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BACKGROUND
Principle:
Microwave ovens work by using microwaves about 12cm in length to force polarised molecules in food to rotate. The interaction of these molecules undergoing forced rotation creates heat and the food is cooked. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy when dispersed as molecular vibration in solids and liquids is heat.
Microwave assisted reactions can be carried out under a variety of conditions, viz. solution phase, on a solid support, without a solvent. The main advantage of solvent free microwave reaction is the reduction or elimination of the use of volatile and toxic organic solvents. This facilitates easy isolation of products and promotes safety associated with handling the reaction in open vessels accumulating in chemical processes with unique attributes such as faster reaction kinetics, higher product yields.1
Aim: To prepare aspirin from salicylic acid by using microwave environment.
Reaction:
Mechanism:
Use:
It has analgesic action. It is recommended for arthritis conditions.
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REQUIREMENTS
Chemicals: Salicylic acid – 1.38 g
Acetic anhydride – 3.06 ml
Sulphuric acid
Sprit
Apparatus: Conical flask
Beaker
Pipette
Glass rod
Buchner funnel
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PROCEDURE
Take 1.38 g (0.01 mol) of salicylic acid, 3.06 g of acetic anhydride, one drop of conc. sulphuric acid or 85% phosphoric acid in a 100 ml beaker and mix well. Cover the beaker with a watch glass and place it in a microwave oven at 600W for 3 min or at a power level of 30 % (probably level 3) or at 70°C for five min. (Also you can place 2 min at 80% power and again a 2 min. of stand maintaining the same condition in microwave oven after few second of stirring). Take the beaker out of the microwave oven, allow it to cool to room temperature and then place in an ice bath. Recrystallize from boiling water along with small quantities of spirit. Report the yield and melting point.
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 acetylsalicylic acid (aspirin) = C9H8O4
Molecular weight of salicylic acid = 138 g/mole
Molecular weight of acetyl salicylic acid (aspirin) = 180 g/mole
Theoretical yield:
1.38 g of salicylic acid gives 180 g of aspirin
Therefore, 5 g of salicylic acid will give ? (X) g of aspirin
= X =( 180 ×1.38)/138 = 1.8 g
Theoretical yield = 1.8 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
Aspirin was synthesized and the yield was found to be …..%.
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REFERENCES
- Cresswell SL, Haswell SJ. J Chem Educ. 2001;78:900-4.
- Loupy A. Microwaves in Organic Synthesis, 1st edition; Wiley-VCR Verlag GbH & Company KGaA, Weinheim, 2002, pp. 35, 115-116.
- Mirafzal GA, Summer JM. J Chem Educ. 2000;77:356-7.
