Thermodynamic properties of donor–acceptor complexes of tertiary amine with aryl ketones in hexane medium

doi:10.1016/j.tca.2012.02.018

Thermochimica Acta

Volume 536, 20 May 2012, Pages 15-23

https://doi.org/10.1016/j.tca.2012.02.018 Get rights and content

The thermodynamic stability of complexes formed between N,N-dimethylaniline (DMANI) and three ketones, namely, acetophenone (ACP), 4-chloroactophenone (ClACP) and 4-methylacetophenone (MACP) in n-hexane is extensively investigated by spectral and ultrasonic methods. The ultrasound scan was carried out in the temperature range 208.15–313.15 K and at atmospheric pressure on solutions containing equimolar concentrations of components ranging from 0.025 to 0.2 M. The existence of solute–solute interactions has also been confirmed through electronic absorption spectra analyzed with Benesi-Hildebrand theory at 303.15 K. The stability constants of the donor–acceptor complexes determined both by spectroscopic and ultrasonic methods are comparable and follow similar trends. The trend in the formation constants is discussed with structures of the components. The thermodynamic behavior of the systems was explained through the computed values of the free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) changes for complex formation are computed and discussed.

Highlights

► Ultrasonic scan is carried out on ternary systems of aromatic tertiary amine and three aryl ketones. ► Formation of CT complexes is found between tertiary amine with aryl ketones. ► Stability constant values are computed by ultrasonic and spectral methods are compared. ► The trend in the ‘K’ suggests that substituents in ketones influence the stabilities of these complexes. ► The thermodynamic parameters suggest CT interaction is exothermic and the complexes are thermodynamically stable.

Introduction

The significance of charge transfer (CT) complexes in various industrial and biochemical reactions was reported in the literature [1], [2], [3], [4]. The formation of 1:1 complexes in solution was detected by the simple and low cost ultrasonic technique by several investigators [5], [6], [7]. The aromatic tertiary amine, N,N-dimethylaniline (DMANI) acts as a strong π-electron as well as n-electron donor due to the presence nitrogen as its basic center and also due to the presence of π-electrons of the aromatic ring. The formation of donor–acceptor complexes between DMANI and different types of acceptors has been investigated by various techniques [8], [9], [10]. In continuation of our earlier work [11], in the present study an attempt has been made to detect the possible interactions between DMANI with one of the strong polar groups viz., the carbonyl group of ketone. The ultrasonic investigation is carried out in n-hexane solutions containing equimolar concentrations of DMANI and aryl ketones at four different temperatures, 298.15, 303.15, 308.15 and 313.15 K. The trend in the thermo acoustic and their excess properties with concentration and temperature is discussed in terms of charge transfer between the components in the liquid mixtures. An attempt is made to confirm the complex formation in these ternary systems through UV–vis spectroscopic analysis at ambient temperature. The stability constants of the donor–acceptor complexes were calculated both by acoustic and spectral methods and the results are discussed. The stability constant values at different temperatures were used to calculate the free energy of formation (ΔG), enthalpy changes (ΔH) and entropy changes (ΔS) for the formation of these complexes.

Section snippets

Experimental

N,N-dimethylaniline and the solvent n-hexane used were of AnalaR grade samples (MERCK) with the purity of > 99.8%. The ketones were of spectrochem purity supplied by SD fine Chemicals. The chemicals were used in the present investigation without further purification. A single crystal variable path ultrasonic interferometer (Model F-81) operating at 2 MHz frequency supplied by Mittal Enterprises Pvt. Ltd., New Delhi, India was used to measure the ultrasonic velocities (u) of liquid mixtures with

Ultrasonic study

The experimental results of ultrasonic velocity, density and viscosity along with the computed acoustical and excess thermo acoustical parameters at temperatures 298.15–313.15 K are listed in Table 1, Table 2, Table 3, Table 4, Table 5, Table 6. The variations in ultrasonic velocity and excess compressibility with concentration are graphically represented in Fig. 1, Fig. 2. An inspection of acoustical parameters in Table 1, Table 2, Table 3 reveals that the measured properties for the three

Conclusion

The present investigation deals with determination of thermodynamic properties of charge transfer complexes formed between an aromatic tertiary amine, namely, N,N-dimethylaniline and three structurally different aromatic ketones in n-hexane medium by ultrasonic and UV–visible spectral methods. The formation constant values computed by ultrasonic and optical methods follow the same trend with structural variation in ketone.

The trend in the acoustical properties and negative values of entropy of

Acknowledgments

The authors are grateful to the Managements of The New College, Chennai 600 014, India and R.K.M. Vivekananda College, Chennai 600 004, India for the facilities provided to carry out the work. We wish to thank Prof. M. G. Mohamed Kamil, Head (Retd.), Department of Physics, The New College for his constant encouragement and useful discussion.

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View full text

Thermodynamic properties of donor–acceptor complexes of tertiary amine with aryl ketones in hexane medium

Highlights

Introduction

Section snippets

Experimental

Ultrasonic study

Conclusion

Acknowledgments

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