The corfect reagent for Etard reaction is $\mathrm{CrO}_2 \mathrm{Cl}_2, \mathrm{H}_3 \mathrm{O}^{+}$.
In case, the side chain contains a group higher than $-\mathrm{CH}_3$ group, the end carbon atom of the chain is oxidised by $\mathrm{CrO}_2 \mathrm{Cl}_2$ to $-\mathrm{CHO}$ group.
e.g.