Chem 4563 Organic Qualitative Analysis Solubility Tests Solubility tests should be performed on every unknown. They are extremely useful in determining the nature of themajor functional groups present in the unknown compound. The tests are very simple and require only a small amount of sample. In addition, the solubility tests will determine whether the compound is a strong base (amine), a weak acid (phenol), a strong acid (carboxylic acid) or a neutral compound (aldehyde, ketone, alcohol ester). The common solvents used to determine solubility are: water, ether, 5% NaOH, 5% NaHCO3 , 5% HCl, 96% H 2 SO4 , and 85% H3 PO4. A summary scheme of solubility classification is given on p 92 and 93 of Shriner. 1. Solubility in Water. Compounds with four or less carbons, and which contain polar groups of oxygen, nitrogen or sulfur, will often be soluble in water. Almost any functional group containing these elements will lead to water solubility for low molecular weight (C4) compounds. Compounds with five or six carbons with those elements will often have borderline solubility or be insoluble. Branching of the alkyl chain of the molecule lowers intermolecular forces between its molecules. Yhis is usually manifested in a lower melting or boiling point, and a greater solubility in water for the branched compound thann for a corresponding straight chain compound. This occurs simply because the molecules of the pranched compound are more easily separated from one another. Thus t-butyl alcohol is more soluble in water than is n-butyl alcohol. When the ratio of O,N or S atoms in a molecule to the carbon atoms is increased, the solubility of that compound in water will likewise increase. This is due to an increase in the number of polar functional groups. Thus 1,5-pentanediol is more soluble in water than is 1-pentanol . As the size of the alkyl chain of a compound is increased beyond 4 carbons, the influence of polar functional groups is diminished and water solubility begins to decrease (this is formulated as the 5-cCarbon Rule). A few examples of these generalizations are given below. SOLUBLE BORDERLINE INSOLUBLE H3C O CH3 O O H2 H3C C C OH CH C H3C C C H3C C OH C C OH CH3 H2 H2 H2 CH3 OH OH H3C OH H3C CH3 The π-electrons of an aromatic ring are more polarizable thus a benzene ring is more soluble than a typical hydrocarbon 2. Solubility in 5% HCl. The possibility of an amine should be considered immediatly if a compound is soluble in dilute HCl. Aliphatic amines (RNH2 , R 2 NH, R3 N) are basic compounds which readily dissolve in acid because they form hydrochloride salts that are soluble in aqueous medium. H R NH2 + Cl R NH2 + HCl The substitution of an aromatic ring (Ar) for an alkyl group (R) reduces the basicity of the amine somewhat, but the amine will still protonate, and it will wtill be generally soluble in dilute aqueous acid. This reduction in basicity is due to resonance delocalization of the unshared electrons on the amino nitrogen of the free-base. The delocalizaton is lost on protonation, a problem which does not exist for aliphatic amines. The substitution of two or three aromatic ringson an amine nitrogen reduces the basicity even further. Di- and tri-aryl amines will not dissolve in HCl since they do not protonate easily. Some very high molecular weight amines like tribromoaniline (MW 330) may also be insoluble in dilute acid. N N N N Delocalization lowers basicity of triaryl amines 3. Solubility in 5% NaOH and 5% NaHCO 3 Compounds which dissolve in sodium bicarbonate, a weak base, are stron acids. Compounds which dissolve in sodium hydroxide, a stron base, may be either strong or weak acids. Thus one can distinguish between weak and strong acids by determining their solubility in both strong (NaOH) and weak (NaHCO3 ) base. For the purpose of this experiment, carboxylic acids (strong acid pKa~5) are generally indicated when the unknown compound is soluble in both bases, while phenols (weak acid pKa~10) are indicated when a compound is soluble only in NaOH. Compounds dissolve in aqueous base solutions because they form sodium salts that are soluble in aqueous medium. However, the salts of some high molecular weight compounds are not soluble and will precipitate out. The salts of long chain fatty acids such as myristic (C14), palmitic (C16) and stearic(C18) acids are in this category, these salts form soaps. Both phenols and carboxylic acids produce resonance stabilized conjugate bases. In phenols, substitution of an electron withdrawing group (nitro or cyano) at the ortho and para positions of the ring increases the acidity. Phenols with two or three nitro groups at the ortho and para positions are strong acids that will dissolve in both strong and weak bases. STRONG ACIDS WEAK ACIDS Carboxylic acids Sulfonic Acids Ortho and para substituted di- and trinitrophenols 4. Phenols Nitroalkanes β-diketones Solubility in Conc. Sulfuric Acid and Phosphoric Acid. Many compounds are soluble in cold, concentrated sulfuric acid. Of the compounds commonly encountered, alcohols, ketones aldehydes and esters fall in this category. Other compounds which also dissolve include alkenes, alkynes, nitroaromatics and amides. Since several different types of compounds are soluble in sulfuric acid, further chemicalt tests are required. Compounds which are insoluble in sulfuric acid are extremely weak bases. It is the extreme acidity of concentrated sulfuric acid along with its powerful ionizing ability that allows these compounds to be soluble in sulfuric acid. Thus sulfuric acid is able to protonate alcohols, ketones aldehydes,ester an unsaturated hydrocarbons. The resulting ions produced are soluble in the medium.. R OH + H2SO4 R OH2 + HSO4 O R C R' + H2SO4 O H R C R' + HSO4 O R C OR" + H2SO4 R H R + H2SO4 R O H R C OR" + HSO4 R R R + HSO4 R R Ionizing Power of Conc. H2SO4 Phosphoric acid, on the other hand is not nearly as strong an ionizing solvent as is sulfuric acid. It is capable of ionizing most of the oxygen containing compounds such as alcohols,aldehydes, and esters, but is not able to ionize unsaturated hydrocarbons such as alkenes,alkynes or aromatics. Thus these compounds will be insoluble in this medium. 5. Inert Compounds Compounds not soluble in conc. Sulfuric Acid or any of the other solvents are said to be inert. These compounds include the alkanes, most simple aromatics, and the alkyl halides. Read: "The Systematic Identification of Organic Compounds", Chapter 5 Section 1 pp 90-112. Know how to use chart on p 93 to determine solubility classes.