Synthesis and Recrystallization of Benzoic Acid from Benzaldehyde

Categories: ChemistryScience


The synthesis of benzoic acid from benzaldehyde is a classical organic chemistry experiment demonstrating the principles of oxidation and recrystallization. This experiment illustrates how benzaldehyde can be oxidized to benzoic acid using potassium permanganate (KMnO_4) in an alkaline medium, followed by the purification of the product through recrystallization. The process not only showcases chemical reactions and separations but also emphasizes the importance of precise measurement and control of conditions to achieve a successful conversion and purification.

Experimental Procedure

In order to do this experiment we first had to weigh 5g of sodium hydroxide with a 250ml flask and dissolve 100ml of distilled water.

Then we weighed out 7g of potassium permanganate into a watch glass and separated it into four equal parts with a spatula. We then added one fourth of the potassium permanganate into the solution in the flask and swirled it around until dissolved.

We then measured 5ml of benzaldehyde into a 10ml graduated cylinder and weighed it.

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We added the benzaldehyde into the KMNO_4/NaOH solution, and weighed the benzaldehyde again to measure the amount of benzaldehyde added. After the benzaldehyde was poured we mixed it with a stirring rod making sure it dissolved. Then another one fourth of KMNO_4was added and mixed; this was done until all the KMNO_4 was poured in.

In order to turn soluble KMNO_4 into insoluble (MnO)_2 we had to add 7ml of ethanol, but before that was done we had to wait for the mixture to cool and then add the ethanol.

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After ethanol was added we used a rubber stirring rod and some distilled water to make sure all the KMNO_4 was being dissolved. We then waited 5 minutes and then filtered it to remove the MnO_2.

After the filtration apparatus was set up we placed a piece of filter paper inside and put some drops inside and turned on the vacuum. We poured the solution inside the funnel making sure it wouldn’t overflow, and then added some water into the flask to remove any MnO_2 excess. We repeated the step, watched sodium and potassium salts collect inside the filter flask, and pour that solution into a 250ml beaker.

Next, we had to precipitate the benzoic acid which was done by adding a few ml at a time of concentrated hydrochloric acid into the filtrated solution. This was stirred after every addition and was done after the solution became acidic. The acidity was checked by using a pHydracid paper and an extra 5ml of HCl was added.

We cooled this new solution in an ice bath, and also cooled 20ml of distilled water. As the solution cooled the filtration apparatus was cleaned and set up again. We poured the solution inside the funnel and a powdery benzoic acid was collected at the top. We washed the flask with cold water and poured it again into the funnel making sure all the solution was filtrated.

After this was done we poured the powdery benzoic acid into a 250ml flask and added 100ml of water. We heated this up until all the powder was dissolved and it was boiling. After that was done we let the heated solution cool in air and then cooled in an ice bath and watched precipitation happen. As this was cooling down we set up the filtration apparatus one last time. We poured the solution into the funnel, collected the recrystallized benzoic acid and again washed the flask twice with distilled water and poured inside the funnel again. We left the vacuum running for 10min in order for the product to dry out. We finally transferred the recrystallization of benzoic acid into a watch glass, covered it with a beaker and weighed it to determine the percent yield of benzoic acid based on the benzaldehyde used.


This experiment successfully demonstrates the oxidation of benzaldehyde to benzoic acid using KMnO_4 in an alkaline medium, followed by the purification of benzoic acid through recrystallization. The procedure highlights the critical aspects of chemical synthesis, including the role of oxidizing agents, the importance of pH in precipitating products, and the effectiveness of recrystallization in purifying chemical compounds. The percent yield of benzoic acid provides insight into the efficiency of the synthesis process, underscoring the importance of precise experimental techniques in achieving high yields.


While the experiment offers valuable learning in organic synthesis and compound purification, variables such as reaction time, temperature, and the purity of reagents could impact the final yield. Future iterations could explore the effect of these variables on yield efficiency, as well as the use of alternative oxidizing agents and purification methods to enhance the synthesis of benzoic acid from benzaldehyde.

Updated: Feb 16, 2024
Cite this page

Synthesis and Recrystallization of Benzoic Acid from Benzaldehyde. (2024, Feb 16). Retrieved from

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