Similia similibus solvuntur

Similarity effect on chemical events may be traced back to the old empirical rule that like dissolves like similia similibus solvuntur). This rule, however, implies the similarity between solute and solvent molecules in polarity or functional group, but not in molecular shape. 

For quite sparingly soluble substances, formic acid, pyridine, bromobenzene, nitrobenzene, and occasionally also phenol, ethyl benzoate, aniline, and dioxan are used. A distinct relation exists between the constitution of solute and solvent, and is expressed by the old rule similia similibus solvuntur. Thus, as is well known, substances containing hydroxyl (e.g. sugars, carboxylic acids) are soluble in water, whereas hydrocarbons are more soluble in benzene and petrol ether than, for example, in alcohols. 

The application of the HSAB concept to solutions leads to the rule that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents [66], This rule can be considered as a modem version of similia similibus solvuntur . For example, benzene is considered a very soft solvent since it contains only a basic function. Contrary to benzene, water is a very hard solvent, with respect to both its basic and acidic properties. It is the ideal solvent for hard bases and hard acids. The hardness of water is reduced by the introduction of alkyl substituents in proportion to the size of the alkyl group. In alcohols, therefore, softer solutes become soluble. Whereas oxalate salts are quite insoluble in methanol, the corresponding softer bisthiooxalate salts are quite soluble. 

Sinee the formation of a liquid-liquid biphase system is due to a sufficient difference in the intermolecular forces of two liquids [2], the selection of a nonaqueous catalyst phase depends primarily on the solvent properties of the product phase at a high conversion level. For example, if the product is apolar the catalyst phase should be polar, and vice versa if the product is polar the catalyst phase should be apolar. The success of any nonaqueous biphase system depends on whether the catalyst could be designed to dissolve preferentially in the catalyst phase. Perhaps the most important rule for such design is that the catalyst has to resemble the catalyst phase, since it has been known for centuries that similia similibus solvuntur of like dissolves like [3]. 

An old alchemist maxim, similia similibus solvuntur" ( like dissolves like"), is the oldest rule for selecting suitable solvents, meaning that the nature of the solute determines the nature of the solvent. Due to the classification of phase systems in Section 4.2.2, organic and aqueous solvents are distinguished. Non-polar to medium-polar substances show best solubility in typical organic solvents while medium-polar to polar substances show best solubility in aqueous solvents. 

The buzzword polarity , derived from the dielectrie approach, is certainly the most popular word dealing with solvent effects. It is the basis for the famous rale of thumb similia similibus solvuntur ( like dissolves like ) applied for diseussing solubility and miseibility. Unfortunately, this rale has many exceptions. For instanee, methanol and toluene, with dielectric constants of 32.6 and 2.4, respectively, are miscible, as are water (78.4) and isopropanol (18.3). The problem lies in exactly what is meant by a like solvent. Originally, the term polarity was meant to be an abbreviation of statie dipolarity and was thus associated with solely the dielectric properties of the solvent. Later on, with the advent... 

The question of what solvent to employ and to what extent a given substance is soluble in it is one of the greatest importance to the chemist who has to experiment with the particular substance. The problem is as old as the science of chemistry itself and was answered by the alchemists before the scientific era of chemistry in the following terms Similia similibus solvuntur. ... 

This discussion may be regarded as a certain qualitative foundation for the statement Similia similibus solvuntur. ... 

The saying Like dissolves like ( Similia similibus solvuntur ) applies to polymer solutions as well as to liquid mixtures of small molecules. For example, polar liquids are more likely to be better solvents for polar polymers than are nonpolar liquids, and aromatic liquids are more likely to dissolve a polymer that is predominantly aromatic than are aliphatic liquids. Also, solution is favored when specific interactions such as hydrogen bonding are formed between solvent and polymer. 

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