Synthesis of Salicylic Acid

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Purpose:
  The purpose of this experiment was to perform a base-catalyzed hydrolysis reaction while converting oil of wintergreen to salicylic acid. The full balanced molecular equation for the salicylic acid synthesis is C₈H₈O₃ + 2 NaOH + H₂SO₄  -> C₄H₆O₃ + Na₂SO₄ + CH₃OH + H₂O. An illustrated equation showing compounds to be isolated or synthesized is shown below.

Salicylic Acid is a white crystalline compound with a melting point of 160°C and molecular weigh of 138 grams/mole, it is not symmetrical and typically gives 7 diff C-13 Nuclear Magnetic Resonance Signals.   

Methyl salicylate has a relatively weak phenol and carboxylic functional groups that are important in this experiment; it also has a mint like smell smell, and reacts rapidly with NaOH. The C-13 NMR has extra signal compared to that of Salicylic Acid, a melting point of 223°C, a molecular weight of 152 grams/mole and a density of 1.17 g/mL.  Structures of molecules mentioned above are shown below.

Procedure

             All procedures were carried out under a hood, all individuals involved wore nitrile gloves and safety goggles. Glassware was cleaned, rinsed with deionzed water and if necessary acetone before usage. Remember...be safe.

            First 3.5 mL of water were poured into a 50mL round bottom flask. Next .448 g Sodium Hydroxide (NaOH), which happened to be in pellet form, was then weighed out using a weigh boat and was added to the water, which was then swirled until the solid dissolved.  The pellet form of the NaOH made it difficult to acquire the .48 grams indicated by the manual. After the pellets dissolved .23 mL of Methyl salicylate (measured with a graduated pipette) was added to the NaOH solution. A boiling stone was then dropped to the bottom of the round bottom flask.

            Next, using grease on the glass joint, a water jacketed condenser was added to the round bottom flask. They were connected and held in place by a screw cap at the joint of intersection between the two. Two water tubes were attached to the condenser, the top arm attached to a tube allowing for water to go out, the bottom arm of the condenser was connected to a tube lead from the water source in the hood, allowing for the water to come in and run through. A clamp was used to hold the apparatus steady and hold fast to a vertical rod in the hood. The round bottom flask was kept in place by an aluminum block, placed on a heating plate, which was turned on to about 1/3 of its maximum power. The mixture was heated for about 15 minutes as the white solid dissolved.

            After reflux, the equipment was allowed to cool to room temperature by separating it from the heat source, it sat till it was only slightly warm and comfortable to handle. The condenser was removed. Next, 3M Sulfuric Acid (H2SO4) was added in .5mL increments till a white precipitate formed and stayed as the solution continued to be stored.  An additional .5mL were added to allow completion of the precipitation reaction.

            The contents of the round bottom flask then underwent vacuum filtration through a Hirsch Funnel fit with filter paper and connected with an integral adapter to a 25mL filter flask. The crude product from the Hirsch funnel was then transferred to weigh paper using a spatula. The salicylic acid crystals were then placed in a 10mL Erlenmeyer flask. Next, 5mL of water were added to the Erlenmeyer flask containing the crystals. The solution was put on the hot plate till the solid dissolved and was immediately taken off.  The flask and its contents were allowed to sit at room temperature while crystallization occurred. The crystals were collected via a second filtration using a Hirsch funnel with the same set up as the one done earlier in the experiment. Upon completion of the filtration the crystals were placed in an empty vial and stored for a week. The following week the crystals were packed into a capillary tube with only one open side. The sample was placed in apparatus, Mel Temp, that provided a magnified view of the sample and temperature, allowing for the assessment of the samples melting point and range. Another sample was prepared with acetone with deutero Hydrogen by an instructor. This sample underwent C-13 Nuclear Magnetic Resonance Spectroscopy (C-13 NMR). Samples from multiple lab groups ran through the machine, the goal was to learn how the C-13 NMR helps determine molecular structure, and look at the induced magnetic field.

Things that are good to know for this lab:

For Reference:       C₈H₈O₃ + 2 NaOH + H₂SO₄  -> C₄H₆O₃ + Na₂SO₄ + CH₃OH + H₂O.

Finding the limiting reagent (plug and chug)

Note: There is a 1:2 molar ratio of Methyl salicylate (C₈H₈O₃) to Sodium Hydroxide, respectively. Note, Methyl salicylate is the limiting reagent; this will be used to calculate the theoretical yield.

_____ g NaOH x      1 mole      = ____moles NaOH x 1 mole Methylsalicylate  = .____ moles Methyl Salicylate needed

                           39.99 grams                                              2 moles NaOH

___ g Methyl Salicylate x 1 mole   = ____ moles Methyl Salicylate x 2 moles NaOH     = ____ moles NaOH needed

                                     152 grams                                                 1 mole Methyl salicylate

Theoretical Yield: Referring to the balanced molecular equation there is a 1:1 molar ratio                                        between Methyl Salicylate and the Salicylic Acid.

___ moles Methyl Salicylate x   1 mole Salicylic Acid = ____ moles Salicylic acid expected

                                                  1 mole Methyl Salicylate

____ moles Salicylic Acid expected x       138 grams       =    ____ grams Salicylic Acid expected

                                                              1 mole Salicylic Acid

Questions?

After NMR give two pieces of evidence that salicylic acid was the solid produced by reacting methyl salicylate with the aqueous base. Which piece of evidence gives the direct structural information? Which gives only indirect evidence?

C13 NMR of methyl salicylate has 8 unique carbon peaks, while salicylic acid has 7. The C that is lost was bonded to 2 Oxygen molecules making it down stream whihc is congruent with the chemical equaiton. C13 NMR gives indirect structural information.

Measuring the melting point and range allows you to compare it to the known melting point of salicylic acid telling you if the compound has the same melting point and if it has impurities. Impurities cause a depression and increase in range. The larger the range the more impurities it has.  Measuring the melting point gives direct information.

Why is it that dilute sulfuric acid does not allow the reaction to proceed while concentrated sulfuric acid does?

Sulfuric Acid is used as a reaction catalyst. This means that you need to overcome an activation energy to create the product. The dilute sulfuric acid does not have as fast and as high dissociation as the higher concentration sulfuric acid, thus the reaction does not finalize because it does not overcome the energy of activation.

On the other hand....

What is the purpose of adding concentrated sulfuric acid in the preparation of methyl salicylate?

Sulfuric acid is used in esterfication reactions as a catalyst.

When methyl salicylate is prepared from salicylic acid and methanol, an OH- is the leaving group from the salicylic acid. The presence of the acid provides a large amount of H+ ions to stabilize the leaving group to H2O, and speeds up the rate of the reaction.

What are  the structures of each of the following compounds

i)-The organic species present immediately after methyl salicylate is first added to the aqueous sodium hydroxide solution and before heating.  
ii)-The organic species present at the end of the reflux period just before the sulfuric acid is added.  iii)-The organic compound present after the sulfuric acid is added.
 Label each  as being soluble or insoluble in water at room temperature.

The Methyl Salicylate is the option "i"it is insoluble in water at room temperature.
The image shown after adding NaOH but before H2SO4 is the organic species present at the end of the reflux period it is soluble in H2O. Salicylic acid, the product is not soluble in water (H2O) during room temperature.

What are the relative chemical shifts in parts per million and their assignment on the Salicylic Acid molecule for C-13 NMR.

Remember C-13 NMR looks at unique carbons. For more on C-13 NMR click here.
 

Peak, δ, ppm	Assignment
112.3	A
117.3	B
119.1	C
130.4	D
136.0	E
162.1	F
171.6	G

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