Thermodynamic Study of p-Bromobenzoyl Acetone Complexes from a Temperature Variation Study

INTRODUCTION

The presence of a -carbonyl group with proton on the carbon atom allows a keto⇌enol tautomerism to take place (Fig. 1). By the replacement of proton by a metal ion -ketoenolate complexes are obtained. Given widespread applications of -diketones and their derivatives p-Bromobenzoylactone (p-BBA) has been chosen to study the interaction with bivalent metal ions. -Diketones are commonly used reagents for the solvent extraction of some metal ions like lanthanides.

p-Bromoacetophenone (20 g) was mixed with 200ml of ether and 2.5 g of sodium metal. Then 22 g of ethyl acetate was added and the mixture was refluxed for about five hours. Then it was cooled. The residue obtained was dissolved in water and acidified with acetic acid. Light yellow coloured crystals were obtained, recrystallized with alcohol (Fig. 2)

The ligand was characterised by elemental analysis, infrared and PMR studies. The solution of p-Bromobenzoyl acetone (ligand) was prepared in freshly distilled dioxane. The titration was carried out in a 75:25 (V/V) dioxane water mixture. Chemically pure sodium perchlorate was used to maintain the ionic strength. All other chemicals used were of reagent grade. 0.04 M solution of tetramethylammonium hydroxide (TMAH) in 75% (V/V) aqueous dioxane was used as the titrant. Dioxane used was purified by refluxing with sodium metal for about 24 hours and was freshly distilled over sodium before use. The EC-digital pH-meter, model pH 5651 with a combined glass electrode was used for pH measurements. The instrument was standardized by using potassium hydrogen phthalate and phosphate buffers.

by Irving and Rossetti has been used to obtain the values of proton-ligand and the metal-ligand formation constants. For carrying out the studies these solutions with a total volume of 20 ml were titrated against TMAH at 20, 30, 40 and 50C. (i) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml K2SO4 (0.02 M) + 15 ml dioxan + 0.5 mL H2O (ii) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml K2SO4 (0.02 M) + 10 ml ligand (0.01 M) + 5 ml dioxan + 0.5 ml H2O (iii) 3.0 ml HClO4 (0.02 M) + 1.0 ml NaClO4 (2.0 M) + 0.5 ml metal ion solution (0.02 M) + 10 ml ligand (0.01 M) + 5 ml dioxan + 0.5 ml H2O.

The pKa values were obtained from the plot of log HHn/1n vs pH. These values are given in table 1. The valuesn obtained lie below 2.0 in all the cases. So only two types of complexes 1:1 and 1:2 are obtained in each case. The concentration of metal ion used was very low (5.0104 M) so the possibility of the formation of polynuclear complexes is ruled out. The number of ligands combining with metal atom depends upon its valency. Each ligand is equivalent to two units so the complexes will have two ligands attached to the metal atom. On complexation a very stable six-membered ring is obtained and the metal atom links the two ends of the ligand through oxygen atoms. The order of increasing stability constant of the metal complexes is Cd(II) < Mn(II) < Co(II) < Zn(II) < Ni(II) < Fe(II) < Cu(II). With the increase in temperature the stability constants and the pKa values decreases which shows that lower temperature favours the formation of complexes.

The values of the overall changes in free-energy (G), enthalpy (H) and entropy (S) are calculated using these equations- G =  RT ln K The values of G, H and S obtained are given in table 2.

From the values of the thermodynamic parameters (Table 2) it is concluded that the reactions are exothermic. The values of enthalpy change (H) are negative, so there is release of energy during complex formation. The values of entropy change (S) are positive, so it favours complex formation. Hence the chelation effect is an entropy effect. The higher positive values of entropy are there due to greater availability of coordination sites on the metal ions.

The author is thankful to University Grants Commission, New Delhi for the award of fellowship. Organic Chemistry, 3rd edn. Longmans. London, p.177. 2. J. Bjerrum (Ed.) (1941), Metal Ammine Formation in Aqueous Solution. Hasse Copenhagen. 3. H. Calvin and K.W. Wilson (1945), J. Am. Chem. Soc., 67, pp. 2003. 4. H. Irving and H.S. Rossotti (1954), J. Chem. Soc., pp. 2904. 5. J.C. Sullivan, J. Rydberg and W.F. Miller (1959), Acta Chem. Scand., 13, pp. 2023. 6. K.B. Yatsimirskii and V.P. Vasi (1960). 'L' Ex. Instability Constants of Complex Compounds, Van Nostrand, New York, p. 63.