Exp 9B: From Oil of Wintergreen to Salicylic Acid Introduction • • • • • In a chemical reaction bonds between atoms are broken and formed resulting in the formation of new chemical compounds A rearrangement of atoms and bonds results in a compound (the “product”) with different properties than the original compound (the “reactant”) Difference are sometimes large and easy to detect The reactant in this reaction, methyl salicylate, is a liquid; the product, salicylic acid, is a solid at room temperature The compounds in this reaction are organic compounds, in which carbon and carbon-carbon bonds are the main element 1 Exp 9B: From Oil of Wintergreen to Salicylic Acid Purpose • • • • • Study the conversion of methyl salicylate (“Oil of Wintergreen”) to salicylic acid Salicylic acid is the basic compound to prepare aspirin, acetylsalicylic acid Like aspirin, salicylic acid has pain-relieving properties Aspirin is easier on the stomach than salicylic acid The actual yield (“How much do you get?”) of salicylic acid in this experiment will be compared to the theoretical yield (“How much could you have gotten, if…?”) 2 Exp 9B: From Oil of Wintergreen to Salicylic Acid Reaction + H2O + CH3OH (methanol) “hydrolysis” + H2O + CH3OH (methanol) 3 Exp 9B: From Oil of Wintergreen to Salicylic Acid Reaction • The reaction will go faster when concentrated sodium hydroxide, NaOH, is used instead of H2O • Excess NaOH will be removed with sulfuric acid, H2SO4 (“neutralization”) • Oil of Wintergreen is soluble in water, but salicylic acid is not soluble in water it will precipitate (“crystallize out”) • Recover the precipitate by suction filtration • Purify by recrystallization from boiling water 4 Exp 9B: From Oil of Wintergreen to Salicylic Acid Recrystallization Purification technique for solids • Solids are more soluble in solvent at higher temperature • Dissolution of solids in appropriate heated solvent • Reformation/precipitation of crystals upon cooling – If impurities remain in solution pure crystals – If impurities do not dissolve filtration before cooling • Determine purity, e.g., by melting point, or other analytical techniques • Recrystallization may have to be repeated to get pure crystals • Working range: – Upper limit = boiling point of solvent – Lower limit = depends on experiment; often ice-water mix (0oC) or ice-salt (-20oC) 5 Exp 9B: From Oil of Wintergreen to Salicylic Acid A. Preparation of reaction • Add 60 mL of distilled water to a 250-mL beaker. • Mark the level of the water with a Sharpie or wax pencil • Pour out the water and dry the beaker • Put a flask with DI H2O on ice to cool down (you will need this later in the experiment!) 6 Exp 9B: From Oil of Wintergreen to Salicylic Acid B. Reaction 1. 2. 3. 4. Measure 4.0 mL methyl salicylate with a pipet and pour in a 10-mL graduated cylinder Measure the total mass on an electronic balance and record in your notebook (How many decimals?) Pour as much as possible of the methyl salicylate in the dry 250 mL beaker. Measure the mass of the graduated cylinder. • Calculate the amount of methyl salicylate that you used 5. 6. 7. 8. Add 40 mL of 6 M NaOH (sodium hydroxide) to the beaker (use a graduated cylinder) (Observation: what happens?) Stir the mixture with a glass stirring rod. Leave the rod in the beaker! Place the beaker on wire gauze and bring to boil. Boil gently for 15 minutes after the precipitate has dissolved. Stir the solution regularly 7 Exp 9B: From Oil of Wintergreen to Salicylic Acid B. Reaction (continued) 9. 10. 11. 12. 13. 14. Rinse solids from the wall of the beaker with DI water from a water bottle. Make sure the volume does not exceed 60 mL (the mark on the beaker). After boiling for 15 min let the beaker cool for a few minutes, then put in ice Add 50 mL DI water to the beaker Cautiously add 50 mL 8 M H2SO4 (sulfuric acid) to the beaker while stirring Cool the beaker in ice Make sure that you have 50-100 mL of DI water in an Erlenmeyer flask in an ice bath!! 8 Exp 9B: From Oil of Wintergreen to Salicylic Acid C. Filtration 1. 2. Pour the cold mixture on a paper filter in a Büchner funnel Filter the mixture by vacuum filtration over a Büchner funnel http://orgchem.colorado.edu/hndbksupport/filt/filtration.html 3. 4. Wash the crude extract with ice-cold water Continue the suction for several minutes to dry the precipitate (Wash and dry the 250-mL beaker during this time) 9 clamp a side arm flask securely to a ring stand place the rubber adaptor in the side arm flask get a piece of filter paper … . . . and put it in the Buchner funnel place the Buchner funnel on the adaptor connect the side arm flask to a vacuum source 10 Wet the paper with a small amount of the solvent to be used in the filtration, turn on the vacuum source Rinse the flask with a little fresh solvent and pour it into the filter funnel Filter the solution Vacuum pulls the solvent through the filter and into the filter flask Disconnect the vacuum AT THE FLASK and before turning off the water aspirator. This prevents water from being sucked into the vacuum flask 11 Carefully remove the filter paper and solid from the Buchner funnel Set the filter cake onto a watch glass to air dry Crude filtrate needs to be further purified by recrystallization 12 Exp 9B: From Oil of Wintergreen to Salicylic Acid D. Recrystallization http://orgchem.colorado.edu/hndbksupport/cryst/crystproc.html 1. Selection of appropriate solvent (in this case: diH2O) 2. Dissolve solid in boiling solvent 3. Formation of crystals by cooling for about 5 min at room temperature • Record: what do you observe, what type of crystals do form 4. Cool further in an ice-water bath 5. Make sure that you have enough ice-cold DI water (50 mL dH2O) in an Erlenmeyer flask 6. Isolate purified crystals by suction filtration as done before Weigh the dry filterpaper before you do the filtration step! 7. Wash the crystals with ice-cold water. Keep suctioning! 8. Keep suctioning for about 10-15 minutes after washing! 13 After a while, crystals should appear in the flask You can now place the flask in an ice bath to finish the crystallization process The crystals are now isolated from the solution by vacuum filtration (see 14 before) Filter paper with crystals on it Carefully scrape the crystals onto the watch glass Let the crystal finish drying on the watch glass 15 Exp 9B: From Oil of Wintergreen to Salicylic Acid Recrystallization E. Drying the Crystals 1. 2. 3. 4. Determine the mass of a 250-ml beaker Carefully transfer crystals with the filterpaper to the beaker Cover the beaker with a watch glass Write your name on the beaker and let the crystals dry in the air Next Monday: 1. 2. 3. Measure and record the mass of the beaker and the contents without the watch glass Determine yield: calculate the yield compared to the theoretical yield (% recovery) Determine melting point: this is a property that will indicate how pure the reaction product is 16 Exp 9B: From Oil of Wintergreen to Salicylic Acid Chemical name Methyl salicylate 2-Hydroxybenzoic acid, Salicylic acid Chemical formula C8H8O3 C7H6O3 Molecular mass 152.1494 g/mol 138.12 g/mol Melting point 8.6 °C 159 °C Boiling point 220 - 224 °C 211 °C (at 20 mmHg) Density 1.174 g/cm Solubility Appearance 3 <0.1 g/100 mL (at 20oC) Colorless, yellowish to reddish oily liquid 3 1.44 g/cm (at 20 °C) 0.2 g/100 mL (at 20oC) Whitish crystals, light-sensitive 17 Exp 9B: From Oil of Wintergreen to Salicylic Acid Sulfuric acid Formula H2SO4 Mole mass 98.0734 Melting point 3 oC Boiling point 280 oC Density 1.84 g/cm3 Solubility Soluble. Miscible/Reactive Appearance Colorless (pure) to dark brown, oily, dense liquid with sharp, acrid odor. Comments Corrosive Sodium hydroxide Formula Mole mass Melting point Boiling point Density Solubility Appearance Comments NaOH 39.99707 318 oC 1390 oC 2.13 g/cm3 50 g/100 mL. Highly soluble Colorless, odorless solid Corrosive 18 Determination of Melting Point INTRODUCTION • The melting point of a pure compound is an intensive property, like density and boiling point. Intensive properties are independent of the amount of substance present. • The melting point of a compound is the temperature at which it changes from a solid to a liquid. • Experimentally, melting point is actually recorded as the range of temperatures in which the first crystal starts to melt until the temperature at which the last crystal just disappears. 19 Determination of Melting Point Reasons for determining melting points: 1. The melting point indicates the level of purity of a sample. An impure compound melts over a wider range of temperatures, usually greater than 2 degrees. 2. The melting point helps to identify unknown samples, narrowing the number of possibilities, because a pure solid melts reproducibly over a narrow range of temperatures. 3. The melting point helps to characterize new compounds. 20 Filling the Capillary and Packing the Sample 21 Loading Sample into Apparatus 22 Onset point 23 Meniscus Point 24 Clear point (U.S. - based melting point) 25 Side–by-side comparison