Among the simplest examples are formic acid H-COOH, which occurs in ants, and acetic acid CH3-COOH, which gives vinegar its sour taste. Acids with two or more carboxyl groups are calleddicarboxyllc, tricarboxylic, etc. The simplest dicarboxylic example Is oxalic acid (COOH)2, which Is Just two connected carboxyls. Mellitic acid is an example of a hexacarboxylic acid. Other important natural examples are citric acid (in lemons) and tartaric acid (in tamarinds). Salts and esters of carboxylic acids are called carboxylates. When a carboxyl group Is deprotonated, Its conjugate base forms a carboxylate anion.
Carboxylate ions are resonance stabilized and this increased stability makes carboxylic acids more acidic than alcohols. Carboxylic acids can be seen as reduced or alkylated forms of the Lewis acid carbon dioxide; under some circumstances they can be decarboxylated to yield carbon dioxide. Physical properties Solublllty Carboxylic acids are polar. Because they are both hydrogen-bond acceptors (the carbonyl) and hydrogen-bond donors (the hydroxyl), they also participate in hydrogen bonding. Together the hydroxyl and carbonyl group forms the functional group carboxyl.
Carboxylic acids usually exist as dimeric pairs In nonpolar media due to their tendency to “self-associate. ” Smaller carboxylic acids (1 to 5 carbons) are soluble in water, whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain. These longer chain acids tend to be rather soluble In less-polar solvents such as ethers and alcohols. Bolling points Carboxylic acids tend to have higher boiling points than water, not only because of their increased surface area, but because of their tendency to form stabilised dimers.
Carboxylic acids tend to evaporate or boil as these dimers. For boiling to occur, either the dimer bonds must be broken, or the enure dimer arrangement must be vaporised, both of which increase enthalpy of vaporization requirements significantly. Acidity Carboxylic acids are typically weak acids, meaning that they only partially dissociate Into cations and RCOO- anions In neutral aqueous solution. For example, at room temperature, In a I-molar solution of acetic acid, only 0. 4% of the acid molecules are dissociated. Electronegative substituents give stronger acids.
This reagent is useful for classifying compounds known to contain halogen. Add 1 drop or a couple of crystals of the unknown to 2 mL of the 2% ethanolic silver nitrate solution. If no reaction is observed after 5 min standing at room temperature, heat the solution to boiling and note if a precipitate is formed. If there is a precipitate, note its color. Add 2 drops of 5% nitric acid, and note if the precipitate dissolves. Silver halides are insoluble in dilute nitric acid; silver salts of organic acids are soluble. Positive Test alkyl halide – Production of solid silver halide salt is a positive test. yl halide – Production of solid silver carboxylate salt is a positive test. This solid should redissolve in dilute nitric acid. carboxylic acid – Production of solid silver carboxylate salt is a positive test. This solid should redissolve in dilute nitric acid. sulfonyl chloride – Production of solid silver sulfonate salt is a positive test. This solid should redissolve in dilute nitric acid. Complications The time and temperature required to form the solid salt can vary widely. Carboxylic Acid Sodium bicarbonate test for carboxylic acids.
A few drops or a few crystals of the unknown sample are dissolved in 1 mL of methanol and slowly added to 1 mL of a saturated solution of sodium bicarbonate. Evolution of carbon dioxide gas is a positive test for the presence of the carboxylic Complications None. Schotten-Baumann Reaction Acid Anhydride- Schotten-Baumann reaction for anhydride. Acyl Halide- Schotten-Baumann reaction for acyl halide. In a small flask, place 0. 5 mL of ethanol, 1 mL of water, and 0. 2 mL or 0. 20 g of the unknown compound. To this solution add in portions, with vigorous shaking, 2 mL of 0% sodium hydroxide solution.
Stopper the flask and shake the mixture for several minutes and then test the solution with litmus paper to make sure that it is still alkaline. The anhydrides and acyl halides will undergo a reaction with alcohols under basic conditions to form esters. Esters are both insoluble in water and less dense than water and thus will form a layer on top of the water. Formation of an oily layer on top of the water is a positive test Complications Schotten-Baumann reaction for carboxylic acids. A mixture of 0. 20 g of the compound, 0. 40 mL of absolute ethanol, and 0. mL of concentrated sulfuric acid is warmed over a steam bath for 2 min. The mixture is poured slowly into an evaporating dish containing 2 mL of saturated sodium bicarbonate solution. A second layer should be formed. Carefully smell the mixture. rhe presence of a sweet, fruity smell in the product, where no such smell existed in the original unknown, indicates that the original compound was a carboxylic acid and the acid was esterified. Large molecular weight carboxylic acids produce esters that are odorless. Formation of an oily layer on top of the water is a positive test.