Effect of Dielectric Constant of Medium on Conductance for Trimethylsulfonium Halides in Normal and Branched Alcohols

The conductance parameters, Λo (equivalent conductance at infinite dilution) and KA (association constant) for trimethylsulfonium halides (Bromide and Iodide) in methanol, ethanol, n-propanol, 2-propanol and n-butanol have been evaluated at 25°C from the conductance measurements. The values of Λo for different salts decrease with decreasing dielectric constant of medium (D) with some variations and are in the order of MeOH > EtOH > n-PrOH > 2-PrOH > n-BuOH. The association constants of the same salts in different alcohols increase with decreasing dielectric constant of medium and are in the order of n-BuOH > 2-PrOH > n-PrOH > EtOH > MeOH.


I. INTRODUCTION
Recent theories of the behaviour of electrolytes in solution suggest that the dielectric constant of the medium plays a very important role.Debye and Pauli [1] discussed the various factors that influence the dielectric constant of the medium in the immediate neighborhood of an ion.It was found that in very dilute solutions the dielectric constant of the pure solvent is the proper value to use for so-called "Limiting Law".
Furthermore, it has been shown that changes of dielectric constant of the medium are closely connected with corresponding changes of the velocity of homogeneous reaction in solutions.An approach to a theoretical interpretation of the relation of these medium changes to the simultaneous changes in the velocity of certain catalyzed reactions was given by Harmed and samaras [2].
The measurement of the dielectric constant [3] of conducting solutions is still very much a matter of controversy and definite, reliable results seem yet not to have been obtained.On the other hand, the measurement of the dielectric constant of a pure non-conducting solvent does not offer any particular difficulties and may be carried out by different methods.However, even the purest solvent exhibits some conductivity and the reliability of the measurement decreases rapidly with increasing conductance.Many solvents act as very weak acids or bases, showing large increases of the dissociation constant with increasing temperature and this gives less reliable data for their dielectric constant.
Electrolytes in the primary alcohols [4,5] and amides [6] appear to exhibit many of the properties previously observed only in water [7,8] and were attributed to the unique structure of that solvent.In alcohols, ionic association was found to increase with anionic size, a result contrary to the predictions of the electrostatic theory.This was interpreted in terms of a multiple-step association process involving hydrogen bonded solvation of anions in the homologous series methanol to 1-propanol.

II. EXPERIMENTAL PART
All salts were highly purified reagent grade and used without further purification.where trimethylsulfonium bromide (Me3S.Br) and trimethylsulfonium iodide (Me3S.I) are Analar analytical reagent "BDH".Methanol (BDH), ethanol (BDH), n-propanol (BDH), isopropanol (BDH) and n-butanol (BDH) were used without further purification.The specific conductance ೫° for methanol, ethanol, n-propanol, iso-propanol and n-butanol at 25°C are listed in Table (I).The densities, viscosities and dielectric constant values were depicted [9][10][11][12][13] 25 C [19-13] All solutions were prepared by weight.Salts were weighed using microbalance which reads to ± 0.1 mg.Dilution was carried out successively into the cell by siphoning the solvent by means of weighing pipette.Conductivity Bridge was model Crison GLP31+ and the cell with bright platinum electrodes was used.The cell constant was 0.1 cm-1 for dilute solutions.
The association constant of Trimethylsulfonium halides increases with decreasing the dielectric constant of medium according to the order: It also increases with increasing anionic size in each case following the order: I -> Br -According to the prediction of electrostatic theory, the association of electrolyte in solvent containing hydroxyl group appears to increase with ionic size.Analysis of the concentration dependence of conductance in methanol solutions shows an association constant for alkali metal and tetra-alkylammonium halides, which increases as the size of anion increases [14].For example, constants of 0, 4 and 18 have been obtained for tetra-alkylammonium chloride, bromide and iodide respectively [4].
Recent studies of Kay and co-workers on conductivity of several alkylammonium halides in methanol and ethanol [4,14] have clearly supported our findings concerning the influence of the solvent on the association of ion-pairs.
Hafez et al. [15] have found that equivalent conductance at infinite dilution (Λ0) for s-n-alkylisothiouronium (bromides, iodides and picrate) ethanol, n-propanol, and n-butanol decrease while association constant (KA) increase with decreasing the dielectric constant of the medium.
El-Hammamy et al. [16] have found similar behavior for Λ0 and KA values N, N-diphenyl-s-n-alkylisothiouronium bromides in methanol, ethanol and n-propanol.The decrease of Λ0 is attributed to decrease in ion mobility as the dielectric constant decreases, while the increase of KA may be due to the increase of the number of non-conducting ion pairs under such condition of decreased mobility.
The effect of solvent is twofold: (a) It can stabilize the pair due to the hydrogen bond chains in the alcohol.(b) It can solvate anions or cations by hydrogen bond [17], so that the expected KA values order increases with anionic radius for salts with the same cation as well as increasing with cationic radius for salts with the same anion.
The participation of alcohols in the ion-pair formation equilibrium therefore, should involve both steric effect and columbic effect.On the basis of this approach, the structure modifications of the alcoholic polymers are generated by added solvents should result in a variable influence of the alcohol molecules on the ion-pair association of electrolytes.
When the higher alcohols are chosen as the solvent system, the pattern of ionic association of hydroxyl solvents may be investigated without such complications as threedimensional structured effects or small association constant [4].Consequently, the KA values in mixed solvents should vary with the mixture composition in a way different from that expected on the basis of the short range ion-ion interactions which depend only on the dielectric constant of the solvent.
The association constant should be only a function of the dielectric constant and their values increases with decreasing the dielectric constant of medium.This is found to be true in studying the behavior of trimethylsulfonium halides in methanol ethanol, n-propanol, iso-propanol and n-butanol.The association constant was also generally found to increase with increasing anionic size in each alcohol as in Table (II).The same behavior of Λ0 and KA was obtained for tetrabutylammonium halides and perchlorate in different alcohols [4,14]   From Figure (1), the variation of KA with dielectric constant (case of alcohols), one can conclude that the sphere in continuum model cannot be applied to these systems.Kay and his co-workers [14] studied some salts of tetraalkylammonium in methyl alcohol.They obtained values of KA higher than those expected by using the Bjerrum-Fuoss theory [18] and they explained their results on the hypothesis that the ion-pair association is affected by the particular structure of the alcohol.
It can be readily seen from Table (IV) [19,20] that the association constant KA increase with increasing cationic size, i.e., in the order of: CsI > RbI > KI > NaI.This can be explained by adopting the Lee and Wheaton assumption namely, the formation of solvent-separated ion -pair.Lee and Wheaton obtained a similar behaviour for alkali iodides in different alcohols.Figure (1) shows the plot of log KA versus 1/D, which is linear as expected from the relation [21]: Accasina et al. [22] studied the conductance of LiClO4 in (dioxane-water) mixtures at 25°C and they found a slight curvature.Hafez et al. [15], measured the conductance of salkylisothiouronium (bromides, iodides and picrate) in ethanol, n-propanol, and n-butanol.They found that in most cases straight lines were obtained according to ionic association equation [22].In other cases, a slight curvature was due to the interference of ion-solvent interaction [23].El-Hammamy et al. [16] measured the conductance of N, Ndiphenyl-s-n-alkylisothiouronium bromides in methanol, ethanol, n-propanol and n-butanol.They found a curvature in the same plot.This behavior was explained as being due to solutesolvent interaction.El-Hammamy et al. [24,25] measured the conductance of acetylcholine halides and perchlorate in normal and branched alcohols (methanol, ethanol, n-propanol, 2-propanol and n-butanol) at 25 o C and found that in all cases straight lines were obtained according to ionic association equation.El-Hammamy et al. [26] measured the conductance of s-acetylthiocholine halides and perchlorate in normal and branched alcohols (methanol, ethanol, n-propanol, 2-propanol and n-butanol) at 25 o C and found that in all cases straight lines were obtained according to ionic association equation.


Abstract-The conductance parameters, Λo (equivalent conductance at infinite dilution) and KA (association constant) for Trimethylsulfonium halides (Bromide and Iodide) in methanol, ethanol, n-propanol, 2-propanol and n-butanol have been evaluated at 25°C from the conductance measurements.The values of Λo for different salts decrease with decreasing dielectric constant of medium (D) with some variations and are in the order of MeOH > EtOH > n-PrOH > 2-PrOH > n-BuOH.The association constants of the same salts in different alcohols increase with decreasing dielectric constant of medium and are in the order of n-BuOH > 2-PrOH > n-PrOH > EtOH > MeOH Index Terms-Conductance Measurement, Dielectric, Trimethylsulfonium Halides.

TABLE I :
SOLVENT PROPERTIES AT Effect of Dielectric Constant of Medium on Conductance for Trimethylsulfonium Halides in Normal and Branched AlcoholsIt can readily be seen from Table (II) that the equivalent conductance at infinite dilution Λ0 of Trimethylsulfonium halides decreases with decreasing the dielectric constant of medium.The ionic mobility of ions decreases with decreasing the dielectric constant of medium according to the order: MeOH

TABLE II :
Λ• AND KA VALUES IN DIFFERENT ALCOHOLS AT 25˚C at 25°C and is illustrated in Table (III).