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An organic chemistry laboratory experiment involving an equilibrium dehydration reaction where two hydroxyl groups of a tetra-alcohol, pentaerythritol, react with benzaldehyde in aqueous acid to form a benzal, 5,5-bis(hydroxymethyl)-2-phenyl-1,3-dioxane (an acetal derived from benzaldehyde) is described. This reaction can be used to illustrate the mechanism for the formation of ketals (and acetals) and their use as protecting groups for ketones (and aldehydes). Since the product contains nonequivalent geminal protons this experiment is appropriate for the analysis of inequivalent protons by NMR spectroscopy. The temperature sensitivity of this reaction also enables this experiment to be used as a group exercise to illustrate optimization of reaction conditions to maximize product yield. The green components of this procedure illustrate the use of inexpensive, nontoxic starting materials and water as a solvent.
Supplemental materials are provided on the J. Chem. Educ. website and include laboratory procedures, sample questions and instructor notes.
Summary prepared May 2005 by Julie A. Haack, Department of Chemistry at the University of Oregon.
Collard, D. M.; Jones, A. G.; Kriegel, R. M. J. Chem. Educ., Print 2001, 78, pp 70-72.