Formal lab report

Formal lab report

• 1.
  Synthesis of methylsalicylate (oil of wintergreen) using Fischer-Speiers esterification method Jacob scholes 1. Introduction Methyl salicylate is an ester which are widely used for their unique properties which include pleasant smells and flavours and they are found in many plants and herbs. Methyl salicylate is more commonly known as “oil of wintergreen” because it is found within wintergreen plant species (Gaultheria procumbens)1 however the majority of the methyl salicylate we use today in such products as “deep heat” and “olbas oil” (which gives the strong aroma) is synthesized in laboratories. This is because the wintergreen plant does not meet the demand when it comes to supplying enough methyl salicylate. The reason methyl salicylate is inlcuded in pain relief products like mentioned is because it is thought to have a numbing effect on the area of external use. Esters like methyl salicylate can be absorbed easily into the skin which makes it extremely useful for treating pain. Due to the strong smell of methyl salicylate it can be used in small quantities for hygenic properties such as toothpaste to give a “minty smell”. There are many ways to produce esters in a laboratory however the method I used for the synthesis of methyl salicylate was Fischer-Speier esterification. This involves a carboxylic acid being refluxed with an excess of alcohol in the presence of an acid catalyst. For this synthesis, salicylic acid and methanol were used with an acid catalyst of concentrated sulfuric acid. This reaction is an equilibrium so to push the equlibrium towards the products, we have to use le chatelier’s principle and using it we can see that an excess of methanol will push the reaction in our favour to achieve a greater percentage yield of methyl salicylate. We can use an excess of methanol in this reaction because methanol can be evaporated off very easily using a rotary evaporator. Figure 1 – Shows reaction of salicylic acid and methanol in the presence of concentrated sulfuric acid catalyst to form methyl salicylate (oil of wintergreen) and water.
• 2.
  Figure 2 –Mechanism of methyl salicylate synthesis Firstly in the mechanism, we can see that a proton from the sulfuric acid catalyst has accepted a pair of electrons from the oxygen which then leaves the oxygen with a partial positive charge. Now, the oxygen on the methanol can attack and form a bond with the carbon which is bonded to the partially charged oxygen. This leads to a double bond being broken between the carbon and oxygen. This causes water to fall off the molecule and left behind is methyl salicylate.2 2. Results and discussion ALTHOUGH I DID NOT PRODUCE ANY SPECTRUMS FOR MY METHYL SALICYLATE I WILL BE ANALYSING THE FIGURES BELOW WHICH SHOW ACCURATE SPECTRUMS FOR METHYL SALICYLATE AND SALICYLIC ACID. Figure 3 – IR spectrum of salicylic acid Figure 4 – IR spectrum of methyl salicylate IR spectrums can show thedifferences and similarities within thefunctional groups of molecules. This is possiblebecausedifferent bonds within thefunctional groups can absorb different wavelengths. Thedifferences between salicylic acid and methyl salicylatearethat thecarboxylic acid group in salicylic acid has been converted into an ester in methyl salicylate. Wecan see this from thefigures 3 and 4 shown above, we can
• 3.
  see that thereisnot as much absorbancebetween 3000-4000cm-1 which indicatesthat theOH in thecarboxylic acid group has been converted into an ester. Another difference indicates by thefigures is thesharp peak at 1673cm-1 which is clearly seen in themethyl salicylatespectrum but not In thesalicylic acid spectrum. This is due to theconjugated ketone. Yield of methyl salicylate obtained = 8.19g % 𝑦𝑖𝑒𝑙𝑑 = 𝑎𝑐𝑡𝑢𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 x 100 Theoretical yield: mass of salicylic acid used = 6.998g Mr of salicylic acid = 138 Mr of methyl salicylate = 152 Mass = Mr x Mol | Moles of salicylic acid = (6.998/138) = 0.0507 moles 0.0507 x 152 = 7.708g Theoretical yield = 7.71g % Yield = (8.19g / 7.71g) x 100 % Yield = 106.2 % Yield obtained may be greater than theorized as not all of the solvent had been evaporated off. Figure 5 – Thin layer chromotography of Figure 6 – Thin layer chromotography of compounds or mixtures labelled above. compounds or mixtures labelled above. Salicylic Reaction Methyl Acid Mixture Salicylate Salicylic Crude Pure Acid Methyl Methyl Salicylate salicylate
• 4.
  To confirm formationand purity of a product we can use TLC (thin layer chromotography). The TLC plate on the left in figure 5 shows the reaction mixture after only 20 minutes under reflux so it was expected that there was methyl salicylate formed and some unreacted salicylic acid. If the reflux was left much longer, the reaction would have gone to completion more compared to the 20 minutes that was done. In figure 6 (after work up) we can see that the crude methyl salicylate was relatively pure because it looks very similar to the pure methyl salicylate sample given. We can see that the crude methyl salicylate was relatively pure because there is only one spot and not two. This indicates that there was only methyl salicylate present in the crude sample and it was free of any unreacted salicylic acid. In the work up, water was firstly added to remove the concentrated sulfuric acid catalyst still present in the mixture. This also diluted excess methanol used initially. Then a solution of sodium bicarbonate was added to the organic layer to react with any salicylic acid present in the mixture. 3. Experimental method Salicylic acid (6.998g, 0.051mol) was placed in a 100ml round bottomed flask containing a stirrer bar which was in a heating mantle, along with methanol (30ml, 0.75mol), and was stirred with a stir bar until all the salicylic acid had dissolved into the methanol. To this mixture concentrated sulphuric acid (8ml) was added dropwise very slowly with constant stirring. This is because a lot of heat was produced when the sulfuric acid was added so it was done slowly to avoid boiling and splashing. The round bottomed flask was connected to a water condenser vertically and was allowed to reflux for about 20 minutes. After, the reaction mixture was disconnected and cooled in an ice bath and the purity was checked against salicylic acid and pure methyl salicylate by thin-layer chromatography (TLC) the solvent used for this was ethyl acetate and heptane in the ratio 1:4. The reaction mixture was then transferred to a separating funnel and water (150ml) was added. The mixture was extracted twice with dichloromethane (2 x 75ml). This was vented frequently to release pressure from building when shaking the funnel to mix the contents properly. The combined organic layer was put back into the separating funnel and washed with saturated bicarbonate solution (75ml) again with frequent venting to release CO2 gas that was building up. This was then seperated off and the organic layer was then dried over anhydrous magnesium sulfate to remove any water still present. This was then filtered into a weighed round bottomed flask and the dichloromethane was evaporated on the rotary evaporator. The purity of the crude product was checked again by TLC using the same solvent.3
• 5.
  4. References 1 https://www.britannica.com/plant/wintergreen-plant (accessed– 26/11/2020) 2 Organic chemistry, Clayden, Jonathan, Greeves, Nick, Warren, Stuart, Wothers, 2000 3 Lab book used for experimental method on the day (19/11/2020)