Benzaldehyde

Benzaldehyde

Benzaldehyde distilled from bitter almond oil, with traces of water.
Names
IUPAC name Benzaldehyde
Systematic IUPAC name Benzenecarbaldehyde
Other names Benzenecarbonal Benzenecarboxaldehyde Benzoic aldehyde Phenylmethanal
Properties
Chemical formula	C7H6O C6H5CHO
Molar mass	106.12 g/mol
Appearance	Colorless liquid
Odor	Almond-like
Density	1.050 g/cm3 (15 °C) 1.044 g/cm3 (20 °C)
Melting point	−57.12 °C (−70.82 °F; 216.03 K)
Boiling point	178.1 °C (352.6 °F; 451.2 K)
Solubility in water	0.3 g/100 ml (20 °C) 0.695 g/100 ml (25 °C)
Solubility	Miscible with diethyl ether, ethanol, methanol Soluble in anh. ammonia
Vapor pressure	1.27 mmHg (25 °C)
Thermochemistry
Std enthalpy of formation (ΔfHo298)	−36.8 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	64 °C
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	1,300 mg/kg (rat, oral)
Related compounds
Related compounds	Phenylacetaldehyde Hydrocinnamaldehyde
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Benzaldehyde is an organic chemical compound, the simplest aromatic aldehyde. It has the formula C₆H₅CHO.

Properties

Chemical

Benzaldehyde will slowly oxidize in air, forming mainly benzoic acid.

C₆H₅CHO + ½ O₂ → C₆H₅COOH

Hydrogenation of benzaldehyde yields benzyl alcohol.

Physical

Benzaldehyde is a colorless liquid, which has a characteristic almond-like odor. It is immiscible with water, but miscible with organic solvents, such as alcohols and ethers.

Availability

Benzaldehyde is difficult to acquire in some countries, due to its use in the manufacturing of illegal amphetamine-class drugs. In US it is considered a List I chemical, meaning it cannot be legally purchased without a permit. In the EU however, its sale is not restricted and can be bought online or from chemical suppliers.

A good source of benzaldehyde is bitter almond oil, which is mostly crude benzaldehyde. It can be purified via vacuum distillation or steam distillation in an inert atmosphere. However, keep in mind that since the bitter almond oil is basically crude benzaldehyde, legally there's no distinction between the oil and impure benzaldehyde. In US it's illegal to import bitter almond oil without proper paperwork.

Preparation

Perhaps the most straightforward preparation of benzaldehyde is through the oxidation of benzyl alcohol in an aqueous solution containing an oxidizer. Benzyl alcohol is slightly soluble in water, although benzaldehyde's solubility in water is even lower. So with good stirring, proper stoichiometry, and plenty of time, it should be easy to produce using anything from household bleach (sodium hypochlorite solution) to potassium permanganate. As a bonus, potassium permanganate is reduced to MnO₂, which can also oxidize benzyl alcohol, so it's important to take this into account when calculating stoichiometry. Good yields have been reported by using nitric acid as the oxidizer.

Benzaldehyde can be prepared via the Sommelet reaction, using benzyl chloride and hexamine. Here is a practical procedure posted by Vesp, from TheVespiary.org:

"Into a 1000 ml flask, add 45 g of hexamine. Prepare either a 60% ethanol solution by mixing 80 ml of ethanol in 55 ml of distilled water, or a 50% acetic acid solution by mixing 80 ml glacial acetic acid with 80 ml of distilled water. Swirl the flask at least enough 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 35 ml 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 20 ml of 31.45% with 40 ml of distilled water, or 30 ml of 20% with 30 ml 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 15-20 min. 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.

Set the flask up for simple distillation and begin heating it on an oil bath. You will now be steam distilling the benzaldehyde from the reaction mixture/by products. After many tests, I have found this to be the most efficient (and easiest method) to obtain the benzaldehyde. If you used ethanol, collect the first fraction all the way up to ~98 °C. Now, collect everything else distilling over (whether you used ethanol or acetic acid) until the distillate is running out is mostly clear and benzaldehyde free. You may need to add more water to the distillation flask to aid in the steam distillation.

If you used ethanol, take the first fraction you collected and add it to a 500 ml separatory funnel. Dilute it with about the same volume of distilled water and shake. Let the layers separate and drain the bottom (aqueous) layer. Add the rest of the distillate you collected (with a very obvious difference in layers) and dilute with enough water to fill the funnel up. Drain off the lower (aqueous) layer, saving the benzaldehyde. If you used ethanol, wash the benzaldehyde a few times with plain distilled water. Be conservative on the water you use though, because benzaldehye is a little soluble in pure water. If you used acetic acid, you must neutralize the benzaldehyde by washing it a few times with dilute sodium (bi)carbonate solution and distilled water. Clearly, it is much more beneficial to use the ethanol solution as opposed to the acetic acid solution. Once the benzaldehyde has been washed/neutralized, dry it with magnesium sulfate. It should be a clear yellow to clear orange liquid now. Distill the benzaldehyde under reduced pressure to minimize decomposition. Use a monograph 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 178-180 °C. Theoretical yield is 29.4 g. Last time I attempted this, I obtained ~22 g which is about a 75% yield; 70% yields are standard for this procedure."

Another common route to benzaldehyde is the oxidation of toluene with chromyl chloride, process known as Étard reaction.

Another more accessible route involves the retro-aldol reaction of cinnaldehyde with sodium carbonate. The yield of this reaction is pretty low, but cinnamon oil is more readily available than bitter almond oil. The reaction takes place in distilled water.

Oxidation of benzyl alcohol with a solution of 15% hydrogen peroxide, done by refluxing both reagents has been shown to produce benzaldehyde, with a reported yield of ~90%. There are some reports on the SM forum that sodium persulfate can also be used to oxidize benzaldehyde.

Carbonylation of benzene is another route, but may not give a good yield and uses toxic carbon monoxide. It aso requires expensive rhodium-based catalyst.

Projects

• Flavoring agent
• Make mandelic acid
• Cinnamaldehyde synthesis
• Make bee repellent

Handling

Safety

Benzaldehyde has low toxicity, though it may be irritant. The lethal dose for a 70 kg adult is considered to be 50 ml. Benzaldehyde does not accumulate in any tissues, and is metabolized and then excreted in urine.

Some studies indicate that benzaldehyde might have anti-cancer properties.

Storage

Since benzaldehyde is sensitive to air and light, it should be stored in amber bottles, which are then made air-tight by sealing them with sealing tape, like parafilm. Schlenk flasks can also be used to store this compound.

Disposal

Benzaldehyde should be mixed with a more volatile solvent and safely burned outside. Another more lengthy route involves oxidation to benzoic acid, which is less volatile and can be safely dumped in trash or poured down the drain.

Gallery

• 

  Benzaldehyde condenses with hydrazine to form yellow benzalazine

References

Relevant Sciencemadness threads

• toluene --> benzaldehyde
• Toluene to Benzaldehyde with MnO2
• Preparation of Benzaldehyde from Benzyl Alcohol
• Purification of benzaldehyde?
• Benzene --> benzaldehyde
• bitter almond benzaldehyde
• Benzaldehyde synthesis using aqueous nitric acid