Ultrasonics Sonochemistry 16 (2009) 308–311

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Ultrasonics Sonochemistry

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Short Communication

Ultrasound assisted regioselective sulfonation of aromatic compoundswith sulfuric acid

Ziyauddin S. Qureshi, Krishna M. Deshmukh, Sachin R. Jagtap, Nitin S. Nandurkar, Bhalchandra M. Bhanage *

Department of Chemistry, Institute of Chemical Technology, University of Mumbai, N. Parekh Marg, Matunga, Mumbai 400 019, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 June 2008Received in revised form 4 October 2008Accepted 8 October 2008Available online 17 October 2008

PACS:*43.35.�c81.20.Ka

Keywords:UltrasoundSulfonationAromatic hydrocarbonSulfuric acid

1350-4177/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ultsonch.2008.10.001

* Corresponding author. Tel.: +91 22 24145616; faxE-mail address: bhalchandra_bhanage@yahoo.com

A simple and convenient methodology for selective sulfonation of aromatic compounds using sulfuricacid under sonication is described. The present methodology shows a considerable enhancement inthe reaction rate along with improved selectivity compared with the reactions performed under silentconditions. The effect of various parameters such as agitation speed, sulfuric acid concentration, and tem-perature on reaction system have been investigated and are explained on the basis of ultrasonically gen-erated cavitational effects.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The sulfonation of aromatic compounds is one of the importantreactions in organic transformations and many aromatic hydrocar-bons have been sulfonated [1]. Several methods have been reportedin the literature for the sulfonation of aromatic compounds such as,sulfonation of aromatic compounds with concentrated sulfuric acid[2], trifluoro acetic acid–sulfuric acid [3], sulfur trioxide in dichloro-methane [4], sulfonation of Grignard and lithium reagents [5],nucleophilic aromatic substitution with sodium sulfite [6], and sul-fonation with sulfur tri oxide–dioxane complex [7]. Recently silicasupported sulfuric acid sulfonation of aromatic compounds havebeen reported [8]. But the methods employed so far for the sulfona-tion of aromatic compounds have several disadvantageous such aslonger reaction time, high temperature, higher concentration andlower yield.

In recent years the use of ultrasound in organic transformationsis well known as it can enhance the rate, yield and selectivity of thereactions. It can also facilitate reactions at ambient conditionseliminating requirement of drastic conditions such as temperature,pressure or concentrations [9]. The driving force for the ultrasounddevelopments in organic synthesis has many facets, The increasingdemand for environmentally clean technologies that can mini-

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: +91 22 24145614.(B.M. Bhanage).

mizes the production of waste at source. Thus ultrasound may offercleaner reactions by improving product yield, selectivity, enhance-ment in product recovery and purification processes. Sonicationalso allows the use of non-activated and crude reagents as wellas an aqueous solvent system; therefore it is environment friendly.Ultrasound is widely used for improving the traditional reactionsthat require expensive reagents, strongly acidic conditions, longerreaction time, high temperatures, unsatisfactory yields and incom-patibility with other functional groups. Although the use of ultra-sound has been wide spread, the specific reasons for the rateenhancement in the presence of ultrasound are not known. In lit-erature various mechanisms such as single electron transfer, stresscreated due to implosion of cavities, an increase in the mass trans-fer coefficients due to increase in the contact surface area [10], hightemperature and pressures reached during cavitational collapseare held responsible for the spectacular effects observed as a resultof ultrasound. It has also been speculated that the free radicals pro-duced during cavitation initiate many reactions in the bulk, thusenhancing the rate of reactions [11]. The presence of a supercriticalregion at the interface of the solution and cavity is also supposed toenhance the rate of some reactions.

In continuation of our special interest in ultrasound assisted or-ganic transformations [12], we herein report for the first time sul-fonation of aromatic compounds using sulfuric acid as sulfonatingagent under solvent free conditions by sonication. The presentmethodology shows a considerable enhancement in the reaction

SO3H

+ Conc. H2SO4)))))

25 ºCRR

R = H, CH3, OCH3, -X, Ar etc

Scheme 1. Sulfonation of aromatic compounds under sonication.

Z.S. Qureshi et al. / Ultrasonics Sonochemistry 16 (2009) 308–311 309

rate along with improved selectivity compared with reactions per-formed under silent conditions.

2. Experimental

Fig. 1 shows a typical experimental setup used for ultrasound-mediated reactions. Various materials and equipment used in theexperimental methodology have been described in the followingsections.

2.1. General

All the chemicals were obtained from commercial source andused as it is. The progress of the reaction was monitored usingHPLC [Jasco LC-900 series with uv/vis detector (Jasco-975) at280 nm, Column (Jasco Model: Finebak SIL C8–10), Acetonitrile:water (60:40) as a mobile phase and 0.2% ammonium acetate buf-fer solution]. Mechanical agitator (six-blade, pitched-blade madeup of glass) with automated speed control was used. All productswere confirmed by LC using authentic standards and also by theirphysical constant reported in the literature [13].

2.2. Typical procedure for sonochemical sulfonation

In a typical reaction, a 100 ml round bottom flask equipped withmechanical stirrer was placed at the centre of ultrasonic bath hav-ing maximum cavitations. To it toluene (0.1 mol) was added andafterwards concentrated sulfuric acid (98 wt%, 0.12 mol) wasadded drop wise. The reaction mixture was then sonicated at roomtemperature (25 oC) up to desired time. The ultrasonic bath usedhad a frequency of 33 kHz and electric power rating of 100 W. Aftercompletion the reaction mixture was subjected to HPLC analysisand the experimental error is of ±5%.

3. Results and discussion

The sulfonation of aromatic compounds was performed usingsulfuric acid in presence of ultrasound and the results were com-pared with those obtained without ultrasound. The typical reactionscheme is shown in Scheme 1.

Two sets of experiments have been carried out: (1) experimentswere carried out under silent conditions with mechanical agitationand (2) same experiments were carried out under ultrasonic irradi-ation with mechanical agitation. Using this comparison the

Fig. 1. Experimental setup for sulfonation of aromatic compounds undersonication.

efficiency of using ultrasound over a conventional system can bequantified. The methodology was applied to various aromatic com-pounds and the results are summarized in Table 1. It has been ob-served that the sulfonation of toluene under silent conditionrequired 24 h for 70% conversion and the selectivity towards p-tol-uene sulfonic acid was up to 90% (Table 1 entry 1). However, whenthe same reaction was performed under sonication the reactionshowed a significant rate enhancement and the reaction got com-pleted within 45 min providing 76% conversion and the selectivitytowards p-toluene sulfonic acid was improved up to 100% (Table 1entry 2). The protocol was further extended to various activatedaromatic compounds such as o-xylene, m-xylene, p-xylene and ani-sole and all were found to provide good conversion and selectivity.The system was further extended to deactivated aromatic com-pound like chloro benzene and was also found to provide similarresults. The probable reason for the significant rate enhancementcould be due to the well-discussed cavitation effect observed undersonication. The cavitation effect, which plays an important role ofenhancing the interaction between the aromatic compound andsulfuric acid thus providing higher conversion, which is otherwisedifficult to achieve under silent condition.

The effects of various parameters such as agitation speed, sulfu-ric acid concentration, and temperature on reaction system havebeen investigated and are discussed as follows.

3.1. Effect of agitation speed

The agitation speed plays an important role under sonicationand showed interesting behavior. Here again the two sets of exper-iments were carried out to study the effects of agitation. (1) Exper-iments without agitation (2) experiments with agitation speed inthe range of 200–800 rpm (Fig 2). It was observed that in the ab-sence of agitation under sonication, the conversion of toluenewas 66% and did not proceed further. The reaction with mild agita-tion speed of 200–400 rpm shows increase conversion to 76%, indi-cating that there is a role played both by the microlevel masstransfer caused due to the turbulence created by ultrasound andmacrolevel mass transfer caused by agitator. However the majorrole is played due to the turbulence created by ultrasound. How-ever when the agitation speed was further increased to 600 and800 rpm conversion of toluene was lowered considerably (below50%) and the probable reason may be that, higher agitation speedmight be affecting the microlevel mass transfer generated by ultra-sound hindering the formation of active SO3H radicals responsiblefor sulfonation. This indicates that ultrasonic irradiation and agita-tion in combination does give synergistic effects up to certain agi-tation speed with major role played by ultrasound [12b].

3.2. Effect of sulfuric acid concentration

The effects of sulfuric acid concentration on the reaction systemhave been investigated and the results are presented in Table 2.Sulfonation of toluene with different mole ratio of concentratedsulfuric acid (0.8–1.4 mole) was carried out. It was observed thatwith increase in the concentration of sulfuric acid there is a consid-erable increase in the conversion of toluene toward p-toluene

Table 1Sulfonation of aromatic compounds with concentrated H2SO4 with and without ultrasound.

Entry Substrate Condition Time Conversion (%) Selectivity (%) Product Mp (oC)

1 Toluene Silent 24 h 70 90 Toluene-4-sulfonic acid 99–1022 Toluene ))))) 45 min 76 100 Toluene-4-sulfonic acid 99–1023 Benzene Silent 5 h 13 80 Benzenesulfonic acid 45–474 Benzene ))))) 45 min 28 100 Benzenesulfonic acid 45–475 Naphthalene Silent 5 h 11 87 Naphthalene-1-sulfonic acid 79–816 Naphthalene ))))) 45 min 66 100 Naphthalene-1-sulfonic acid 79–817 o-Xylene Silent 5 h 37 90 3,4-Dimethyl-benzenesulfonic acid 81–838 o-Xylene ))))) 45 min 70 100 3,4-Dimethyl-benzenesulfonic acid 81–839 m-Xylene Silent 5 h 10 90 2,4-Dimethyl-benzenesulfonic acid 61–6310 m-Xylene ))))) 45 min 82 100 2,4-Dimethyl-benzenesulfonic acid 61–6311 p-Xylene Silent 5 h 70 90 2,5-Dimethyl-benzenesulfonic acid 89–9112 p-Xylene ))))) 45 min 76 100 2,5-Dimethyl-benzenesulfonic acid 89–9113 Anisole Silent 5 h 60 84 4-Methoxy-benzenesulfonic acid 88–9014 Anisole ))))) 45 min 80 100 4-Methoxy-benzenesulfonic acid 88–9015 Chloro benzene Silent 5 h 2 70 4-Chloro-benzenesulfonic acid 85–8716 Chloro benzene ))))) 45 min 10 100 4-Chloro-benzenesulfonic acid 85–87

Reaction condition: substrate (0.1 mol); concentrated H2SO4 (98 wt%) (0.12 mol); agitation speed 200 rpm; temperature 25–30 oC; conversion is based on HPLC analysis.

Effect of agitation

0

20

40

60

80

100

0 200 400 600 800 1000

Agitation speed (rpm)

Con

vers

ion/

pro

duct

for

mat

ion

(%)

Fig. 2. Effect of agitation under sonication. Reaction condition: Toluene (0.1 mol);concentrated H2SO4 (98 wt%) (0.12 mol); temperature 25–30 oC; time 45 min.Conversion is based on HPLC analysis.

Table 2Effect of sulfuric acid concentration under sonication

Entry Toluene:H2SO4 Time (min) Conversion (%)

1 1:0.8 45 442 1:1 45 493 1:1.2 45 764 1:1.4 45 79

Reaction condition: toluene (0.1 mol); concentrated H2SO4 (98 wt%) moles of sul-furic acid to 1.25, 1, 0.83, 0.71, 0.62, agitation speed 200 rpm; temperature 25–30 �C; Conversion is based on HPLC analysis.

Table 3Effect of temperature under sonication.

Entry Temperature (�C) Time (min) Conversion (%)

1 25–30 45 762 50–55 45 783 80–85 45 43

Reaction condition: toluene (0.1 mol); concentrated H2SO4 (98 wt%) (0.12 mol);agitation speed 200 rpm; time 45 min. Conversion is based on HPLC analysis.

310 Z.S. Qureshi et al. / Ultrasonics Sonochemistry 16 (2009) 308–311

sulfonic acid. The 1:1.2 mole ratio of toluene: H2SO4 was found tobe effective. Further increase in mole ratio of sulfuric acid with re-spect to toluene did not have any effect on the reaction system.

3.3. Effect of temperature

To study the effect of temperature on sulfonation of toluenethree sets of experiments were carried out in the temperaturerange 25–30, 50–55, 80–85 oC under sonication and the resultsare presented in Table 3. The effect of temperature was done tonullify the error of rise in temperature in the reaction. It was ob-served that when reaction was carried out at 25–30 oC the conver-sion of toluene was 76%. Further increase in temperature from 50to 55 oC did not have much effect on the conversion of toluene.Further increase in temperature from 80 to 85 oC inhibits the con-version, the probable reason may be that at higher temperature the

cavitational effect generated by ultrasound gets affected leading tolower conversion.

4. Conclusion

In conclusion, this paper reports an efficient protocol for regio-selective sulfonation of aromatic compound under solvent freeconditions using ultrasound as promoter. The present methodol-ogy shows a considerable enhancement in the reaction rate alongwith improved selectivity, when compared with the similar reac-tions performed under silent conditions. The effect of variousparameters such as agitation speed, sulfuric acid concentration,and temperature on the reaction system was investigated andshows interesting behavior.

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