Esterification of carboxylic acids with alcohols catalyzed by polyaniline salts

Abstract

Polyaniline salts such as polyaniline hydrochloride, sulfate, nitrate, phosphate and p-toluenesulfonate are used as catalysts in the esterification of carboxylic acids with alcohols. The activity, recovery, reusability and handling of the catalysts are found to be good. This process is being reported for the first time.

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Green Context

Esterifications are important reaction types which often rely on the use of homogeneous acids. This contribution describes the use of a polymeric salt of a strong acid and a weak base which is shown to be an easily recoverable and effective catalyst for the esterification of lauric acid. While reaction times are moderate, the catalyst can be recovered and reused readily, making product isolation and catalyst reuse simple.

DJM

Introduction

In view of environmental mandates, there is a global effort to replace conventional catalysts by eco-friendly catalysts. The replacement of current chemical processing with more environmentally benign alternatives is an increasingly attractive subject.¹ Esterification is one of the most fundamental and important reactions in organic synthesis (see ref. 2 and references cited therein). Esters are useful in a wide variety of industrial applications, such as coatings, adhesives, resins, fragrances, perfumes, plasticizers etc. Although several methods have been explored and developed,² most of them are not suitable to meet the stringent specifications which are being applied in the chemical industry. The most acceptable method of preparing an ester is to react an acid with an alcohol in the presence of a catalyst.

The catalysts employed in the esterification reaction of carboxylic acids with alcohols include mineral acids,^3,4 anhydrous magnesium sulfate and catalytic amounts of sulfuric acid,⁵ tosyl chloride, pyridine,⁶ boron trifluoride etherate alcohol,⁷ titanium salts,⁸ tin salts,⁹ hafnium salts^10,11 aluminium phosphate molecular sieve,¹² NaX and NaY zeolites,¹³ polymer protected reagent,¹⁴ graphite bisulfite,¹⁵ diorgano tin dichloride,¹⁶ Filtrol-24, Amberlyst-15, sulfated zirconia,¹⁷ and ion exchange resin.^18,19

Results and discussion

Recent advances in the field of electrically conducting polymers have led to a variety of materials with significant potential applications. Electrically conducting polymers form a unique class of materials, offering the possibility of controlled electrical conductivity combined with their good processing characteristics, low cost and stability. Among them, polyaniline is one of the most interesting materials because of its moderately high conductivity upon doping with acids, well behaved electrochemistry, easy preparation, possible processability and good environmental stability (see ref. 20 and references cited therein). In this work, the application of polyaniline in catalysis was attempted. Polyaniline salts such as polyaniline hydrochloride, sulfate, nitrate, phosphate and p-toluenesulfonate were prepared by chemical oxidative polymerization of aniline by ammonium persulfate in the presence of acid.²¹ The structure of polyaniline salt is generally represented as shown below.

We first investigated the esterification of lauric acid with different amounts of methanol. The esterification of lauric acid (1 g) and different amounts of methanol (1, 2, 3, 4 and 5 ml) was carried out in a 10 ml round bottom flask with 150 mg of polyaniline sulfate powder. The reaction mixture was refluxed at 70 °C for 20 h. The reaction mixture was filtered and washed with chloroform to recover the catalyst. The chloroform solvent and unreacted methanol were evaporated off. The compound was loaded onto a column containing silica gel of finer than 200 mesh and eluted with 20∶80 chloroform–hexane (400 ml). The solvent mixture was recovered to obtain the pure ester (Table 1). More than 3 ml of methanol gave the ester quantitatively, but the use of 1 ml of methanol gave the corresponding ester in low yield.

Table 1 Direct esterification of lauric acid and methanol catalyzed by polyaniline sulfate salt

Entry	Amount of methanol/ml	Yield (%)
1	1	55
2	2	90
3	3	92
4	4	99
5	5	99

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We investigated the catalytic activities of various polyaniline salts which promote the reaction of lauric acid with methanol at reflux. The esterification of lauric acid (1 g, 5 mmol) and methanol (4 ml, 98.7 mmol) was carried out with 150 mg (15 wt% based on the amount of the acid) of polyaniline salt powder. The reaction mixture was refluxed at 70 °C for 20 h. Five different catalysts were employed in esterification. Polyaniline hydrochloride, sulfate, nitrate, p-toluene sulfonate gave esters in good yield, whereas the polyaniline phosphate gave the corresponding ester in low yield (Table 2). This is due the low strength of phosphoric acid when compared to the other acids.

Table 2 Direct esterification of lauric acid and methanol using polyaniline salt catalysts

Entry	Polyaniline salt	Yield of ester (%)
1	Polyaniline hydrochloride	99
2	Polyaniline sulfate	99
3	Polyaniline nitrate	99
4	Polyaniline phosphate	75
5	Polyaniline p-toluene sulfonate	99

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To explore the generality and scope of the polyaniline salt catalyzed esterification, we examined the reaction of various structurally diverse alcohols with carboxylic acids (Table 3). The esterification of carboxylic acid (1 g) and alcohol (4 ml) was carried out with 200 mg of polyaniline sulfate salt as catalyst. The reaction mixture was refluxed at 70 °C for 24 h. The esters of aliphatic carboxylic acids with aliphatic alcohols gave quantitative yields. The yield of the esters was found to decrease (99, 26 and 11%) in going from primary, to secondary to tertiary alcohols, respectively. Phenoxyacetic acid (α-substituted acid) gave the ester in 85% yield while cinnamic acid (α,β-unsaturated acid) gave a 99% yield of ester. Menthyl acetate (57%) was prepared in the same configuration using menthol (chiral alcohol) and acetic acid. The methyl ester of Naproxen (81%) was obtained in the same configuration from Naproxen (chiral acid) and methanol.

Table 3 Direct esterification of carboxylic acids and alcohols catalyzed by polyaniline sulfate salt^a

Entry	Acid	Alcohol	Yield of ester (%)
a The products were analyzed by ^1H NMR spectra and authenticity of the products was established.
1	Lauric acid	Methanol	99
2	Lauric acid	Ethanol	98
3	Lauric acid	Propanol	98
4	Lauric acid	Butanol	98
5	Lauric acid	1-Decanol	98
6	Lauric acid	Isopropyl alcohol	26
7	Lauric acid	tert-Butyl alcohol	11
8	Caprylic acid	Methanol	95
9	Caproic acid	Methanol	99
10	Myristic acid	Methanol	98
11	Stearic acid	Methanol	99
12	11-Bromoundecanoic acid	Methanol	99
13	Cinnamic acid	Methanol	99
14	Phenoxyacetic acid	Methanol	85
15	Naproxen	Methanol	81
16	Acetic acid	Menthol	57

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Reusability of catalyst was checked by the esterification of lauric acid with methanol catalyzed by polyaniline sulfate salt and resulted in a stoichiometric yield (99%). The experiment was repeated nine times with the same filtered catalyst and it gave the corresponding ester in the same high yield (99%). After completing ten cycles, the catalyst, polyaniline sulfate salt was recovered and characterized. The results for the used polyaniline sulfate salt catalyst were found to be essentially the same as that of the polyaniline sulfate salt prepared originally (Table 4).

Table 4 Comparison of polyaniline sulfate salt used ten times as catalyst in the esterification reaction with that of fresh polyaniline sulfate salt

	Polyaniline sulfate salt
Properties	Fresh^a	Used as catalyst
a Ref. 21.
Amount of acid present in the polyaniline sulfate salt (%)	25.3	24.7
Pellet density/g cm^−3	1.20	1.18
Particle size/μm	10–100	1–90
Conductivity/S cm^−1	4	5

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The esterification reaction has been carried out using monoaliphatic carboxylic acids, α-substituted acid, α,β-unsaturated acid; primary, secondary and tertiary alcohols; chiral alcohols and chiral acids. The advantages of polyaniline salt catalysts over the conventional catalyst are: (i) preparation of polyaniline salts through a simple synthetic route, (ii) various types of polyaniline salts can be synthesized, (iii) the amount of acid in the polyaniline chain can be varied, (iv) quick regeneration of the catalyst, and (vi) better reusability of the catalyst.

Conclusion

Polyaniline salts are used as catalysts in the esterification of carboxylic acids with alcohols. Preparation and handling of the catalyst are easy. This catalyst can be easily separated and reused, there is no acidic waste and catalyst life time can be maximized. Polyaniline salts are environmental-friendly. However, esterification reactions take longer (20 h) and also require higher amount of catalyst (15 wt%) to obtain the ester in quantitative yield. Work is in progress to modify the polyaniline system and thereby increase the efficiency of the catalyst. The use of polyaniline salts as catalysts in organic transformations may open up future work in the catalysis field.

Experimental

In a typical experiment, esterification of lauric acid (1 g) and methanol (4 ml) was carried out in a 10 ml round bottom flask with 200 mg of polyaniline sulfate powder. The reaction mixture was refluxed at 70 °C for 24 h. The reaction mixture was filtered and washed with chloroform to recover the catalyst. The chloroform solvent and unreacted methanol were evaporated off. The compound was loaded a column containing silica gel of finer than 200 mesh and eluted with 20∶80 chloroform–hexane (400 ml). The solvent mixture was recovered to obtain the pure ester.

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