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.

         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.

         (ii) Rosenmund reaction :

         (iii) By dry distillation of a mixture of calcium benzoate and calcium formate

         (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 350^o C.

This process is used for commercial production of benzaldehyde.

         (v) By oxidation of Toluene

         Commercially the oxidation of toluene is done with air and diluted with nitrogen (to prevent complete oxidation) at 500^o C in the presence of oxides of Mn, Mo  or Zr as catalyst.

         (vi) Etard's reaction : 

         (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.

         (viii) Gattermann reaction

         (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

         (xi) Grignard reaction  

         Other reagents like carbon monoxide or HCN can also be used in place of ethyl formate.

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