Benzaldehyde


 * Benzaldehyde ** (C 6 H 5 CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond -like odor . In fact, benzaldehyde is the primary component of bitter almond oil and can be extracted from a number of other natural sources. Benzaldehyde was first extracted from bitter almonds in 1803 by the French pharmacist Martrès. In 1832 German chemists Friedrich Wöhler and Justus von Liebig first synthesized benzaldehyde.





Occurrence
Almonds, apricots , apples and cherry kernels  , contain significant amounts of amygdalin. This glycoside breaks up under enzyme catalysis into benzaldehyde, hydrocyanic acid and two molecules of glucose :

Reactions
On oxidation, benzaldehyde is converted into the odorless benzoic acid, which is a common impurity in laboratory samples. Benzyl alcohol can be formed from benzaldehyde by means of hydrogenation. Reaction of benzaldehyde with anhydrous sodium acetate and acetic anhydride yields cinnamic acid, while alcoholic potassium cyanide can be used to catalyze the condensation of benzaldehyde to benzoin. Benzaldehyde undergoes disproportionation upon treatment with concentrated alkali ( Cannizzaro reaction ): one molecule of the aldehyde is reduced to the corresponding alcohol and another molecule is simultaneously oxidized to sodium benzoate.

Uses
It is commonly employed to confer almond flavor. Benzaldehyde is used chiefly as a precursor to other organic compounds, ranging from pharmaceuticals to plastic additives. The aniline dye malachite green  is prepared from benzaldehyde and dimethylaniline. It is a precursor to certain acridine dyes as well. Via aldol condensations, benzaldehyde is converted into derivatives of cinnamaldehyde and styrene. The synthesis of mandelic acid starts from benzaldehyde: First hydrocyanic acid is added to benzaldehyde, and the resulting nitrile is subsequently hydrolysed to mandelic acid. (The scheme above depicts only one of the two formed enantiomers ). Glaciologists LaChapelle and Stillman reported in 1966 that benzaldehyde and N-heptaldehyde inhibit the recrystallization of snow and therefore the formation of depth hoar. This treatment may prevent avalanches caused by unstable depth hoar layers. However, the chemicals are not in widespread use because they damage vegetation and contaminate water supplies. Benzaldehyde is a bee repellant and is used to drive bees out of their hives when harvesting honey.