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Principle: The reaction between aromatic aldehyde and an aliphatic anhydride capable of providing an ‘active methylene‘ moiety in the presence of a basic catalyst, such as: acetate ion and a hydronium ion, which yields an α, β-unsaturated carboxylic acid and a mole of acetic acid i.e. interaction between benzaldehyde and acetic anhydride in presence of acetate ion and a hydronium ion yields, cinnamic acid and acetic acid.1,2
Aim: To prepare cinnamic acid from benzaldehyde.
Cinnamic acid is used in the flavors, synthetic indigo and certain pharmaceuticals. It is used in the manufacture of methyl, ethyl, and benzyl esters for perfume industry. It is also used as a precursor of the sweetener, aspartame.1
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Chemicals: Benzaldehyde – 10.5 g
Potassium acetate – 6 g
Acetic Anhydride – 15 g
Sodium carbonate – 20 g
Conc. Hydrochloric Acid – q.s.
Rectified Spirit – 50 ml
Apparatus: Double naked round-bottomed flask with guard tube system
Reflux condenser set
Another 500 ml RBF with steam distillation system
Buchner funnel etc
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About 10.5 g (10 ml) of benzaldehyde, 15 g (14 ml) of acetic anhydride and 6 g of finely powdered potassium acetate are transferred into a dry 250 ml round bottomed flask, provided with CaCl2-guard tube at its top-end. Contents of the RB-flask are mixed thoroughly and the reaction mixture is heated in an oil bath at 160°C for 60 minutes; and further at an elevated temperature of 170-180°C for about 3 hours. While still hot (90°-100°C) the contents of the flask is poured into a 500 ml round-bottomed flask containing about 50 ml of water, which is fitted for steam-distillation operation. The contents of the flask-1 is rinsed with a little hot water and poured into the flask-2. The resulting solution in the 500 ml RB-flask is made alkaline by adding gradually a saturated solution of Na2CO3 with vigorous shaking. The solution is subjected to steam-distillation until all the ‘unreacted benzaldehyde‘ is removed and the distillate is absolutely clear. The contents of the distillation flask is cooled and filtered by suction to get rid of most resinous unwanted by-products. Carefully, the filtrate is rendered to acidic pH by adding concentrated HCl gradually in small lots at intervals, and with continuous shaking until the evolution of CO2 ceases completely.3 The resulting solution is chilled, cinnamic acid gets separated as colourless crystals, filtered in the Buchner funnel, washed with a little cold water, drained well and dried at 100 °C.
Calculation of yield:
106.12 g of Benzaldehyde yields Cinnamic Acid = 148.16 g
10.5 g of Benzaldehyde shall yield Cinnamic Acid = 148. 16 / 106. 12 x 10. 5 = 14.66 g
Hence, Theoretical yield of Cinnamic Acid = 14.66 g
If reported Practical yield = 9.5 g
Then, Percentage Practical yield = Practical yield / Theoretical yield × 100
= 9. 5 / 14. 66 x 100 = 64.8%
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The percentage yield of the Cinnamic acid is about 64.8% of melting point 132 °C
[ps2id id=’references’ target=”/][ps2id id=’1′ target=”/]
Kar Ashutosh, Advanced Practical Medicinal Chemistry, New Age International (P) Limited Publication, New Delhi, Page No. – 182.
Advanced Organic Chemistry: Reaction Mechanisms by Reinhard Bruckner, Published by Harcourt Academic Press: 2002; Page No. 419.
Systematic Lab Experiments in Organic Chemistry by Arun Sethi: New Age International Publishers; First Edition- 2003, Reprint- 2006, Page No. 706.