Synthesis of Some Benzocrown Ether Derivatives and Their Solvent Extraction with Organic Salts of Alkali Metals

 

Rajeev Ranjan¹* and Ramjatan Sinha²

¹PG Department of Chemistry, Ranchi College, Ranchi-834008

²University Department of Chemistry, Patna University, Patna-800005

 

ABSTRACT:

 

INTRODUCTION:

The macrocyclic polyethers having hydrophobic exteriors show a remarkable range of specificity for a wide variety of cations.^1-4 The chemical profile of a macrocycle as a complexing agent has often been determined via solvent extraction. It is extensively used an indirect method for assaying the complexing tendency of various polyether-salt systems, and known as the picrate extraction method.⁵ The cation selectivities of macrocyclic ligands can be determined by different types of experiments, among which are solvent extraction,^6-8 complex stability constant determination and permeabilities of cations through macrocycle-containing liquid membranes.⁹ The formation constants for cryptates, which show greatly enhanced stability and selectivity compared to crown ethers, can be ~10⁶ times greater than those of the ethers. Uncommon complex stoichiometry, substitution effect, ring size effect, sandwiching complexation of a series of crown ethers by solvent extraction experiments have been reported.^10-12 In our study, we report cation binding behavior of selected benzocrown ether derivatives (figure: 1.1) with some univalent alkali metal salts by using solvent extraction method and investigation of their behavior.

 

MATERIALS AND METHODS:

The 2,4,6-trinitrophenol used was of E. MERCK (AR grade). Other chemicals used were also of AR grade. The commercially available regents were used without further purification. The metal contents were estimated by flame photometric method. Results of elemental analysis of synthesized compounds agreed with required value within experimental error. The melting point of the synthesized compounds was determined on electrical tempo T-1150 apparatus. Molar conductivities of the compounds were measured using Systronic Conductivity Meter-306. The conductivities of the compounds were measured at the concentration 10^-3 M in methanol solvent at 30(±0.5)⁰C. IR spectra were recorded by Perkin elmer spectrometer RX1 (4000-450 cm^-1). UV-visible spectral data were recorded through Systronic double beam spectrophotometer-2203 (600-200 nm). The ¹H−NMR spectra of ligand and crown ether complexes were recorded in CDCl₃ by Bruker DRX-300.

 

EXPERIMENTAL:

Preparation of salt of 2,4,6-trinitrophenol, M(TNP) :

About 4.58 gm (0.02 mol) of 2,4,6-trinitrophenol was taken in a conical flask and dissolved in 25 ml of ethanol with constant stirring with the help of glass rod. Further 0.02 mol of appropriate alkali metal hydroxide was dissolved in ethanol and was slowly added to the alcoholic solution of 2,4,6-trinitrophenol with constant stirring. The mixture was continuously refluxed on hot plate fitted with magnetic stirrer for 50 minutes and the temperature was maintained at 78⁰C. The solution in conical flask was corked and kept undisturbed. Then coloured crystalline product was obtained. It was filtered, washed with absolute ethanol and dried in an electric oven at 80⁰C. Some physical properties of synthesized alkali metal salts are given in table-1.1.

 

Table – 1.1 Physical properties of alkali metal salts

Compound	Colour	Melting point (0C)	% Nitrogen Found
Li(TNP)	Deep orange	260 e	17.91
Na(TNP)	Light orange	270 e	16.53
K(TNP)	Orange	260 e	15.39
Rb(TNP)	Yellow	290 d	14.14

e – explosion temp, d – decomposition temp

 

Preparation of crown ethers:

Preparation of crown ether which may work as a strong complexing host molecule was one of the important part of this research work. crown ethers, were prepared by the known synthetic methods as reported in literature. Benzo-15-crown-5 (1)^13-17, 4′-iodo-benzo-15-crown-5 (2), 4′-amino-benzo-15-crown-5 (3),¹⁸ dibenzo-18-crown-6 (5),^13-17 2,14-diamino-6,7,9,10,17,18,20,21-octahydro-5,8,11,16,19,22-hexaoxadibenzo[b,k]cyclooctadecene (6) 2,13-diamino-6,7,9,10,17,18,20,21-octahydro-5,8,11,16,19,22-hexaoxadibenzo[b,k]cyclooctadecene (7)¹⁹ and benzocrown ether derivative 4²⁰,  were prepared according to the reported procedure.

 

Figure : 1.1

 

Solvent extraction is a method of separation based on the transfer of a solute from one immiscible solvent into another. In solvent extraction method, univalent metal cations were extracted by crown ether from a water phase into an organic phase.^21-24 The solvents, CH₂Cl₂ and H₂O were saturated with each other prior to use for preventing volume changes of both phases during extraction. Equal volumes of CH₂Cl₂ solution (10 ml) of the respective  crown ether (0.3 mmol/l) and an aqueous solution of  alkali metal picrate (0.03 mmol/l) were introduced into a stoppered Erlenmeyer flask and the mixture was shaken for 20 minute in an incubater thermostated at (28.0±0.2) ^oC. This mixture was then allowed to stand for 4 hour at that temperature for complete phase separation. The concentration of alkali metal picrates in the aqueous phase was determined through measuring the absorbance at 354 nm by UV-vis spectroscopy. The extractability of univalent alkali metal ions (Li⁺, Na⁺, K⁺ and Rb⁺) by crown ethers is listed in table 1.2.

 

Table – 1.2 Solvent extraction of aqueous alkali metal salts with benzo-15-crown-5, single-armed 15-crown-5, dibenzo18-crown-6 and double-armed dibenzo18-crown-6^a

Ligand	Extractabilityb %
	Li+	Na+	K+	Rb+
1	0.59	1.27	1.18	1.42
2	1.69	1.48	0.49	0.37
3	0.56	0.98	1.69	1.84
4	0.53	1.28	1.76	1.12
5	1.04	0.98	1.16	0.74
6	7.11	9.26	9.14	4.34
7	3.82	7.89	9.45	4.59

^aTemperature (28.0±0.2) ^oC; aqueous phase (10 ml), [picrate] = 0.03 mmol/l; organic phase (CH₂Cl₂, 10 ml), [Ligand] = 0.3 mmol/l. ^b Percent of picrate extracted into the organic phase. Average of two independent runs.

 

RESULTS AND DISCUSSION:

The obtained results, when compared with the relevant data for compounds 1–3 and 5–7, has furnished further understanding of the complexation behaviour of the benzocrown ether derivatives with alkali metal ions.^25-27 All of the crown ethers gave relative higher extractabilities with Na⁺ and K⁺ because of their size-fit. In the series of crown-5 compounds, 3 and 4 reversed the selectivity of Na⁺/K⁺ to give higher binding ability towards K⁺ than Na⁺ as compared with parent crown ether 1, which is attributed to the introduction of electron-donating substituent groups. These results indicate that electronic effect of the side arm attached to benzocrown ether alters not only the cation binding ability, but also cation selectivity. In conclusion, the crown ethers possessing electron-donating amido group, benzocrown ethers 3, 6 and 7 gave the highest extractability but the lowest selectivity towards univalent cations. On the other hand, although novel benzocrown ether derivatives 4 gave the moderate extractability, it shows enhanced distinct cation selectivity.

 

ACKNOWLEDGEMENT:

The authors thank to the Chairman, UGC, New Delhi, for providing financial assistance to this research programme under UGC-Minor Research Programme. We further extend our sincere thank to the Head, SAIF, CDRI, Lucknow, for providing IR-spectra, ¹H-NMR spectra and necessary facilities.

 

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Received on 28.09.2012        Modified on 08.10.2012

Accepted on 14.10.2012        © AJRC All right reserved