Introduction
The procedure below describes the oxidation of benzyl alcohol to benzaldehyde in high yield using aqueous nitric acid as an oxidizer. Other methods of oxidizing benzyl alcohol to benzaldehyde are well-known, including those using chlorochromates, persulfate, or activated manganese dioxide. The method is advantageous in that it gives a high yield while using relatively simple equipment and more common, easy-to-obtain reagents than others.
Equipment and glassware:
- Round-bottom flask, 250 ml;
- Salted ice bath (-10 °C);
- Pasteur pipette and/or drip funnel, 100 ml (optional);
- Separating funnel 250 ml;
- Retort stand and clamp for securing apparatus;
- Magnetic stirrer (optional);
- Beaker 100 ml (x2) and 200 ml (x2);
- Measuring cylinder for 100 ml;
- Laboratory scale (1 g - 100 g is suitable).
Reagents:
- 50 g 90% nitric acid (density 1.48 g/ml);
- 66 g (610 mmol) of benzyl alcohol;
- 30 g Sodium bicarbonate (NaHCO3);
- 50 g Sodium sulphate (NaSO4);
- 30 g Sodium chloride (NaCl);
- 2 L Distilled water.
Procedure
50 g (714 mmol) of 90% nitric acid (density 1.48 g/ml) were placed in a 250 ml round-bottom flask. The nitric acid was cooled in a salted ice bath (-10 °C) and a couple of ml of benzyl alcohol (technical grade or better) were added to the flask using a Pasteur pipette.
The flask was manually swirled by hand to mix the reagents, and an immediate change in color from pale yellow to a bright yellow-green was noted. As more benzyl alcohol was added, an upper layer separated and the color of this gradually deepened to an intense blue-green. The release of brown nitrogen oxide vapors was also noted. The classic almond extract of aroma was already heavy upon the air between additions by this point.
Upon each further addition of benzyl alcohol and with stirring of the mixture, the color disappeared and became milky yellow, but after being allowed to react, the blue color returned; the color change was used to follow the reaction, with each addition of benzyl alcohol occurring after the upper layer had regained the unusual coloration. With some difficulty, I managed to get enough light passing through the deep green mixture for a picture.
Upon each further addition of benzyl alcohol and with stirring of the mixture, the color disappeared and became milky yellow, but after being allowed to react, the blue color returned; the color change was used to follow the reaction, with each addition of benzyl alcohol occurring after the upper layer had regained the unusual coloration. With some difficulty, I managed to get enough light passing through the deep green mixture for a picture.
A total of 66 g (610 mmol) of benzyl alcohol were added over the course of about 4 hours. While an ice salt bath was used to regulate temperature at the beginning of the experiment, as the reaction continues, and the concentration of nitric acid falls, the reaction slows significantly and it is sufficient to carry out the last third or so of the reaction at room temperature. For a reason to avoid benzyl alcohol contamination of the synthesis product, addition of benzyl alcohol ceased when the blue-green color didn't return, at least to its full strength, after allowing the reaction mixture to sit for a period of 30 minutes.
After being left to sit overnight (I recommend you to stir overnight, if it is possible) in an airtight container, the two-layer mixture was placed in a separatory funnel (250 ml) and the lower aqueous layer removed. The top layer was washed twice with saturated sodium bicarbonate solution, followed by distilled water, and finally partially dried with a wash of saturated sodium chloride. The pale green upper layer as well as the aqueous layer became immediately red-orange in color after the sodium bicarbonate solution was added for the first time, though the color of the aqueous layer is noticeably less with each successive wash. The brine wash came out colorless, though the benzaldehyde layer is still strongly colored.
The crude benzaldehyde, a translucent reddish-orange liquid, was placed into a pre-weighed container with a little anhydrous sodium sulfate (NaSO4) and the weight was recorded as 55.3 g. If pure benzaldehyde was assumed, this would correspond to an 85% yield (dirty)! Upon prolonged storage over the sodium sulfate, the color actually lightened to a golden yellow color, though this isn't pictured.
Discussion
GC-MS analysis has shown chromatogram which was contained 3 major peaks. The largest peak, benzaldehyde, was measured at 92% of the peak area, with an additional peak for residual benzyl alcohol making up another 3%. The final yield of this synthesis in pure benzaldehyde came out to about 79%. This synthesis may be scaled up easily. I strongly recommend to use vacuum distillation before utilizing this substance in further syntheses.
Benzaldehyde vacuum distillation
Conclusion
Given limited equipment and space, this synthesis, while proceeding without any major hitches or obstacles, could likely be improved in a few ways. Magnetic stirring and a drip funnel would almost certainly be preferable to hovering over the reaction mixture and swirling it manually, for example. Also, use laboratory grade reagents to increase yield and decrease side products, if it is possible.
Benzyl Alcohol Oxidation to Benzaldehyde Catalyzed by Acid in DMSO
Introduction
A represented here method shows how to synthesize quite pure benzaldehyde from benzyl alcohol with almost quantitative yield. It should be highlighted, that this way doesn't take controlled nitric acid and low temperature in compare with the method above. Both method are carried out for about the same time. Nevertheless, the HBr/DMSO approach produce higher yield of height purity benzaldehyde.
Equipment and glassware:
- 20 L Batch chemical reactor, equipped with top stirrer, pressure equalized drip funnel and jacket;
- Chiller;
- Plastic or glass funnel;
- Beakers 5L x4;
- Bucket 10 L x2;
- Vacuum pump;
- Rotary evaporator;
Reagents:
- Dimethylsulfoxid (DMSO) 9 L;
- Benzyl alcohol 1 L;
- Aqueous hydrobromic acid (HBr) 48% 1 kg (671.1 ml);
- Brine 9 L (saturated NaCl aq solution);
- Suitable solvent (ether, ethyl acetate, toluene, DCM, etc.) for extraction 2-3 L;
Benzyl Alcohol Oxidation to Benzaldehyde Catalyzed by Acid in DMSO
Caution: Benzaldehyde is highly flammable substance. Its vapours are explosive. Carry out distillation at safety conditions with a personal protective equipment using.
1. A 20 L batch reactor is equipped with a stirrer and jacket with heating. DMSO 9 L is poured into the reactor. Next, benzyl alcohol 1 L is poured there. The stirrer is turned on.
2. Aqueous hydrobromic acid (HBr) 1 kg 48% is added carefully with constant stirring.
3. Then, the reaction mixture is heated to 100 °C with the heating jacket. The reaction mixture is stirred at this temperature for 4 h.
4. After a while, the reaction mixture is cooled by the jacket to room temperature. Brine 9 L (saturated NaCl aq solution) is added with constant stirring.
5. Then, a suitable solvent (ether, ethyl acetate, toluene, DCM, etc.) for extraction 2-3 L is poured into the reactor with constant stirring.
6. The stirrer is turned off. Layers are separated. It is advisable to carry out the extraction procedure 2 times. Second extraction can be carried out with ½ solvent volume.
7. The separated extract (benzaldehyde is dissolved in a solvent) is distilled in a rotary evaporator. Using a vacuum, extract is transferred into the rotary evaporator. The solvent is distilled off at a maximum temperature 60 °C. It is not recommended to exceed the temperature regardless of vacuum degree. There is an explosion risk of benzaldehyde vapors. If the vacuum is high, the temperature can be lowered.
8. After the distillation procedure, acceptable purity benzaldehyde ~ 900 g (95%) is obtained in the evaporation flask. Benzaldehyde can additionally be purified by means a sodium bisulfite aq solution washing.
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