Physicochemical Studies of Some Schiff Bases Derived From 6-Ethylbenzene 1 , 3-diol

The schiff bases were synthesized and their characterization was done by CHN analysis, IR and NMR spectra. The physicochemical properties such as density, refractive index, conductance, heat of solution etc., of these schiff bases were determined


I Introduction ntroduction
5][6] This draws our attention to study their physicochemical properties.Thus, in the present work, the physicochemical properties such as density, refractive index, conductance, heat of solution etc., of some schiff bases were determined.

E Experimental xperimental
The schiff bases were synthesised in our laboratory.Their characterization were done by CHN analysis, IR and NMR.
The solvents dimethylformamide (DMF), dimethylsulphoxide (DMSO) and 1,4-dioxane used for the physicochemical studies were purified by standard methods reported earlier. 7All the schiff bases were recrystallized from methanol.
The density and refractive index were measured in DMF and 1,4-dioxane whereas the conductivity was measured in DMF and DMSO.Conductivity in 1,4-dioxane was very less, so dioxane was not used in conductivity measurements.Both the measurements were done at 35 0 C.
For the determination of density, refractive index and conductance, a series of solutions were prepared of different concentrations for each schiff base in different solvents.The density, conductance and refractive index were measured at 35 0 C by pyknometer, systronics conductometer (Model No 306) and Abbe refractometer respectively.
The solubility of each schiff base was determined by transferring 25 ml of saturated solution into pre weighed 50 ml beaker and solvent was evaporated to dryness till constant weight is obtained.Three replicate measurements were carried out at a particular temperature and average value of weights was determined.The solubility was determined at 35 0 C. The amount of solvent in 25 ml solution was calculated by determining the weight of the solution in a stoppered conical flask and the weight of the solute.Finally, the moles of solute and solvent were determined in the liter of saturated solution.The results are given in Table 4.

R Result and Discussion esult and Discussion
The density of the solution is related with the densities of the solvent, solute and their weight fractions g 1 and g 2 , according to the equation (1) 1/ ρ 12 = g 1 /ρ 1 + g 2 /ρ 2 -----(1) where ρ 12 is the density of the solution and ρ 1 and ρ 2 are the densities of the solvent and solute respectively.The density of each base was determined from the slope of the plot of 1/g 1 ρ 12 verses g 2 /g 1 .The inverse of the slope gives ρ 2 .
Further, from the knowledge of structural aspects 8 , the density of schiff bases were also calculated by the following equation: where ρ is the density of the compound, K is the packing fraction (=0.681),M is the molecular weight of the compound, N A is the Avogadro's number and ∆V i is the volume increment of the atoms and atomic groups present in the compound.The theoretical and calculated densities are given in Table 1 Table 1.The density is not much affected by intermolecular interaction.However, due to polar substituent, there may be changes in the volume as well as in the molecular weight of the compound.It is observed from Table 1 that difference between experimental values from equation (2) in solutions (of DMF and 1,4-dioxane) and between experimental and calculated values may be due to solvation of the ions in solutions.
(MRD) 12 = (n 2 -1)/(n 2 +2).((x 1 M 1 +x 2 M 2 )/ ρ 12 ) -----(4) where n 12 and ρ 12 are the refractive index and density of the solutions respectively.x 1 and x 2 are the mole fractions of the solvent and the solute respectively.M 1 and M 2 are the molecular weights of the solvent and solute respectively.From the density and refractive index data, the molar refraction of all the compounds at a specified temperature were determined and are plotted against concentration.
From the least square analysis, the value of molar refraction were determined from the intercepts.From the density and molar refraction data, the refractive index of each schiff base was calculated from equation (3).The refractive index and molar refraction for each schiff base are given in Table 2 Table 2.The conductivities of schiff base in DMF and DMSO solutions are given in Table 3 Table 3. Further, the equivalent conductance (λ C ) of each solution was determined by the equation : where K is the specific conductance and C is the concentration (g.equi/lit) of the solution respectively.The equivalent conductance values for all schiff bases are reported in Table 3.The equivalent conductance is plotted against √C and is shown in Fig. 1 for DMF and DMSO systems.It is observed that in DMF, λ C curves for SOSR2 and SOSR4 do not increase uninterruptedly but bend downward at low concentrations giving rise to a maximum at 0.0064 and 0.0042 M concentration.However, the normal behavior i.e., increase of equivalent conductance with dilution is observed for SOSR1 and SOSR3 bases in DMF.This typical behavior of SOSR2 and SOSR4 can be explained in terms of specific solvation characteristics.Both SOSR2 and SOSR4 have methyl groups, which may form hydrogen bonding within  (10) .In DMSO, all the schiff bases show normal behavior i.e., equivalent conductance increases with decreasing concentration.

Compound
A close perusal of equivalent conductance curves of schiff bases shows the following order of λ C : In DMF : SOSR1 > SOSR4 > SOSR2 > SOSR3 In Further, it is obvious from figure that all the schiff bases behave as weak electrolytes.So equivalent conductance at infinite dilution (λ 0 ) can not be determined by extrapolation of the plot of λ C verses √C.Thus, an alternative procedure 10 for extrapolation is considered by using the equation: 6) where κ and κ 0 are the electrolytic conductivities of the solutions and solvent respectively.C is the equivalent concentration and the function φ(c) denotes the effect of interionic interactions.The slope dκ/dc of the plot of κ verses C approximates the limiting conductivity (λ 0 ), provided other derivatives dκ 0 /dC and d[Cφ(c)]/dC in the differntial form of equation ( 7) are neglected as compared to λ 0 .However, λ 0 can be determined from conductivity data obtained at extreme dilution, provided the κ verses C curve is linear over a sufficiently wide range of concentration .11dκ/dC = dκ 0 /dC The concentration dependence of electrolytic conductivity (κ) in DMF system is shown in Fig. 2.This shows that the term dκ 0 /dc has a dominating influence on eq. ( 7) due to the presence of polyion.All the systems have considerable interionic interactions through out the wide range studied.Therefore, the derivative d[Cφ(c)] /dC cannot be ignored in comparison to λ 0 .Despite these restrictions, limiting equivalent conductivities were evaluated from the limiting slope of small linear portions of the κ verses C curve, assuming that the interionic interactions in this range of concentration are limited.The calculated λ 0 values for all the systems are given in Table 4.The accuracy of the evaluated λ 0 values appeared highly questionable and erroneous throughout this treatment.8) where ∆H is the heat of solution, R is gas constant , N 2 is the mole fraction and T and T m are the temperature of the experiment and melting temperature of the schiff base.The heat of solution for schiff bases in DMF and 1,4-dioxane are given in Table 5.It is observed from Table 5 that heat of solution is negative.Thus, the dissolution of a schiff base in the solvent is accompanied by the evolution of heat indicating thereby exothermic behavior of schiff bases.

Table 1 Table 1
Experimental and calculated densities of all the schiff bases at 35 0 C.

Table 3 Table 3
The conductance (κ) and equivalent conductance (λ C ) of all schiff bases in DMF and DMSO at 35 0 C.
DMSO : SOSR4 > SOSR1 > SOSR2 ≈ SOSR3 SOSR1 and SOSR3 have the same molecular weight but in DMF, high λ C values are observed for SOSR1 whereas minimum λ C values are observed for SOSR3.In DMSO, no regular trend is observed.SOSR4 has lowest molecular weight but shows highest λ C values.Next higher values are for SOSR1, which has larger molecular weight.The lowest values are for SOSR2 and SOSR3 bases.SOSR3 has higher molecular weight (equal to SOSR1) whereas SOSR2 has intermediate molecular weight.This ensures that λ C is based largely on the solvent and nature of solute under study rather than on its molecular weight.10

Table 4 Table 4
The limiting equivalent conductance (λ 0 ) for all the schiff bases in DMF and DMSO at 35 0 C The solubility of each schiff base was determined in DMF and 1,4-dioxane at different temperatures and is given in Tab Table5.le 5.It is evident from the Table that solubility of schiff bases decreases with temperature in both the solvents.The heat of solution is determined by the following

Table 5 Table 5
The solubility and heat of solution of all schiff bases in DMF and 1, 4-dioxane at different temperatures.