A Novel Dinitrophenylhydrazine-Free\nSynthetic Approach\nfor Identifying Aldehydes and Ketones in the Undergraduate Laboratory

A renewed experimental procedure for the unknown carbonyl\nexperiment\nhas been developed using hydrazone synthesis, thin layer chromatography\n(TLC) analysis, and melting point determination to differentiate between\nstructurally similar ketones and aldehydes in undergraduate organic\nchemistry practical courses. Benzaldehyde, <i>p</i>-tolualdehyde,\nacetophenone, and <i>p</i>-methylacetophenone can be differentiated\nfrom one another by first forming the respective carbomethoxyhydrazone,\nfollowed by confirming the presence of a ketone or an aldehyde moiety\nin the initial unknown by TLC analysis, and finally, identifying the\ninitial unknown by determining the melting point of the hydrazone\nand comparing it to literature values. Purification by simple recrystallization\nyields the pure carbomethoxyhydrazone in high yield, and the reaction\nand analysis of the product can easily be completed within a single\n8-h\nlaboratory class (or within two 4-h sessions). Students not only learn\nbasic synthesis techniques such as refluxing and recrystallization\nand analysis techniques such as TLC and melting point determination\nbut also are exposed to elements of critical thinking in the need\nfor and use of nuclear magnetic resonance (NMR) in everyday analysis,\nespecially in light of conducting analysis when NMR or other sophisticated\ninstrumentation is not available.