH on azo coupling reactions in the synthesis of azo com- sis in continuous flow reactors was thus made. This was
pounds under acidic and alkaline reaction conditions in LTF- achieved in LTF-MS reactors with the aid of statistical
MS microreactors however, was successfully demonstrated. We modeling where the continuous flow synthesis of Sudan II azo
therefore went ahead to fully make good use of the benefits that dye was optimized and used a model reaction.
microreactor technology offers to organic syntheses such as this
one by performing both reaction steps in continuous flow reac- Based on the statistical experimental central composite design
tors.
used for the optimization of this synthesis, no descriptive trends
showing the effect of the flow rates of reactants on the conver-
sion of 2-naphthol could be obtained. At all varied reaction pa-
rameters in the 20 experiments carried out, the response was
Continuous flow synthesis of Sudan II azo
dye in LTF-MS microreactors
Having determined the reaction parameters that affect the azo relatively the same with no clear cut trends observed as is
coupling reaction in the synthesis of Sudan II azo dye, an shown in Table 5. A quadratic regression model was then fitted
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Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Figure 4: pH profile plot at constant flow rate of 0.7 mL/min.
Figure 5: Temperature profile plot at constant pH 5.66.
onto the observed data and no outliers were found during this independent variables (see Supporting Information File 1). On
model fitting procedure in addition, there were also no indica- further statistical analysis of this data, it was found that there is
tions of either synergistic or antagonist interactions between the some evidence that supports the notion that the flow rate of the
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Beilstein J. Org. Chem. 2016, 12, 1987–2004.
The continuous flow synthesis of
2-naphtholic, phenolic and similar azo
Table 5: Data showing conversions attained at various reaction condi-
tions.
compounds in LTF-MS microreactors
At the reactant flow rates stated in the confirmatory experiment
in Table 6, similar azo compounds were synthesized. To our
delight, the reaction conditions were robust since comparable
high conversions were also attained for the synthesis of similar
azo compounds regardless of the substituent groups present on
the coupler as well as diazotizable amine. This is shown in
Table 7.
Amine + HCl Sodium nitrite Coupler Conversion
Run
(mL/min)a
(mL/min)b
(mL/min)c
(%)
1
0.20
0.09
0.30
0.26
0.20
0.26
0.13
0.26
0.20
0.13
0.20
0.20
0.20
0.20
0.13
0.20
0.20
0.13
0.20
0.26
0.01
0.03
0.03
0.02
0.03
0.04
0.02
0.04
0.03
0.04
0.05
0.03
0.03
0.03
0.04
0.03
0.03
0.02
0.03
0.02
0.07
0.07
0.07
0.03
0.07
0.03
0.10
0.10
0.07
0.10
0.07
0.12
0.01
0.07
0.03
0.07
0.07
0.03
0.07
0.10
98
97
97
99
98
99
93
98
98
96
96
95
96
95
96
95
95
98
98
97
2
3
4
5d
6
7
The continuous flow scaled-up synthesis of
2-naphtholic, phenolic and similar azo
compounds in LTF-MS microreactors
8
9d
10
11
12
13
14d
15
16d
17d
18
19d
20
In the synthesis of azo compounds, there is usually formation of
a precipitate, which renders this reaction problematic in
microreactors due to blockages. The geometrical specifications
of the microchannel are very important, so much that these
dictate the concentration of the reagents used for the reaction,
amount of solvent added to facilitate quick dissolution of partic-
ulates as they are formed as well as the output of the reaction,
i.e., conversion, yield and or even selectivity. The scaled-up
synthesis of these compounds in PTFE tubing of 1.5 mm
internal diameter is herein reported. The experimental set up of
the scaled-up synthesis is shown in the experimental section
(Figure 8). A comparison of the two microreactor systems, i.e.,
LTF-MS and the PTFE tubing microreactor systems is
presented in Table 8. The reactions were conducted at appro-
priate flow rates so as to maintain the same residence time used
in the LTF-MS microreactors.
aAmine-flow rate of HCl + amine (2,4-dimethylaniline) solution, bnitrite-
flow rate of sodium nitrite solution, ccoupler-flow rate of coupler
(2-naphthol) solution. dcenter point.
(amine + HCl) solution in this particular experimental domain
has the most effect on the conversion (Partial correlation: 0.406,
p = 0.094). The R-square value corresponding to the three vari-
ables investigated was also found to be nil which indicated that
there was no correlation between the variables investigated.
It was found that the synthesis of these compounds in a simple
set up comprising of PTFE tubing (i.d. 1.5 mm) provided
The reaction conditions at the center point (run with letter 'd' in comparable conversions to those attained in the LTF-MS
the Table 5 above) of the central composite design used for the microreactors despite the geometrical specifications of the two
optimization were therefore used to generate a small library of reactor systems being quite different. It should also be noted
compounds. Before embarking on this task, a confirmatory ex- that the PTFE tubing is much cheaper than the glass LTF-MS
periment to ascertain the reproducibility of the reaction output and at the internal diameter of the PTFE tubing used there were
at these reaction conditions was performed and indeed a similar no occurrences of blockages witnessed due to precipitate forma-
result was obtained (Table 6).
tion. This is particularly important since the amount of solvent
used in the reactions can be kept at a bare minimum. Despite
the fact that the geometrical specifications of the two reactor
systems (LTF-MS microreactors and PTFE tubing) were quite
different, the drop in conversion is not massive and can be
circumvented by increasing the residence time in order to
achieve even better conversions.
Table 6: Confirmatory experiment for the chosen reaction parameters
for the synthesis.
Amine + HCl Sodium nitrite Coupler Conversion
Run
(mL/min)a
(mL/min)b
(mL/min)c
(%)
1
2
0.2
0.2
0.03
0.03
0.07
0.07
98
98
Conclusion
From our proof of concept investigation, the desired tempera-
ture and pH in the azo coupling of hydroxylated or aminated
couplers in the synthesis of azo compounds was determined. It
aAmine-flow rate of HCl + amine (2,4-dimethylaniline) solution, bnitrite-
flow rate of sodium nitrite solution, ccoupler-flow rate of coupler
(2-naphthol) solution.
1996

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Table 7: Azo compounds synthesized under alkaline and acidic azo coupling conditions.
Run
1
Diazotizable amine
Coupler
Product
Conversion
97%
2
3
88%
95%
4
5
80%
92%
6
80%
7
88%
8
84%
1997

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Table 7: Azo compounds synthesized under alkaline and acidic azo coupling conditions. (continued)
9
87%
79%
10
11
12
89%
90%
13
88%
14
82%
was found that at slightly alkaline conditions (pH 8.55) and at a due to precipitation of the product in the microreactor channels.
temperature of 25 °C, excellent conversions were attained in the This was particularly observed in the synthesis of 1-((2,4-
azo coupling reaction of the diazonium salt solution of 2,4- dimethylphenyl)azo)naphthalen-2-ol at slow flow rates, temper-
dimethylaniline to 2-naphthol whereas the azo coupling reac- atures close to 25 °C and pH greater than 7. Nonetheless, the
tion of the diazonium salt solution of p-nitroaniline to diphenyl- effect of the reaction parameters on the azo coupling reaction in
amine was found to thrive at a pH of 5.71 and at a temperature the synthesis of azo compounds was shown. To the best of our
of 25 °C. It should be noted that the data generated was ob- knowledge, this investigation is the first of its kind to expound
tained in a very short time. On the down side, for a couple of the effect of pH, flow rate and temperature on the azo coupling
experiments, samples could not be collected due to the limita- reaction in the synthesis of azo compounds. A simple and fast
tions of the microreactor used. There were blockages observed continuous flow process was also thereafter developed for the
1998

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Table 8: Comparison of reaction conversion attained from two continuous flow reactor systems.
Run
Diazotizable amine
Coupler
Product
Conversion
LTF-MS
PTFE
(CD: 1.0 mm)
(i.d.: 1.5 mm)
1
2
97%
88%
90%
91%
3
95%
69%
4
5
80%
92%
71%
70%
6
80%
70%
7
88%
67%
8
89%
80%
1999

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Table 8: Comparison of reaction conversion attained from two continuous flow reactor systems. (continued)
9
90%
72%
10
88%
78%
11
82%
66%
synthesis of naphtholic, phenolic and similar azo dyes. The coupling component solutions) were fed to an LTF-MS
robustness of the process was clearly demonstrated. In addition, (Volume: 0.2 mL, channel size: 1 mm, geometry: 115 × 60 ×
an easy scale-up strategy was also established where it was 6 mm) microreactor plate (Figure 6). The microreactor plate
found that the synthesis of these compounds in a simple contin- was dipped into a temperature control bath and delivery of the
uous flow set up consisting of T-mixers and PTFE tubing (i.d.: reactants to the plates was enabled by two Chemyx fusion 100
1.5 mm) provided relatively satisfactory reaction conversions classic syringe pumps.
moreover no occurrence of blockages was observed when this
set up was in use. This finding is of importance especially when
it comes to an increasing reaction throughput by the numbering
up technique. Ideally, in evaluating the performance of two
reactor systems in a chemical synthesis, it is important to keep
most factors constant especially those pertaining to the geome-
try of the reactor channel as this can affect the reaction output.
Albeit comparable reaction conversions were attained from the
two reactor systems investigated, it is worth determining the
role that the difference in geometrical structure had to play in
the data observed in this study.
Experimental
All chemicals and solvents used were of analytical grade. 1H
and 13C NMR spectra were recorded on a Bruker Avance-400
(400 MHz).
Figure 6: Schematic representation of the microreactor set up.
Microreactor set up; azo coupling reactions in
the synthesis of azo compounds
Preparation of reactant solutions
Using two (1 mL) SGE glass syringes and PTFE tubing of Solution A (diazotized primary aromatic amine): 2,4-dimethyl-
0.5 mm internal diameter, the reactants (diazonium salt and azo aniline (0.2918 g) was dissolved in approximately 0.8 mL of
2000

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
Microreactor set up; continuous flow
concentrated 32% HCl and cooled. To this cooled solution,
3 mL of cold sodium nitrite solution (0.29 g in 5 mL of distilled synthesis of azo compounds
water) was added drop wise until the potassium starch iodide Using three (5 mL) SGE glass syringes and PTFE tubing (i.d.
paper test was positive. DMF (20 mL) was added to this, after 0.5 mm, length: 340.2 mm connecting from the first reactor
which the solution was made up to a volume (100 mL) with plate to the second reactor plate and 380.7 mm connecting from
distilled water.
the second reactor plate to the sample collection bottle) reactant
solutions A (amine + HCl solution), B (sodium nitrite solution)
Solution B (coupler): 2-naphthol (0.35 g) was dissolved in 10% and C (coupler) were fed into two LTF-MS microreactor plates
aqueous NaOH (10 mL) to which DMF (15 mL) was added. (reactor volume: 0.2 mL, channel size: 1 mm, geometry: 115 ×
The pH of the solution was buffered to the appropriate pH 60 × 6 mm) joined by PTFE tubing (i.d. 0.5 mm). The delivery
intended for the investigation (pH 5.11, 7.39 and 10.83). The of the reactants was enabled by three Chemyx Fusion 100
solution was then made up to a volume (50 mL) with distilled classic syringe pumps as shown in Figure 7. The reaction tem-
water.
perature for the diazotization (0 °C) and azo coupling (25 °C)
reactions was maintained with the aid of an ice and water bath
Similarly, the reactant solutions A and B (diazotized primary ar- respectively.
omatic amine and coupler) in the azo coupling of diazonium salt
solution of p-nitroaniline to diphenylamine were prepared as
follows:
Solution A (diazotized primary aromatic amine): p-nitroaniline
(0.2918 g) was dissolved in hot concentrated 32% HCl (0.8 mL)
and cooled. To this cooled solution, cold sodium nitrite solu-
tion (0.29 g in 5 mL of distilled water) was added drop wise
until the potassium starch iodide paper test was positive. DMF
(20 mL) was added to this after which the pH of the solution
was buffered to afford the various working pH intended for the
study (pH: 3.5, 5.66 and 6.94). The solution was then made up
to a volume (100 mL) with distilled water.
Solution B (coupler): diphenylamine (0.35 g) was dissolved in
methanol (50 mL).
Figure 7: Schematic representation of the microreactor set up.
Azo coupling reactions in microreactors
A central composite experimental design with a total of 12 ex-
periments was used for each of the optimization studies at the Preparation of reactant solutions
various pH levels for the two reactions. The experiments were Solution A (amine + HCl solution): 2,4-dimethylaniline
performed in a randomized manner. In addition, the flow rate of (0.2918 g) was dissolved in approximately 0.8 mL of concen-
the solutions A and B was also varied as is shown in Table 9 trated 32% HCl. DMF (20 mL) was added to this, after which
below. The temperature of the batch diazotization reaction was the solution was made up to a volume (100 mL) with distilled
kept constant at 0 °C while that of the azo coupling reaction.
water.
Solution B (sodium nitrite solution): Sodium nitrite (0.2914 g)
was dissolved in DMF (5 mL) and made up to a volume
(50 mL) with distilled water.
Table 9: Experimental domain.
Reaction parameters
Minimum
Maximum
Solution C (coupler): 2-naphthol (0.35 g) was dissolved in 10%
aqueous NaOH (10 mL) to which DMF (15 mL) was added.
The pH of the solution was adjusted to 8.55–9 with 10% glycine
solution. The solution was then made up to a volume (50 mL)
with distilled water.
Flow rate (mL/min)
Temperature (°C)
0.03
0
0.7
50
2001

Beilstein J. Org. Chem. 2016, 12, 1987–2004.
For the azo coupling reactions performed in acidic reaction
media, the reactant solution C was prepared as follows.
Table 10: Experimental domain.
Reaction parameters
Minimum
Maximum
Solution C (coupler): 1-naphthylamine (0.3477 g) was dis-
solved in glacial acetic acid (10 mL of 10% glacial acetic acid
solution) and buffered to a pH of approximately 5.77–6. The
solution was then made up to volume (50 mL) with distilled
water.
HCl + amine (mL/min)
Sodium nitrite (mL/min)
Coupler (mL/min)
0.09
0.01
0.01
0.3
0.05
0.12
Microreactor diazotization and consequent
azo coupling reactions
1.5 mm, length: 150.93 mm and 155.43 mm for the diazotiza-
tion and azo coupling reactions, respectively). Using three SGE
A central composite experimental design with a total of 20 ex- glass syringes and two 3-Way-Tee mixers (Omnifit labware,
periments was used for this optimization study, where the Pore size: 8.0 mm i.d., 0.5-4 mm OD), the delivery of reactant
diazotization of 2,4-dimethylaniline and its in situ azo coupling solutions A (amine + HCl solution), B (sodium nitrite solution)
to 2-naphthol was used as a model reaction. The experiments and C (coupler) into the PTFE tubing (i.d. 1.5 mm) was enabled
were performed in a randomized manner. The temperature of by three Chemyx Fusion 100 classic syringe pumps (Figure 8).
the diazotization reaction was kept constant at 0 °C while that The reaction temperature for the diazotization reaction was kept
of the azo coupling reaction was kept at 25 °C (Table 10).
at 0 °C with the aid of ice. The azo coupling reaction was per-
formed at room temperature (25 °C).
Scaled up microreactor diameter set up:
PTFE tubing i.d. 1.5 mm
Sample preparation
Keeping the residence time established in the optimization reac- The microreactor set up was stabilized for 10 minutes between
tions constant, the scale up was carried out in PTFE tubing (i.d. each experiment. In order to obtain substantial amount of sam-
Figure 8: Scaled up microreactor set up: PTFE tubing i.d. 1.5 mm a) Chemyx Fusion 100 classic syringe pump, b) SGE glass syringe, c) T-mixer,
d) Ice bath: Diazotization reaction and e) Room temperature: Azo coupling reaction.
2002

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each in a sample vial containing HCl (0.2 mL of 1 M). The mix-
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C18 100 A (250 × 4.60 mm × 5 microns) column under the
following conditions; flow rate: 1.2 mL/min, mobile phase
(acetonitrile 0.1% formic acid (75:25)) equipped with a vari-
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external standard calibration HPLC method was used to quan-
tify the amount of coupler utilized in the reaction. The wave-
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Supporting Information File 1
Additional diagrams and NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-186-S1.pdf]
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doi:10.1021/op200347k
Acknowledgements
We would like to thank the South African National Research
Foundation, InnoVenton and Nelson Mandela Metropolitan
University for the financial support and Mr Coos Bosma for
statistical support.
27.De Zani, D.; Colombo, M. J. Flow Chem. 2012, 2, 5–7.
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