Benzaldehyde the most commonly used aromatic aldehyde in chemical industry. At room temperature it exists a colorless liquid with a special smell of almonds. Benzaldehyde is the main component of bitter almond oil extract, also from apricot, cherry, laurel, extracted in peach.
Benzaldehyde is also obtained by oxidation of acidic solution of toluene’s manganese dioxide(MnO₂). In the presence of vanadium pentoxide(V₂O₅) at 350°C, oxidation of toluene by air oxygen also yields benzaldehyde.
Benzaldehyde is obtained from oxidation of toluene by chromyl chloride CrO2Cl2. This reaction is called Etard’s Reaction.
C6H5CH3 + 2CrO2Cl2 → C6H5CH(OCrOHCl2)2 → C6H5CHO
Preparation Method
To make benzaldehyde in the laboratory, take 10 grams of benzene chloride, 8 gram lead nitrate and about 50 ml of water in a round flask. Attaches a reflux condenser to the flask. CO2 gas flows into the flask. So that the atmosphere of CO2 remains in the flask.
Heats the flask for about 6 hours in this condition. The mixture is then cooled and extracted by ether. Benzaldehyde enters the ether layer. Separating the ether layer and adding a saturated solution of sodium bisulfite(NaHSO3) to it.
Benzoic acid to benzaldehyde
Crystals of benzaldehyde sodium bisulfite are obtained. They are filtered and dried. Benzaldehyde is released upon heating these crystals with sulfuric acid. It is extracted by ether. Distill the ether solution and separate the ether. After benign distillation of the remaining fluid, pure benzaldehyde is obtained at 179°C.
Benzal Chloride: Benzaldehyde is obtained by reaction of benzal chloride(C7H6Cl2) with aqueous NaOH or KOH.
C6H5CHCl2 → C6H5CH(OH)2 → C6H5CHO
Benzene Alcohol: benzaldehyde can be obtained by oxidation of benzene alcohol by acidic potassium dichromate.
C6H5CH2OH + O → C6H5CHO + H2O
Benzaldehyde is also obtained when the vapors of benzene alcohol are precipitated at 300°C heated copper.
Benzoyl Chloride: benzaldehyde is obtained from rosenmund reduction of Benzoyl chloride.
C6H5COCl + H2 → C6H5CHO + HCl
Benzene: benzaldehyde is also obtained from benzene by gattermann koch aldehyde synthesis.
C6H6 + CO → C6H5CHO + HCl
Benzaldehyde after long time storage.
Benzaldehyde will be oxidized, resulting in impurities benzoic acid.
Many aldehydes, such as acetaldehyde and Benzaldehyde in the air can be oxidized, which is called the self oxidation reaction (autoxidation). Ater a long time storage ,there appears white needle crystal at the bottle of the Benzaldehyde containers. It is benzoic acid.
The reaction formulation of the oxidation of Benzaldehyde by air is as follows:
2C6H5CHO+O2 - 2C6H5COOHHow to remove the Benzoic acid from Benzaldehyde?
The commonly used method is to wash with saturated potassium carbonate solution. After washing, the benzoic acid can be removed.
Benzoic acid can also be removed by distillation, the boiling point of the Benzaldehyde is 178 degrees.