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BACKGROUND
Principle:
Methyl salicylate (oil of wintergreen) is an organic ester. When an acid (containing the –COOH group) reacts with an alcohol (a compound containing an –OH group) formed an ester. The type of reaction may be known as condensation reaction because the small molecule of H2O is eliminated from the reactants while the remaining bits of the reactants condense together to give the main product. This reaction is also termed as an esterification, since the product of the reaction is an ester, a compound containing the COOR group.1
Aim:
To prepare methyl salicylate from salicylic acid.
Reaction:
Mechanism:
Use:
It is used to treat muscular and joint pain.
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REQUIREMENTS
Chemicals: Salicylic acid
Dry methanol
Concentrated sulphuric acid
Carbon tetrachloride
Magnesium sulphate etc.
Apparatus: Round bottomed flask – 500 ml,
Reflux condenser
Thermometer
Separatory funnel
Beaker
Buchner funnel
Measuring cylinder
Filter paper
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PROCEDURE
Take a mixture of 28 g (0.2 mol) of salicylic acid, 64 g (81 ml, 2 mol) of dry methanol and 8 ml of concentrated sulphuric acid in a round bottomed flask of 500 ml. Put a few small chips of porous porcelain, set a reflux condenser to the flask and boil the mixture gently for 5 h. Distil off the excess amount of alcohol on a water bath and allow to cool. Pour the residue into approximately 250 ml of water contained in a separatory funnel and rinse the flask with a few ml of water which are also poured into the separatory funnel. If, owing to the comparatively slight difference between the density of the ester and of water, difficulty is experienced in obtaining a sharp separation of the lower ester layer and water, add 10-15 ml of carbon tetrachloride (2) and shake the reaction mixture in that funnel vigorously upon standing, the heavy solution of methyl salicylate in the carbon tetrachloride separates sharply and rapidly at the bottom of the separatory funnel. Run off the lower layer cautiously, discard the upper aqueous layer, return the methyl salicylate to the funnel and shake it with a strong solution of sodium hydrogen carbonate until all free acid is removed and no further evolution of carbon dioxide occurs. Wash once with water, and dry by pouring into a small dry conical flask containing about 5 g of magnesium sulphate. Stopper the flask, shake for about 5 min and allow standing for at least half an hour with occasional shaking. Filter the methyl salicylate solution through a small fluted filter paper directly into a round bottomed flask fitted with a still-head carrying a 360 °C thermometer and an air condenser. Put some boiling chips and distil from an air bath; increase the temperature slowly at first until all carbon tetrachloride has passed over and then heat more strongly. Collect the methyl salicylate (a colourless oil of delightful fragrance, ‘oil of wintergreen’) at 221— 224 °C; the yield is 25 g (81%). The ester may also be distilled under reduced pressure; the b.p. is 115 °C/20 mmHg and a 2 °C fraction should be collected.
Calculation:
Here limiting reagent is salicylic acid; hence yield should be calculated from its amount taken.
Molecular formula of salicylic acid = C7H6O3
Molecular formula of methyl salicylate = C8H8O3
Molecular weight of salicylic acid = 138 g/mole
Molecular weight of methyl salicylate = 152 g/mole
Theoretical yield:
138 g salicylic acid forms 152 g methyl salicylate
Therefore, 28 g salicylic acid will form …….? (X) g methyl salicylate
X =( 152 ×28)/138 = 30.84 g
Theoretical yield = 30.84 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
Methyl salicylate was synthesized and the percentage yield was found to be………..%
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REFERENCES
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Vogel’s Textbook of Practical Organic Chemistry by Brian S. Furniss, Antony J. Hannaford, Peter W. G. Smith & Austin R. Tatchell; Fifth Edition; Page No. 1078.
