Notes

These notes contain procedure hints. They also describe some of the "behind the scenes" chemistry that occurs during the experiment.

[1] Stacking and clamping equipment in this way is standard procedure. A lab jack is nearly always used to support stirring and heating equipment so that you can safely separate the heater from the flask without having to grab the heater or flask (hot!). The lowest piece of glassware in any apparatus (usually a round bottom flask) must always be clamped to monkey bars; it should never dangle from other glassware. Remember: support apparatus from the bottom, don’t hang it from the top. See Padias p. 23 for description of Thermowell heater, and p. 31 & 33 for drawings (left side Fig. 1-12 shows our setup minus the lab jack; top Fig. 1-13 shows flasks immersed in ceramic heaters). BACK TO PROCEDURE

[2] Liquid reagents are most conveniently measured by volume. You must calculate the desired volume of reagent before making any measurements (see Calculations for help). You must also record the actual volume used in your lab notebook. It is not necessary to hit the desired amounts precisely, but you should record your measurements precisely. BACK TO PROCEDURE

[3] Aqueous NaOH reacts with the compounds that make up glass, etching and dissolving the glass. Two pieces of glassware can even "freeze" into a single unit if aqueous NaOH is present on the surface where the two pieces touch. Therefore, you should always remove this reagent from ground glass surfaces before assembling apparatus. BACK TO PROCEDURE

[4] Methyl salicylate reacts instantly with NaOH to make a white gummy material, but this does not hinder the eventual formation of salicylic acid. The white gummy material may be the product of an acid-base reaction:

BACK TO PROCEDURE

[5] Reflux means to boil a solution at a sufficient rate so that its vapor condenses in the apparatus above the flask and runs back down into the flask. The controller voltage required to initiate and maintain reflux must be established by trial-and-error. See Padias p. 31. BACK TO PROCEDURE

[6] Your glassware is made out of Pyrex and should not break if subjected to modest temperature shocks. You can take advantage of this fact to quickly cool the reaction mixture by immersing the flask in a large beaker of cold water. BACK TO PROCEDURE

[7] The reaction between methyl salicylate and NaOH produces the dianion of salicylic acid. This dianion, because of its electrical charge, is water-soluble. It is also a weak base, so it reacts with strong acids, like aqueous H₂SO₄, to give neutral, water-insoluble salicylic acid. BACK TO PROCEDURE

[8] Use 3 M H₂SO₄ (do not use concentrated sulfuric acid). You do not need to calculate how much acid to use. Rather, you should add acid until the solution's pH (measured with pH paper) is sufficiently below the pK_a of salicylic acid to guarantee that nearly all of salicylic acid has been neutralized. The pKa of salicylic acid is approx. 3, so any pH of approx. 1 or less is satisfactory. BACK TO PROCEDURE

[9] See Padias p. 25 for description of vacuum aspirator. The knob for the aspirator water is located under the hood. Open the knob completely to create a useful vacuum. See Padias p. 26-30 for description of vacuum filtration. BACK TO PROCEDURE

[10] Salicylic acid precipitates from a solution containing a large number of impurities: Na⁺ and SO₄^2- ions and methanol molecules. Some of these impurities get trapped in the solid, but others can be removed by rinsing with water. Use small amounts of ice-cold water to minimize loss of salicylic acid. BACK TO PROCEDURE

[11] See Padias p. 107-115 for information about recrystallization. This experiment uses a single recrystallization solvent. Later in the year, you will use a mixture of two solvents to bring about recrystallization. BACK TO PROCEDURE

[12] Recrystallization separates a product from impurities by 1) dissolving everything in hot solvent and 2) cooling the resulting solution. If all goes well, the desired product crystallizes as the temperature falls, but the impurities (now present in relatively small amounts) do not. Unfortunately, some product is always lost during recrystallization because some remains dissolved in the cold solution. Therefore, the "trick" for a successful recrystallization is to use just enough solvent to get everything to dissolve, but no more than that.

Here are some common mistakes that you can avoid. 1) Do not remove the "acid" flask from the hot plate to examine it; the solution will cool and the product will crystallize. 2) The product does not dissolve instantly in hot water, but do not wait too long for it to dissolve because the water will boil away while you wait. 3) It is better to use slightly too much water (and lose some of your product) than too little (and fail to separate the impurities from your product). BACK TO PROCEDURE

[13] See Padias p. 45-51 for information about characterization procedures, including accepted sources of scientific data, phase diagrams, and melting point measurements. Several different apparatuses are available for use. Safety tip: every melting apparatus contains a heater that can ignite; turn off the apparatus after use. BACK TO PROCEDURE