A Novel Dinitrophenylhydrazine-Free Synthetic Approach for Identifying Aldehydes and Ketones in the Undergraduate Laboratory

A renewed experimental procedure for the unknown carbonyl experiment has been developed using hydrazone synthesis, thin layer chromatography (TLC) analysis, and melting point determination to differentiate between structurally similar ketones and aldehydes in undergraduate organic chemistry practical courses. Benzaldehyde, p-tolualdehyde, acetophenone, and p-methylacetophenone can be differentiated from one another by first forming the respective carbomethoxyhydrazone, followed by confirming the presence of a ketone or an aldehyde moiety in the initial unknown by TLC analysis, and finally, identifying the initial unknown by determining the melting point of the hydrazone and comparing it to literature values. Purification by simple recrystallization yields the pure carbomethoxyhydrazone in high yield, and the reaction and analysis of the product can easily be completed within a single 8-h laboratory class (or within two 4-h sessions). Students not only learn basic synthesis techniques such as refluxing and recrystallization and analysis techniques such as TLC and melting point determination but also are exposed to elements of critical thinking in the need for and use of nuclear magnetic resonance (NMR) in everyday analysis, especially in light of conducting analysis when NMR or other sophisticated instrumentation is not available.