Benzaldehyde
Benzaldehyde is a useful chemical in many pharmaceutical syntheses. A great it example is it's use to prepare mandelic acid, or cinnamic acid. It is also used as an artificial flavor, specifically almonds, or bitter almonds. It has a very sweet smell, not unlike nitroaromatics. Pure benzaldehyde is a clear, colorless liquid. It can be prepared by the sommelet reaction of benzyl chloride, or by the hydrolysis of benzal chloride.
Materials |
Equipment |
| Benzyl chloride | 1000ml flask with 24/40 joints |
| Hexamine (Prep / OTC) | Reflux equipment |
| Ethanol, or Glacial acetic acid | 500ml Separatory funnel |
| Hydrochloric acid | Vacuum distillation equipment |
| Distilled water | |
| Diethyl ether |
Into the 1000ml flask, add 45g of hexamine. Prepare either a 60% ethanol solution by mixing 80ml of ethanol in 55ml of distilled water, or a 50% acetic acid solution by mixing 80ml glacial acetic acid with 80ml of distilled water. Swirl the flask at least enought to dissolve all visible solids - it doesn't have to become a completely clear solution, you just don't want a cake of hexamine at the bottom. Add 35ml of benzyl chloride to the flask and fit the reflux column to the flask. If you used ethanol, you can use a water bath, otherwise, use an oil bath to heat it to reflux and hold it there for about two hours. Prepare a ~10% solution of hydrochloric acid by mixing 20ml of 31.45% with 40ml of distilled water, or 30ml of 20% with 30ml of distilled water. After the materials in the flask have refluxed for two hours, add the above solution of hydrochloric acid to it and continue refluxing for at least 15-20min. After this, remove the flask from the bath and let it cool to room temp. You should see two layers, the bottom layer is usually a light orange and the top layer can be yellow to orange. The top layer is your benzaldehyde.
Add the contents of the flask to a 500ml separatory funne and pour off the aqueous layer (bottom layer) into a 500ml flask. Store the top layer in a separate flask as well. Extract the aqueous layer twice with two 100ml portions of ether. Combine the ether and the benzaldehyde layer (top layer) you separated initially and dry it with magnesium sulfate. It should be a clear yellow to clear orange liquid now. Add this to your distillation apparatus and distill off the ether under normal pressure. Once most of it is gone, distill off the benzaldehyde under reduced pressure to minimize decomposition. Use a nomograph to predict the boiling point of the pure benzaldehyde. You can also distill it under atmospheric pressure, but expect slight decomposition. Collect the portion coming over between 178-180C. Theoretical yeild is 29.4g. Last time I attempted this, I obtained ~22g which is about a 75% yeild; 70% yeilds are standard for this procedure.