Chapter 5 Classification of Organic Compounds by Solubility Deductions based upon interpretation of simple solubility tests can be extremely useful in organic structure determination. Both solubility and spectrometric analyses often lead to the same kinds of structural deduction. Solubility involves the formation of one layer, if the compounds are miscible, or formation of two layers, if the compounds are immiscible. ١
The solubility of organic compounds can be divided into two major categories: Solubility in which a chemical reaction is the driving force Solubility in which simple miscibility is the only mechanism involved, such as dissolving ethyl ether (CH 3 CH 2 OCH 2 CH 3 ) in carbon tetrachloride (CCl 4 ) ٢
Solubility Tests for the Identification of Functional Groups ٣
٤
Test the solubility of a substance in Water 5% NaOH 5% NaHCO 3 5% HCl A substance is said to be soluble if it dissolves to the extent of 3.3 g/100 ml of solvent Cold concentrated H 2 SO 4 (i) This can provide three kinds of information: Presence of a functional group. For instance, because hydrocarbons are insoluble in water, observing that an unknown is partially soluble in water indicates that a polar functional group is present. ٥
(ii) Solubility in certain solvents often leads to more specific information about functional group. i.e., solubility in 5% NaOH of a waterinsoluble unknown is a strong indication of an acidic functional group. (iii) Certain deductions about molecular size and composition may sometimes be made from solubility tests. Example: in many homologous series of monofunctional compounds, the members with fewer than about 5 carbon atoms are water soluble, whereas the higher homologs are ٦insoluble.
Procedure for Determination of Solubility: Solid: R.T. warm for water solubility cool to R.T. Liquids: ~25 mg (0.05 ml one drop) in 0.75 ml (15 drops) Testing with litmus paper Heat should not be applied for ether solubility, and also when solubility in acid or alkali is being determined, because it might cause hydrolysis to occur. If the mixture is shaken thoroughly, the time required for the unknown to dissolve should not be more than 1 to 2 min. Solubility in Concentrated Acid Place 0.6 ml of conc. H 2 SO 4 in a test tube, and add 0.05 ml (~1 drop) or 25 mg of the unknown. Unknowns that react with H 2 SO 4 to produce heat and/or color changes should be classified as soluble even if the sample does not appear to dissolve. ٧
٨ The dielectric constant is the measure of the ability of the solvent to separate ionic charges. The dielectric constant is related to the polarity of the solvent. To dissolve ionic compounds, a high dielectric constant is required, but it is not the only characteristic of an effective ion solvent. Example: HCN is a very poor solvent for salts such as NaCl The high dielectric constant and hydrogen-bonding ability of water (and other hydroxylic solvents) which combine to make it a good solvent for salts, also make it a poor solvent for nonpolar substances.
Like dissolves like Most organic molecules have both a polar and a nonpolar entity, it can be deduced that solubility would depend on the balance of the two parts. As the percentage of the hydrocarbon portion increases, water solubility decreases and ether solubility increases. ٩
Generally, solubility of a substance is inversely related to its melting point: high melting point low solubility. Among cis-trans isomers, the cis form generally is more soluble. ١٠
In general, an increase in molecular weight leads to an increase in intermolecular forces in a solid and decreased solubility. Similarly, ١١ glucose vs. starch or cellulose, and amino acids vs. proteins
Compounds having a branched chain is more soluble than the corresponding straight-chain compound. Branching lowers intermolecular forces and decreases intermolecular attraction. The position of the functional group in the carbon chain also affects solubility. ١٢
Solubility in Water Water is a poor solvent for hydrocarbons. Presence of double bonds, triple bonds or aromatic rings does not affect the polarity greatly similar to alkanes in their solubility. Substituting halogen for a hydrogen decreases the water solubility. Salts are extremely polar and are usually water soluble. Acids and amines are more soluble than nonpolar compounds (due to H-bonding). Solubility of amines decreases as the basicity decreases. Many tertiary amines are more soluble in cold than in hot water (at lower temperatures, the solubility of the hydrates is involved). ١٣
Solubility in 5% Hydrochloric Acid Solution 1 o, 2 o, and 3 o aliphatic amines form polar ionic salts with HCl. Aliphatic amines are readily soluble in 5% HCl. The presence of conjugated aryl groups decreases the basicity of the nitrogen atom. soluble in 5% HCl insoluble in 5% HCl insoluble in 5% HCl Disubstituted amides (RCONR 2 ) of sufficiently high molecular weight to be water insoluble are soluble in 5% HCl solution. ١٤
١٥ Solubility in 5% NaOH and 5% NaHCO 3
K a 1x10-10 insoluble in NaHCO 3 soluble in NaOH soluble in NaHCO 3 soluble in NaHCO 3 ١٦
Aldehydes and ketones are sufficiently acidic to react with aqueous alkali, but they are too weakly acidic to dissolve to any measurable extent in 5% NaOH solution. When two carbonyl groups are attached to the same carbon, the acidity increases sharply. Esters with five or six carbon atoms that are almost completely soluble in water may be hydrolyzed by continued shaking with 5% NaOH solution. The alkali should not be heated and the solubility or insolubility should be recorded after 1-2 minutes. ١٧
Solubility in Organic Solvents The solubility of organic compounds in organic solvents should be determined in order to plan for a variety of laboratory operations. These include choosing solvents for organic reactions, dissolving substrates for spectral analyses, cleaning glassware, extraction, Thin layer and other chromatography, Crystallization. ١٨
١٩
Solubility in Cold, Concentrated Sulfuric Acid Cold, concentrated H 2 SO 4 is used with neutral, waterinsoluble compounds containing no elements other than carbon, hydrogen and oxygen. If the compound is unsaturated, is readily sulfonated, or posses a functional group containing oxygen, it will dissolve in cold, concentrated H 2 SO 4 Alkanes, cycloalkanes, and their halogen derivatives are insoluble in H 2 SO 4 ٢٠
Simple aromatic hydrocarbons, and their halogen derivatives do not undergo sulfonation and are insoluble in H 2 SO 4 However, the presence of two or more alkyl groups on the aromatic ring permits the compound to be sulfonated. ٢١ Dissolve readily in H 2 SO 4
٢٢
٢٣
٢٤
٢٥ THE END