Synthesis of benzoic acid

Goal : Carry out an organic synthesis and prepare benzoic acid from benzyl alcohol

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Hi there, I will walk you through today's lab experiement which is the synthesis of benzoic acid.

Step 1 : Oxidation of benzyl alcohol :

In a 250 mL three-necked flask equipped with a condenser, a dropping funnel and mechanical stirring, I introduced 2.6 mL of benzyl alcohol and 25 mL of a 10% sodium hydroxide solution. I brang the mixture to a gentle boil. Then I slowly poured, over approximately 20 minutes, 100 mL of a 0.30 mol.L-1 potassium permanganate solution while continuing to heat. Once the addition is complete, I heated at reflux for 15 min and then cooled at room temperature. Finally, I added a few drops of ethanol until disappearance of the slight pink tint.

Step 2 : Büchner filtration :

I cooled the mixture in an ice bath (<10°C) and then I filtered it a few times using a Büchner funnel. In the end, I obtained 0.77 g of a clear white looking wet powder that I placed in the oven (80°C) for the rest of the day.

Here is the picture of the reflux assembly :

Step 3 : Purification of the product :

I recrystallized some of the product with an ethanol solution and then dried the purified solid. I obtained 0.56 g of purified product.

Step 4 : Purity controls :

I performed thin layer chromatography using cyclohexane and acetone as eluent. I made 4 deposits :

1. 1% benzyl alcohol solution in ethanol

2. 1% benzoic acid solution in ethanol

3. My solution of benzoic acid

4. My solution of purified benzoic acid at 1% in ethanol

I revealed it under the UV lamp (254 nm). The results showed that my product was not fully purified.

I measured the melting temperature of the purified product using a Kofler bench. My product had a melting point of 120°C which is close to the standard (= 121-123°C) but still show that my product was not pure.

Step 5 : Theorytical purity controls :

Since my school does not own any kind of spectrophotometers, we are using theoretical spectra. Here is the infrared spectrum of the benzoic acid which I analized.

And here is the NMR spectrum of the benzoic acid. I attributed the peaks to each equivalent proton group in the molecule.

Step 6 : Calculus :

After a few calculus, I ended up doing my synthesis yield which was quite bad : 26% !!

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This lab experiment was really fun, let's hope I'll do better next time !