Question

give reaction mechanism for preparation of benzaldehyde from benzene​

Answer 1

Bromination of benzene by standard methods gives bromobenzene, which can be lithiated with n-BuLi or converted to phenylmagnesium bromide. This can then be formylated with DMF (N,N-dimethylformamide, a common solvent), NFM (N-formylmorpholine), or NFP (N-formylpiperidine).

I have given u reaction in 2 pics

Answer 2

there are a few different ways to do it. The first that comes to my mind is Friedel-Crafts Alkylation. You will need benzene, Formyl Chloride, and Aluminum Chloride. The reaction is fairly simple on paper but don't try to do this at home without the proper training. Anywho here is the alkylation reaction  

C6H6+CHOCl----AlCl3----> C7H6O + inorganic products.

2. Gatterman - Koch reaction is use for the preparation of benzaldehyde or substituted benzaldehyde by treating benzene or substituted benzene with carbon monoxide and hydrogen chloride in presence of anhydrous aluminum chloride or cuprous chloride.