Benzaldehyde, C6H5CHO
Benzaldehyde is the simplest aromatic aldehyde. It happens in bitter almonds in the form of its glucoside, amygdalin (C20H27O11N). When amygdalin is boiled with dilute acids, it hydrolyses goes into benzaldehyde, HCN nad glucose.
![234_preparation of benzaldehyde.png](https://www.expertsmind.com/../CMSImages/234_preparation of benzaldehyde.png)
Benzaldehyde is also defined as oil of bitter almonds.
(1) Method of preparation
(i) Laboratory method : It is conveniently prepared by boiling benzyl chloride with copper nitrate or lead nitrate solution in a current of carbon dioxide.
![1976_preparation of benzaldehyde1.png](https://www.expertsmind.com/../CMSImages/1976_preparation of benzaldehyde1.png)
(ii) Rosenmund reaction :
![2378_preparation of benzaldehyde2.png](https://www.expertsmind.com/../CMSImages/2378_preparation of benzaldehyde2.png)
(iii) By dry distillation of a mixture of calcium benzoate and calcium formate
![1854_preparation of benzaldehyde3.png](https://www.expertsmind.com/../CMSImages/1854_preparation of benzaldehyde3.png)
(iv) By oxidation of benzyl alcohol : This involves the treatment of benzyl alcohol with dil. HNO3 or acidic potassium dichromate or chromic anhydride in acetic anhydride or with copper catalyst at 350o C.
This process is used for commercial production of benzaldehyde.
(v) By oxidation of Toluene
![1610_preparation of benzaldehyde5.png](https://www.expertsmind.com/../CMSImages/1610_preparation of benzaldehyde5.png)
Commercially the oxidation of toluene is done with air and diluted with nitrogen (to prevent complete oxidation) at 500o C in the presence of oxides of Mn, Mo or Zr as catalyst.
(vi) Etard's reaction : ![344_preparation of benzaldehyde6.png](https://www.expertsmind.com/../CMSImages/344_preparation of benzaldehyde6.png)
(vii) Gattermann-koch aldehyde synthesis : Benzene is converted into benzaldehyde by passing a mixture of carbon monoxide and HCl gas under high pressure into the ether solution of benzene in presence of anhydrous aluminium chloride and cuprous chloride.
![1234_preparation of benzaldehyde7.png](https://www.expertsmind.com/../CMSImages/1234_preparation of benzaldehyde7.png)
(viii) Gattermann reaction
![690_preparation of benzaldehyde8.png](https://www.expertsmind.com/../CMSImages/690_preparation of benzaldehyde8.png)
(ix) Stephen's reaction : Benzaldehyde is obtained by partial reduction of phenyl cyanide with stannous chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water.
(x) By ozonolysis of styrene
![1885_preparation of benzaldehyde9.png](https://www.expertsmind.com/../CMSImages/1885_preparation of benzaldehyde9.png)
(xi) Grignard reaction
![767_preparation of benzaldehyde10.png](https://www.expertsmind.com/CMSImages/767_preparation%20of%20benzaldehyde10.png)
Other reagents like carbon monoxide or HCN can also be used in place of ethyl formate.
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