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BACKGROUND
Aim: To prepare benzamide from benzaldehyde
Principle:
The first step involves the formation of the anion which transfers a hydride ion on to a carbonyl carbon in another aldehyde molecule. The reaction sequence is completed by a proton transfer, to yield the carboxylate anion and the alcohol. In the second step preparation of acid chloride (benzoyl chloride) is occurring due to the action of thionyl chloride (SOCl2) on the corresponding carboxylic acid (benzoic acid). In the third step nucleophilic attack by NH3 on carbonyl carbon occurs which forms a tetrahedral intermediate. Finally due to regeneration of C=O group explosion of Cl– occurs forming benzamide.1
Reaction:
Step 1:
Step 2:
Step 3:
Mechanism:
Step 1:
Step 2:
Step 3:
Use:
It is used as neuroleptics and/or antipsychotics.
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REQUIREMENTS
Chemicals: Potassium hydroxide
Benzaldehyde
Ether
Conc. HCl
Thionyl chloride
Calcium chloride
Conc. ammonia
etc.
Apparatus: Conical flask
Round bottom flask
Reagent bottle
Separating funnel
Condenser
Beaker
Pipette
Glass rod
Buchner funnel
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PROCEDURE
Step 1: – Preparation of benzoic acid from benzaldehyde
Put 29 g potassium hydroxide and 27 ml water in a conical flask, shake to dissolve and cool to 20° C. Pour the cold solution into a reagent bottle, add 30 ml (32 g) of benzaldehyde, cork the bottle properly and shake vigorously until the mixture converts into a thick emulsion. Allow it to stand for at least 4 hours (preferably overnight) and then add sufficient water (about 105 ml) to dissolve all the solid content
Pour the liquid into a separating-funnel, rinse the bottle with 30 ml of ether and pour into the solution in the funnel. Extract the benzyl alcohol out with ether and separate the lower aqueous solution. Repeat the process twice more, using about 25 ml of ether on each time. Pour the aqueous extract with stirring into a mixture containing 80 ml of concentrated hydrochloric acid, 80 ml of water and 100 g of crushed ice. Filter the precipitate using vacuum pump, wash with cold water, drain and recrystallise from boiling water. The yield of benzoic acid, m.p. 121 °C, is 13.5 g (79%).
Step 2:
Preparation of benzoic chloride from benzoic acid
Take 20 g. dry benzoic acid powder in a round bottom flask, add 15 ml thionyl chloride, some porcelain pieces and then reflux on a boiling water-bath. Heat the mix solution with shaking until the evolution of gas ceases (about one hour). Cool the flask, detach the condenser and fit it to the side arm for distillation, using a 360° thermometer. To the lower end of the condenser fit a small conical flask by a cork carrying a calcium chloride tube. Now distil the contents of flask by heating carefully. A small initial fraction of unchanged thionyl chloride (B. P. 78-80° C) comes first, and then temperature rises rapidly to 194°. Stop the distillation, allow the condenser to drain, and replace conical flask by another duplicate receiver. Run water out of the condenser so that it acts as an air condenser, then continue the distillation process and collect benzoyl chloride fraction at 194-198° C.
Step 3: Preparation of benzamide from benzoyl chloride
Take a mixture of 5 ml concentrated ammonia and 5 ml water in a conical flask with a well-fitting cork. Add 2 ml (2.4 g.) benzoyl chloride, cork the flask and shake vigorously. Heat generates due to the reaction, hence hold the cork securely during shaking. After 15 min not even a trace of oily benzoyl chloride remains. Filter the fine flakes, wash with cold water and recrystallise from hot water: yield, 1-5 g. Colourless crystals, m.p. 130°.2,3
Calculation:
Here limiting reagent is benzaldehyde; hence yield should be calculated from its amount.
Molecular formula of benzaldehyde = C7H6O
Molecular formula of benzamide = C7H7NO
Molecular weight of benzaldehyde = 106 g/mole
Molecular weight of benzamide = 121 g/mole
Theoretical yield:
212 g benzaldehyde forms 121 g benzamide
Therefore, 32 g benzaldehyde will form …….? (X) g benzamide
X =(121 × 32)/212 = 18.26 g
Theoretical yield = 18.26 g
Practical yield = ————- g
% Yield = (Practical Yield)/(Theoretical Yield) × 100
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CONCLUSION
Benzamide was synthesized and the percentage yield was found to be………..%
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REFERENCES
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Practical Organic Chemistry by Frederick George Mann and Bernard Charles Saunders Published by Longan Inc., Fourth Edition; Page No.-231, 140, 119
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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.- 1029
- Experimental Pharmaceutical Chemistry by Anees Ahmad Siddiqui and Seemi Siddiqui, publication year-2005, page no. 191.
