Etard reaction:-In Etard reaction, chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.Ex-\\\\(\\underset{\ ext{Toluene}}{{C}_{6}{H}_{5}-C{H}_{3}} + Cr{O}_{2}{Cl}_{2} \\xrightarrow{C{S}_{2}} {C}_{6}{H}_{5}-CH {\\left( OCrOH{Cl}_{2} \\right)}_{2} \\xrightarrow{{{H}_{3}O}^{+}} \\underset{\ ext{Benzaldehyde}}{{C}_{6}{H}_{5}-CHO}\\\\)Gattermann Koch reaction:-When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.This reaction is known as Gattermann Koch reaction.Ex-\\\\(\\underset{\ ext{Benzene}}{{C}_{6}{H}_{5}} \\xrightarrow[\ ext{Anhy. } {Al{Cl}_{3}}/{CuCl}]{CO + HCl} \\underset{\ ext{Benzaldehyde}}{{C}_{6}{H}_{5}-CHO}\\\\)