Hydrolysis of Methyl Salicylate P.Pradeep M15607
1 AIM To synthesize acid through hydrolysis under basic conditions followed by acidification of methyl salicylate and to determine its yield and purity by measuring the product mass and the melting point range.
2 INTRODUCTION The reagents used in this experiment are NaOH, concentrated hydrochloric acid and Methyl Salicylate. The final product produced is Salicylic acid. Sodium hydroxide is an inorganic compound. It is a white solid and a highly caustic metallic base and alkali salt. It is industrially produced as a 50% solution by variations of the electrolytic chloralkali process. Sodium hydroxide is a popular strong base used in the paper industry, manufacturing of sodium salts and detergents and in the Bayer process of aluminium production. Hydrochloric acid is a clear, colorless, highly pungent solution of hydrogen chloride (HCl) in water. It is a highly corrosive, strong mineral acid with many industrial uses. It is prepared by dissolving hydrogen chloride in water. One of the applications of hydrochloric acid is in the pickling of steel, to remove rust or iron oxide scale from iron or steel. Methyl salicylate is an organic ester naturally produced by many species of plants, particularly wintergreens. It is also synthetically produced and is used as a fragrance, in foods and beverages, and in liniments [1]. Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. It has the formula C7H6O3. Salicylic acid is biosynthesized from the amino acid phenylalanine. It functions as a plant hormone and also has the ability to ease aches and pains and reduce fevers [2]. It is also used as an anti-inflammatory drug. Silver nitrate is an inorganic compound with chemical formula AgNO3 .
This Methyl salicylate is treated with aqueous base and the resulting hydrolysis reaction forms methanol, water, and the sodium salt of salicylic acid. The salt is then acidified with sulfuric acid to convert the organic salt into the protonated carboxylic acid. Methanol and salicylic acid are therefore produced as major products and salicylic acid is isolated and purified by crystallization.
3 OVERALL EQUATION
4 MECHANISM
5 PROCEDURE 2.0 mL of methyl salicylate (from a pre-adjusted dispenser) was added in a 100-mL round-bottom flask (rbf) followed by 20 mL of 2M NaOH and two pieces of boiling chips. The mixture was then reflxed in a hot oil bath until it becomes homogeneous. The mixture was then heated for another 5 min and after which the rbf was lifted up to be cooled. Once it is sufficiently cooled to be touched the water flow was switched off and the condenser was disconnected carefully. The content was further cooled in an ice-water bath while still in the rbf. 3 mL of concentrated HCl (from a preadjusted dispenser) was then cautiously added into the rbf and its acidity was tested using conge Red paper. The crystals were collected using suction filtration. To prepare the suction filtration, a Buchner funnel, filtration flask and rubber adaptor were assembled. Two pieces of filter paper were placed inside the funnel and wet with cold deionized water, a poor solvent. The hose leading to the vacuum source was attached to the side arm of the filtration flask and the vacuum source was turned on. The mixture with the precipitate was slowly poured into the A small amount of cold deionized water was added into the conical flask to transfer the residual compounds left in the flask. The round bottom flask was then rinsed with about 15 mL of cold deionized water and as much of the crystals as possible was recovered. Another 5 mL cold deionized water was used to wash the product on the funnel. The sample was rinsed with a small amount of cold deionised water before the hose was disconnected from the vacuum source. The crystals were washed on the filter paper with cold deionized water one to two more times until the filtrate no longer test positive with silver nitrate solution. The product was purified by recrystallization using water as the solvent and the crystals were collected using suction filtration. A piece of filter paper on a watch glass was weighed on a weighing scale and the mass was recorded. The crystals that were filtered using suction filtration was then spread on a piece of filter paper on a watch glass. They were then dried under IR lamp for about 5 minutes. The yield and the melting point the product was then recorded. Once dried, the crystals were weighed with a weighing scale and the mass was recorded. To determine the mass of crystals, the mass of the filter paper was subtracted from the mass of the crystals and filter paper. To determine the melting point, a melting point capillary tube was inverted and the open end was dabbed into the crystals. The capillary tube was reverted upright and “tapped” to allow the compound to settle down at the closed end of the tube. The capillary tube was inserted into the melting point apparatus and the heating process was initiated. The temperature at which the sample began to melt and the temperature at which the sample had completely turned into a liquid was recorded. These temperatures were then used to find the range of the melting point to determine purity of the product.
6 OBSERVATIONS Step No.
Procedure
Observations
1
Add 2.0 mL of methyl salicylate (from a pre-adjusted dispenser) in a 100-mL round-bottom flask (rbf). Add 20 mL of 2M NaOH and two pieces of boiling chips into the mixture.
White Precipitate Observed
2
3
4 5
6
7
8 9
10
Reflux the mixture in a hot oil bath until it becomes homogeneous (i.e. clear solution). Continue heating for another 5 minutes. Lift up the rbf to cool it. When you can touch the flask comfortably, turn off the water flow and disconnect the condenser carefully. Further cool the content, still in the rbf, in an ice-water bath. Cautiously add 3 mL of concentrated HCl (from a pre-adjusted dispenser) into the rbf. Check if this mixture is acidic to Congo Red paper. Add a few more mL of concentrated HCl, if necessary Collect the crystals using suction filtration. Rinse the round bottom flask with about 15 mL of cold deionized water to recover as much of the crystals as possible. Wash the crystals on the filter paper with cold deionized water one to two more times until the filtrate no longer test positive with silver nitrate solution. Purify the product by recrystallization using water as the solvent. Collect the crystals using suction filtration and dry the crystals under the infra-red lamp. Determine the weight and melting point of the purified product
Clear Solution achieved
-
White precipitate formed
Red Congo paper turned blue
-
White crystals formed after the round bottom flask was left to cool Dry White crystals obtained
Yield Mass: 1.822g M.P range : 156.8∘ 𝐶 − 159.9∘ 𝐶
7 RESULTS AND CALCULATIONS Compound
Methyl Salicylate
HCl
NaOH
Salicylic acid
2
1
Mole ratio
1
1
Theoretical moles/
𝟐. 𝟎 × 𝟏. 𝟏𝟕 𝟏𝟓𝟐. 𝟏𝟒𝟗𝟒
-
moles
= 𝟎. 𝟎𝟏𝟓𝟑𝟖
𝟎. 𝟎𝟐𝟎 × 𝟐 = 𝟎. 𝟎𝟒𝟎
𝒏𝒔𝒂𝒍𝒊𝒄𝒚𝒍𝒊𝒄 𝒂𝒄𝒊𝒅 = 𝟎. 𝟎𝟏𝟓𝟑𝟖
According to stoichiometry, the molar ratio should be 1:2 for methyl salicylate : NaOH. Thus, from calculations, Methyl Salicylate is the limiting reagent
Expected mass of Salicylic Acid crystals: 𝟎. 𝟎𝟏𝟓𝟑𝟖 × 𝟏𝟑𝟖. 𝟏𝟐𝟏 = 𝟐. 𝟏𝟐𝟒𝒈
Mass of filter paper and watch glass: 34.9998 𝑔 Mass of product and filter paper and watch glass: 36.8220𝑔 Actual mass of product: 1.822𝒈
Percentage yield: 1.822 × 100% = 86% 2.124
Expected melting point range: 158∘ 𝐶 − 159∘ 𝐶 Experimental melting point range: 156.8∘ 𝐶 − 159.9∘ 𝐶
8 DISCUSSION Refer to Observations table above for Corresponding Step number: Whit precipitate was formed in the step 1 after the addition of NaOH. This was due to the formation Sodium salicylate and salicylic acid crystals. The reaction was then put under reflux in a hot oil bath until the mixture became homogeneous and clear. The mixture became colorless as the heating resulted in the formation of methanol (CH3OH) and the conjugate base of salicylic acid, both which are colorless. In step 5, HCl was added and by doing so H+ ions were introduced and the conjugate base was re-protonated forming salicylic acid, causing the mixture to become white again. Congo red paper was used to check if the mixture was indeed acidic. Congo red paper turns blue at a pH below 3.0. So if the solution was not acidic, the Congo red paper would remain red. However, our solution was indeed acidic and thus the congo red paper turned blue. Cold deionized water was used to wash the crystals during suction filtration to remove ions such as Na+ and Cl- that are soluble in water. A silver chloride test was carried out to test of the ions have been washed away. This is so that the purity of the salicylic acid crystals are not compromised in the end due to the presence of these ions. A positive silver nitrate test (where a white precipitate, AgCl forms when AgNO3 is added to the filtrate) indicated that there was still Cl- ions in the crystals. Thus, the crystals were continuously washed with cold deionized water until the silver chloride tests turned up negative. The crystals were then left to dry under IR-lamp to dry the crystals to remove water. The yield was about 86% was obtained and the melting point range of the product obtained was from 156.8°C – 159.9°C, which is much wider than the theoretical melting point rang of 158∘ 𝐶 − 159∘ 𝐶 . There are many possible errors in the experiment mentioned below.
There is the possibility of the precipitate being washed down in the filtrate during suction filtration as there could have been gaps at the side where the filter paper did not stick fully and so some precipitate was lost when the solution was transferred to the Büchner funnel. Some of the precipitate was also stuck on the filter paper after suction filtration and could not be removed. This might have caused a loss of product The addition of too much water during recrystallization might have also resulted in the yield to be lower as the amount of Salicylic Acid crystallized would be lower as the solution is no longer saturated thus increasing the solubility of the Salicylic acid even at low temperatures. Also, the mixture was fully cooled down to room temperature before being placed in the ice water bath, which resulted in the crystals not formed in an orderly manner. Thus there could be holes and gaps formed in the crystals and other molecules water may have entered the crystals, causing the product to be impure. Other sources of impurities could have been from the equipment, which may not have been totally clean and as the starting materials are not clean it could result in the product being impure. All these could have resulted in our product’s melting point range being very wide. We had also performed about 3 rounds of drying using filter paper due to crystals still being wet. There was some loss of crystals with each round of drying and that might have contributed to the low percentage yield observed
There are many improvements that could have been made. They are mentioned below:
One improvement would be to use clean equipment and categorized starting reagents to ensure purity of starting reagents to prevent any impurities at the start of the experiment. We should have been more patient and should have waited for the warm water to be full cooled to room temperature before doing crystallization by placing the flask in the ice bath. Other methods such as Fourier Transform infrared (FT-IR) spectroscopy could have been used [4] to determine the purity of the product. It is a technique that allows infrared radiation to pass through a sample, which gives a resulting spectrum. The resulting spectrum is different for each sample just like fingerprints of humans and thus is the resulting spectrum of our product matches the spectrum in literature of salicylic acid, the product is pure.
9 CONCLUSIONS Methyl Salicylate was hydrolyzed under basic conditions followed by acidification to synthesize Salicylic Acid with percentage yield of 86% and melting point range of 156.8∘ 𝐶 − 159.9∘ 𝐶 which
is wider than the theoretical melting point range of158∘ 𝐶 − 159∘ 𝐶. Thus the product is not entirely pure.
10 REFERENCES 1. Park, Sang-Wook, et al. "Methyl salicylate is a critical mobile signal for plant systemic acquired resistance." Science 318.5847 (2007): 113-116.
2. Madan, Raman K., and Jacob Levitt. 'A Review Of Toxicity From Topical Salicylic Acid Preparations'. Journal of the American Academy of Dermatology 70.4 (2014): 788-792. Web
3. Coquoz, J.-L. 'The Biosynthesis Of Salicylic Acid In Potato Plants'. PLANT PHYSIOLOGY 117.3 (1998): 1095-1101. Web. 4. Perkins, W. D. 'Fourier Transform Infrared Spectroscopy. Part II. Advantages Of FT-IR'. J. Chem. Educ. 64.11 (1987): A269. Web.
11 POST LAB QUESTIONS 1. Congo Red paper turns to blue color at a pH below 3.0. Blue Litmus paper on the other hand changes color even at a pH above 4.0. Since the pH of Salicylic acid is around 3.0, using Congo red would be a safer bet as by using a litmus paper, a positive result for the acidity test would be achieved at a pH higher than 3.0 and thus there might be lesser Salicylic acid molecules. Thus the yield would be lesser and the amount of impurities would be more. 2. In step 7, the crystals on the filter paper were washed with cold deionized water one to two more times until the filtrate no longer tested positive with silver nitrate solution. a. In a positive silver nitrate test a white precipitate, AgCl forms when AgNO3 is added to the filtrate. i. Cl- (aq) + Ag+ (aq) AgCl(s) b. The crystals on the filter paper were washed with cold deionized water to remove ions such as Na+ and Cl- which are soluble in water. The test was done so that we are aware when the crystals such as the Na+ and Cl- ions are fully washed off from the crystals. c. If the test is not performed. Na+ and Cl- ions would be present in the crystals, reducing the purity of the product. The yield would also be reduced. The HCl remnants due to the remaining Cl- ions would react with the salicylic acid thus reducing the yield.
