Explain different methods of formation of benzaldehyde by chemical reactions.

(N/A) Benzaldehyde can be prepared by the following methods:
$1$. From acyl chloride (Rosenmund reduction):
Benzoyl chloride is hydrogenated over catalyst palladium on barium sulphate $(Pd/BaSO_4)$ to form benzaldehyde. The reaction is poisoned by sulphur or quinoline to prevent further reduction to alcohol.
$C_6H_5COCl + H_2 \xrightarrow{Pd/BaSO_4} C_6H_5CHO + HCl$
$2$. From aromatic hydrocarbons (Toluene):
$a$. Oxidation by chromyl chloride (Etard reaction): Toluene is oxidised by chromyl chloride $(CrO_2Cl_2)$ to a chromium complex,which on hydrolysis gives benzaldehyde.
$C_6H_5CH_3 + 2CrO_2Cl_2$ $\rightarrow C_6H_5CH(OCrOHCl_2)_2$ $\xrightarrow{H_3O^+} C_6H_5CHO$
$b$. Side chain chlorination followed by hydrolysis: Side chain chlorination of toluene gives benzal chloride,which on hydrolysis gives benzaldehyde.
$C_6H_5CH_3 + Cl_2$ $\xrightarrow{hv} C_6H_5CHCl_2$ $\xrightarrow{H_2O, 373K} C_6H_5CHO$
$c$. Gatterman-Koch reaction: Benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride $(AlCl_3)$ or cuprous chloride $(CuCl)$ to give benzaldehyde.
$C_6H_6 + CO + HCl \xrightarrow{anhydrous AlCl_3/CuCl} C_6H_5CHO$