Chemical Education Today
Letters Assessing Nitration Products of Benzene Derivatives Using TLC Analysis In a recent issue of this Journal (1) Professors Malgorzata and Edward Clennan described a guided discovery experiment in which students use GC–MS analyses to identify isomers formed by the nitration of some mono-substituted benzenes. The novel aspects of this exercise include giving students the opportunity to observe firsthand that predictions based on the “directing” character of a substituent already attached to a benzene ring does not preclude the “less likely” isomer(s) from being formed, nor does it preclude the starting material from remaining largely unreacted, in some cases. The exercise Clennan and Clennan describe also provides students with an interesting example of the use of GC–MS and practice with interpretation of fragmentation patterns. Our students do not have access to GC–MS or NMR, nor does our organic curriculum include use of gas chromatography. Because the pedagogical advantages illustrated by the project Clennan and Clennan described are highly attractive, it was of interest to determine whether thin-layer chromatography (TLC) can be used in conjunction with the data provided by Clennan and Clennan to achieve the same instructional goals. While GC and GC–MS are the preferable tools with which to study these reactions, this illustration of how theory provides us with predictive tools but does not control what “actually happens” when a reaction is carried out is intended for use in the absence of student access to GC or GC–MS. Clennan and Clennan (1) described the nitration of six substituted benzenes they had found to be highly reproducible and to provide results most illustrative for their instructional objectives. Of those six, we carried out the nitration reactions and TLC analyses using anisole and acetophenone, but we substituted methyl benzoate for ethyl benzoate. Using thin-layer chromatographic analysis of authentic samples of the putative nitration products, and correlating the TLC data with the quantitative information provided by the Clennan and Clennan article, we have been able to arrive at the same conclusions regarding the products formed in each reaction. Because TLC is only a qualitative tool, the inclusion of quantitative data (1) by an instructor using this alternative exercise is essential if the maximum learning opportunity is to be provided. The reactions are not difficult and occur rapidly (1). The TLC analyses were carried out using plastic slides precoated with silica gel and impregnated with a fluorescent dye for visual detection. We obtained a supply of 20 cm × 20 cm precoated sheets from Selecto Scientific, which we cut into 3 cm ×
9 cm slides. There are many commercial sources for pre-coated and pre-cut slides. The eluent that provided the best separation of multiple components was 3:1 (v/v) hexanes:ethyl acetate, and the eluted samples were visualized using short wavelength UV light. The starting compounds and the nitro-substituted analogues we used are all readily available from commercial sources. By correlating the Rf values of authentic samples with the Rf values of the products isolated from each reaction, the identify of each product could be determined. The relative amount of each component was assessed as major, minor, or trace. Replicate trials of the TLC analyses were carried out for all samples. A summary of the data are given. A Rf value given for a compound “not detected” refers to the elution of the reference compound: anisole, not detected, 0.85; 3-nitroanisole, not detected, 0.64; 4-nitroanisole, minor, 0.57; 2-nitroaniole, minor, 0.44; 2,4-dinitroanisole, major, 0.26; acetophenone, major, 0.64; 4ʹ-nitroacetophenone, not detected, 0.36; 3ʹ-nitroacetophenone, major, 0.35; 2ʹnitroacetophenone, minor, 0.33; methyl benzoate, major, 0.76; methyl 4-nitrobenzoate, not detected, 0.55; methyl 3-nitrobenzoate, major, 0.51; methyl 2-nitrobenzoate, minor, 0.44.
Tabulated Rf data that show how these results correspond with those reported by Clennan and Clennan (1) are provided in the online supporting material. Literature Cited 1. Clennan, Malgorzata M.; Clennan, Edward L. J. Chem. Educ. 2007, 84, 1679–1681.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Dec/abs1623.html Full text (HTML and PDF) with a link to cited JCE article Supplement Tabulated Rf data Rita K. Hessley Department of Chemical Technology College of Applied Science University of Cinncinati Cincinnati, OH 45206
[email protected] © Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 12 December 2008 • Journal of Chemical Education
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