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The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 22 (2017) pp 137-141

An Improved Laboratory Synthesis of Betaine-30: A Solvatochromic Dye

Magrieta Snyman and Brian P. Rempel*

Department of Science, Augustana Campus, University of Alberta 4901-46th Ave. Camrose, Alberta, T4V 2R3 Canada,
Received June 2, 2017. Accepted July 27, 2017.

Published: 11 August 2017

Abstract. Empirical solvent polarity can be measured with betaine-30, a highly solvatochromatic compound meaning that solutions of betaine-30 in different solvents are different colors in response to the polarity of the solvent. These solutions are excellent tools to visualize solvent polarity, a concept that underlies the important role solvent plays in a chemical reaction or process. By showing how the solution color changes with solvent polarity, students can draw an analogy with non-visible properties that also depend on solvent polarity such as reaction rates. The synthesis of betaine-30 is a straightforward four-step convergent synthesis starting from inexpensive starting materials: 1,3-diphenylpropenone (chalcone), acetophenone and 2,6-diphenylphenol. Herein, an updated and improved synthesis of betaine-30 is presented that is quick, easy, high-yielding, and can be readily accomplished by senior undergraduate organic chemistry students. In particular, a better method for preparation of the first intermediate, 2,4,6-triphenylpyrylium hydrogen sulfate from acetophenone and 1,3-diphenylpropenone (chalcone), is described. As well, nitrosylation of 2,6-diphenylphenol followed by reduction with sodium dithionite produces 4-amino-2,6-diphenylphenol in an efficient reaction sequence. Finally, the greater purity of the intermediates leads to a higher yield for the final coupling to form pure betaine-30.

Key Words: Laboratories and Demonstrations; organic chemistry; laboratory instruction; organic synthesis; solvent and solvent effects; UV-Vis spectroscopy; solvatochromism

(*) Corresponding author. (E-mail:

Article in PDF format (103 KB) HTML format

Supporting Materials:

Complete experimental details, LC-MS analysis of student products, student handouts, evaluative material, sample anonymized student laboratory reports. (8670 KB)

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