$A$ primary amine,$RNH_2$ can be reacted with $CH_3-X$ to get a secondary amine,$RNHCH_3$,but the disadvantage is that $3^o$ amine and quaternary ammonium salts are also obtained as side products. Can you suggest a method where $RNH_2$ forms only $2^o$ amine?

(N/A) The reaction of $1^o$ amine with $CH_3-X$ (alkylation) is difficult to stop at the $2^o$ amine stage because the product is more nucleophilic than the reactant.
To obtain only the $2^o$ amine,we can use the following sequence:
$RNH_2$ $\xrightarrow{CHCl_3 / KOH} RNC$ $\xrightarrow{H_2 / Pd} RNHCH_3$.
In this method,the $1^o$ amine is first converted to an isocyanide $(RNC)$ via the carbylamine reaction. Subsequent catalytic reduction of the isocyanide yields a secondary amine $(RNHCH_3)$ specifically,avoiding the formation of $3^o$ amines or quaternary ammonium salts.