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BACKGROUND
Principle: Here 3-phenylpropionic acid is produced finally from benzylated malonic acid due to decarboxylation on heating. First freshly prepared sodium ethoxide reacts with diethyl malonate giving enolate ion which alkylate with benzyl chloride to produce benzylated malonic ester. Then hydrolysis occurs with the benzylated malonic ester to give benzylated malonic acid in presence of a base like NaOH.1
Aim: To prepare 3-phenyl propionic acid from diethylmalonate.
Reaction:
Mechanism:
Use:
It is used in flavoring agents, food additives, fragrance, spices and medicines as it acts as a preservative.
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REQUIREMENTS
Chemicals: Sodium metal
Dry ethanol
Calcium chloride
Diethyl malonate
Benzyl chloride
Anhydrous sodium sulphate
NaOH
Sulphuric acid
Light petroleum ether
Hydrochloric acid etc.
Apparatus: Three necked flask – 1 litre
Condenser set with separatory funnel
Beaker
Pipette
Glass rod
Buchner funnel etc.
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PROCEDURE
Make a solution of sodium ethoxide from clean sodium 11.5 g (0.5 mol) and super dry ethanol 250 ml in a 1 litre three necked flask with an efficient double surface condenser and a separating funnel; close the central neck with a stopper and clamp the flask by the wide central neck. Attach calcium chloride guard tube at the top of the condenser and the separatory funnel respectively. When the sodium ethoxide solution, which is vigorously stirred, has cooled to about 50 °C, add 80 g (75 ml, 0.49 mol) of redistilled diethyl malonate slowly through the separatory funnel; to the resulting clear solution introduce gradually (60-90 min) 64 g (58 ml, 0.51 mol) of redistilled benzyl chloride. Immediately, reaction occurs and much heat will be evolved. If the reaction becomes violent, cool the flask by giving a stream of cold water over the flask. Reflux the reaction mixture on a water bath until it is neutral to moist litmus (about 2 hours). Remove ethanol as much as possible by distillation under reduced pressure (rotary evaporator) on a water bath. Cool the flask contents to 20 °C, add 200 ml of water to it and shake well. Separate the upper layer of crude ester, dry it with anhydrous sodium sulphate and distil under reduced pressure. A low boiling point fraction passes over first, followed by diethyl benzylmalonate. Hydrolyse the latter with a solution of 75 g of potassium hydroxide in 75 ml of water and isolate the resulting crude 3-phenylpropanoic acid as described previously, using 180 ml of 5 M sulphuric acid in the acidification stage. Distill the product under reduced pressure, collect the purified fractions of b.p. 164-172 °C/25 mmHg which solidifies at room temperature. Recrystallise from light petroleum, b.p. 40-60 °C (or from water containing a little hydrochloric acid), to obtain 20 g (27%) of 3-phenylpropanoic acid of m.p. 47-48 °C.
Calculation:
Here limiting reagent is diethyl malonate; hence yield should be calculated from its amount.
Molecular formula of diethyl malonate = C7H12O4
Molecular formula of 3-phenylpropionic acid = C9H10O2
Molecular weight of diethyl malonate = 160 g/mole
Molecular weight of 3-phenylpropionic acid = 150 g/mole
Theoretical yield:
160 g diethyl malonate forms 150 g 3-phenylpropionic acid
Therefore, 80 g diethyl malonate will form …….? (X) g 3-phenylpropionic acid
X =( 150 ×80)/160 = 75 g
Theoretical yield = 75 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
3-phenylpropionic acid 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. 683.
