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  Home > JCE Print > Journal of Chemical Education > Issues > 2007  > November  >
In the Laboratory
Probing the Rate-Determining Step of the Claisen–Schmidt Condensation by Competition Reactions
Kendrew K. W. Mak, Wing-Fat Chan, Ka-Ying Lung, Wai-Yee Lam, Weng-Cheong Ng and Siu-Fung Lee
Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong SAR
Cover
November 2007
Vol. 84 No. 11
p. 1819
Abstract
Competition experiments are a useful tool for preliminary study of the linear free energy relationship of organic reactions. This article describes a physical organic experiment for upper-level undergraduates to identify the rate-determining step of the Claisen–Schmidt condensation of benzaldehyde and acetophenone by studying the linear free energy relationship. Substituent electronic effects were studied by competition experiments and the product ratios were determined by gas chromatography. Hammett plots were constructed from the relative rates, and the reaction constants (ρ) obtained in varying the substituents at the para position of benzaldehyde and acetophenone were 3.09 and 1.59, respectively. The reaction rates were found to be more sensitive to the substituents on benzaldehyde. The positive reaction constants suggested that nucleophilic addition (the first step) is the rate-determining step as the reaction rates were enhanced by increasing the electrophilicity of benzaldehyde as well as the acidity of the α-H of acetophenone.
Supplement
Instructions for students, notes for instructors, and spectroscopic and chromatographic data of the chalcone products are available.
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Citation
Mak, Kendrew K. W.; Chan, Wing-Fat; Lung, Ka-Ying; Lam, Wai-Yee; Ng, Weng-Cheong; Lee, Siu-Fung. J. Chem. Educ. 2007, 84, 1819.
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Keywords
Aldehydes / Ketones; Aromatic Compounds; Gas Chromatography; Hands-On Learning / Manipulatives; Kinetics; Laboratory Instruction; Mechanisms of Reactions; Organic Chemistry; Second-Year Undergraduate; Synthesis; Upper-Division Undergraduate
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History
Created:
Last Updated:
9/19/2007
9/27/2007
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Experiments, laboratory exercises, lecture demonstrations, and other descriptions of the use of chemicals, apparatus, instruments, computers, and computer interfaces are presented in the Journal of Chemical Education as illustrative of new or improved ideas or concepts in chemistry instruction and are directed at qualified teachers. Although every effort is made to assure and encourage safe practices and safe use of chemicals, the Journal of Chemical Education cannot assume responsibility for uses made of its published materials. Many chemicals are hazardous. Precautions for the safe use of hazardous chemicals and directions for their proper disposal are described in the Material Safety Data Sheets and on the labels. We strongly urge all those planning to use materials from our pages to make choices and to develop procedures for laboratory and classroom safety in accordance with local needs and situations.
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