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The Hammett Equation: Probing the Mechanism W of Aromatic Semicarbazone Formation

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The Hammett Equation: Probing the Mechanism W of Aromatic Semicarbazone Formation In the Laboratory The Hammett Equation: Probing the Mechanism W of Aromatic Semicarbazone Formation Glenn K. Ikeda, Karen Jang, Scott O. C. Mundle, and Andrew P. Dicks* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6; *[email protected] The Hammett equation and its applications are covered in a single-semester, third-year organic chemistry course at this university. Hammett studies provide insight into a par- ticular reaction mechanism by probing structure–reactivity relationships (1). However, many organic reactions are im- practical for demonstrating the Hammett equation in an un- dergraduate environment (2, 3). Both the time constraints of the laboratory period and the requirement for reproduc- ible data with sufficient accuracy to convey the underlying chemical theory eliminate many potential reactions for study. Moreover, collection and analysis of kinetic data often requires Scheme I. Formation of an aromatic semicarbazone. skills possessed by the average upper-level undergraduate. Herein we report a cooperative kinetics experiment that meets the importance of the Hammett equation in studying reac- these criteria while providing students practical insight into tivity of organic molecules, the meaning and interpretation of the Hammett equation. kX The reaction studied is the formation of an aromatic log = ρσX (1) semicarbazone from semicarbazide and a corresponding meta- kH or para-substituted benzaldehyde (Scheme I). To our knowl- where kX is the rate constant for the substituted benzalde- edge, this is a unique undergraduate laboratory where a Ham- hyde, kH is the rate constant for the unsubstituted benzalde- mett plot is developed utilizing rate constants determined by hyde, ρ is the reaction constant, and σX is the substituent UV-vis spectroscopy. Marrs recently reported the measure- constant. Theory underlying this relationship is described in ment of UV-vis absorption spectra to obtain Hammett plots the Supplemental Material.W for ionization of para-substituted phenols (4). Other experi- ments published in this Journal have focused on construct- Experimental Overview and Results ing Hammett plots via a number of other practical The mechanism of aromatic semicarbazone formation techniques, including 13C NMR spectroscopy (2), 1H NMR from semicarbazide and benzaldehyde has been thoroughly spectroscopy (3, 5), and acid–base titration (6, 7). Reviews investigated by Jencks et al. (10–12) and involves two po- by Jaffé (8) and more recently by Hansch et al. (9) outline tential rate-determining steps (defined as rds, Scheme II): The Scheme II. Mechanism of aromatic semicarbazone formation. www.JCE.DivCHED.org • Vol. 83 No. 9 September 2006 • Journal of Chemical Education 1341 In the Laboratory values (9) (Figure 1).2 The slope of this Hammett plot gives ρ = +0.90, in excellent agreement with the literature value of +0.91 (11). The slowest student kinetic run (for p-CH3) takes 20 minutes and the fastest (for m-NO2) 5 minutes, which is extremely convenient from a practical perspective. A comparison of ρ = +0.90 for semicarbazone formation with ρ = +1.00 for benzoic acid dissociations indicates the mag- nitude of electronic effects are similar for two very different systems. The positive value of ρ is consistent with the first step (Scheme II, rdsa) being relatively slow and rate-determining under strongly acidic conditions. At pH 1.7 a significant Figure 1. Student Hammett plot for semicarbazone formation where quantity of unreactive protonated semicarbazide exists in so- + nucleophilic addition is rate-determining, pH = 1.7. lution (pKa H2NC(O)NHNH3 = 3.82) (15). The effective concentration of nucleophilic semicarbazide available for re- action is therefore significantly attenuated.3 Attack at the car- bonyl carbon by the semicarbazide amino group occurs faster first consists of nucleophilic addition to form a carbinolamine with an electron-withdrawing group meta- or para- to the intermediate (rdsa) and precedes the second, which involves aldehyde functionality. This observation involves ground- b dehydration (rds ). One of these steps is rate-determining as state destabilization, whereby such groups (e.g., m-NO2, p- each involves heavy-atom bond formation or cleavage. All CN) pull electron density away from the carbonyl carbon, other mechanistic steps involve faster (proton transfer) events increasing its positive character and activating it further as with every reaction step reversible. The primary goal for stu- an electrophile. Overall, electron-withdrawing groups lower dents is to experimentally deduce which reaction step is rate- the activation energy associated with rdsa and rate accelera- determining under strongly acidic conditions. tion is seen for substituents with more positive σX values, The instructor prepares stock solutions of meta- and leading to ρ = +0.90. At pH 1.7 the dehydration step para-substituted benzaldehydes (∼7 mM, 80%:20% v͞v (Scheme II, rdsb) is relatively fast. The carbinolamine hy- water:ethanol) and semicarbazide hydrochloride (0.4 M, wa- droxyl group is converted to its conjugate acid, allowing H2O ter). Students prepare reactant solutions from these stock so- to act as a good leaving-group and depart in a dissociative- lutions by appropriate dilutions. The semicarbazide type mechanism. hydrochloride reactant solution is made up using 0.1 M HCl such that the pH during a kinetic run is 1.7. Semicarbazone Laboratory Report formation is monitored by the characteristic absorbance in- crease between 280–320 nm after mixing the reactant solu- Students are instructed to submit a formal laboratory tions. As second-order kinetics are followed at pH 1.7 (10), report including a detailed reaction mechanism, Hammett semicarbazide is present in > 1000-fold excess to obtain plot, and interpretation of all results. Reports generally re- pseudo first-order conditions (13) and a simplified rate law: flect a conceptual understanding of the Hammett equation and the electronic effects imparted by various substituent groups. Students are additionally encouraged to use their d[]benzaldehyde rate = = kobs []benzaldehyde (2) Hammett plot for predictive purposes. Rate constants or σX dt values can be determined for a system where the ρ value and one of the two variables are defined. The rate constant of Absorbance versus time kinetic data are subsequently fitted semicarbazone formation from p-hydroxybenzaldehyde is to eq 3 using appropriate software (e.g., GraFit; ref 14), measured by the instructor and the class asked to calculate −ktobs (3) σX for the p-hydroxy substituent. Comparison with the lit- AAAt = ()0 − ∞ e + A∞ ᎑ erature value (σp-OH = 0.37; ref 9) gives an indication as to where kobs is the observed pseudo first-order rate constant, the accuracy of the Hammett plot obtained. The rate con- At is the absorbance at time t, A∞ represents the constant ab- stant for semicarbazone formation from 3,5-dimethoxy- sorbance at the end of the reaction (t = ∞), and A0 is the benzaldehyde can also be predicted from the plot by absorbance at the reaction inception (t = 0). interpolation, assuming that the electron-withdrawing effect Each student is assigned either the parent benzaldehyde of each m-OCH3 substituent is additive. As 3,5-dimethoxy- or one of nine meta- or para-substituted benzaldehyde de- benzaldehyde is commercially available,4 the true rate con- rivatives. Kinetic runs are performed in duplicate or tripli- stant can be measured and compared with the forecasted cate and all calculated rate constants submitted to the value. instructor for tabulation and distribution to the entire class. An essential extension of the post-laboratory report The instructor obtains kinetic data for the p-NO2 and p- deals with the concept of a nonlinear Hammett plot. This 1 OCH3 benzaldehyde derivatives so that a twelve-point emphasizes that ρ values provide information about the rate- Hammett plot is constructed. determining step in a multistep mechanism. Students are A good linear relationship (r 2 = 0.983) is observed be- challenged to consider the same reaction carried out at pH tween average experimental rate constants and literature σX 6.5 rather than under strongly acidic conditions, by con- 1342 Journal of Chemical Education • Vol. 83 No. 9 September 2006 • www.JCE.DivCHED.org In the Laboratory sulting a research article (16). A concave-downwards Hammett plot is observed (Figure 2), indicative of a change in rate-determining step that results from differing elec- tronic contributions of the substituents (17). With electron- donating substituents, the dehydration step (Scheme II, rdsb) is still relatively fast with nucleophilic addition (rdsa) remaining rate-determining (ρ = positive). However, elec- tron-withdrawing groups speed up nucleophilic addition but slow down dehydration, which is now rate-determin- ing. The dehydration step involves build up of positive charge closer to the aromatic ring as the iminium ion gen- erated is represented by two resonance forms (Scheme II). Therefore, rdsb will be slowed by strong electron-withdraw- ing groups and ρ = negative on the right-hand side of the Figure 2. Literature Hammett plot for semicarbazone formation Hammett plot. where either nucleophilic addition or dehydration is rate-determin- The collaborative nature of this experiment makes it ide- ing, pH = 6.5 (data from ref16).5 ally suited to a large upper-level mechanistic laboratory. Ki- netic runs are undertaken on a desirable timescale permitting the acquisition of data for
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