In the Laboratory
Electrophilic Substitution in Naphthalene: Kinetic vs Thermodynamic Control Leslie D. Field, Sever Sternhell,* and Howard V. Wilton Division of Organic Chemistry, The University of Sydney, NSW 2006, Australia; *
[email protected] The electrophilic substitution in naphthalene is a textbook example of kinetic vs thermodynamic control of reaction outcomes (1–6 ). The classical example invariably quoted is the reversible reaction of naphthalene with concentrated sulfuric acid, which gives largely naphthalene-1-sulfonic acid at 80 °C and largely naphthalene-2-sulfonic acid at 160 °C, when equilibrium is attained (7, 8). In this paper we describe a better textbook example of kinetic vs thermodynamic control in naphthalene. The rationalization of the faster substitution at C-1 (kinetic control) is that electrophilic substitution at C-1 (α-position) proceeds via an intermediate for which five valence-bond forms can be drawn, two of which contain intact aromatic sextets (1). Electrophilic substitution at C-2 (β-position) proceeds via an intermediate for which again five valencebond forms can be drawn, but only one of them contains an intact aromatic sextet (Scheme I). The Hammond postulate is then invoked to rationalize the predominance of naphthalene1-sulfonic acid (and indeed the general tendency for electrophilic substitution to give α-products) under the conditions of kinetic control. Analogous arguments can be constructed using qualitative molecular orbital theory (1). The predominance of naphthalene-2-sulfonic acid under the conditions of thermodynamic control requires a separate argument (1) involving the sterically unfavorable peri interaction in naphthalene-1-sulfonic acid, which is absent in naphthalene-2-sulfonic acid. While there is no doubt whatsoever that these arguments are correct, we believe that we have come across a simpler (or at least additional) example that could be used to illustrate the kinetic and thermodynamic control of electrophilic substitution in naphthalene. In connection with another project (9) we required a sample of naphthalene randomly deuterated (i.e., with no preponderance of protium or deuterium at either the α or β position) to the extent of approximately 80%. From previous work (10) we knew that treatment of methyl naphthalenes with a mixture of trifluoroacetic anhydride, D2O, and aluminium tris-trifluoroacetate, at reflux, resulted in randomly deuterated compounds after equilibrium had been attained. Applying the same conditions to naphthalene itself (see experimental section), with the total amount of deuterium available sufficient to effect the desired extent of deuteration and following the reaction by NMR (Figs. 1 and 2), showed that: 1. No thermodynamic effect exists, the final distribution of the remaining protium between the α and β positions (Fig. 1f ) being equal within the experimental accuracy. This is entirely predictable because any thermodynamic (e.g., steric) isotope effect would be expected to be negligible. 1246
Scheme I
2. The distribution of deuterium between the α and β positions in naphthalene itself (Fig. 1a), was also obviously equal (zero), but at all intermediate stages more protium was present at the β position (Figs. 1b–1e), reflecting the more rapid substitution at the more reactive site.
We consider this example to be of educational value for intermediate and advanced students in organic chemistry because it involves no assumptions about thermodynamic
Figure 1. 1H NMR spectra (200 MHz) of naphthalene treated at reflux with 10-fold excess of 2H in the form of CF3COOD in the presence of (CF3CO)2O and Al(COOCF3)3.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Laboratory
reaction was further sampled at 510, 570, 870, 1080, 1380, and 10,000 min and examined by NMR. The residual reaction mixture was then allowed to cool, and 100 mL of ethyl acetate was added to the solution followed by 100 mL of water. The organic and aqueous layers were separated, with the aqueous layer further extracted with ethyl acetate (3 × 50 mL). The organic phases were combined, the solvent removed under reduced pressure, and the residual brown solid sublimed to give deuterated naphthalene as a clear white powder (650 mg, 82%). The total deuterium content (80.6%) and the relative amounts of deuterium at the α (80.6%) and β (80.6%) positions were determined by 1H NMR using 1,3,5-trinitrobenzene as an internal standard. Figure 2. Ratio of residual protium at the α and β positions of naphthalene treated as described in Figure 1. The solid line represents only a simple interpolation of data.
effects—none is predicted and indeed none is found. The kinetic effect is, however, clearly visible from actually following the progress of the reaction rather than by considering reactions carried out under different experimental conditions (7, 8). Experimental Procedure D2O (6.0 g, 0.3 mol) was added slowly to a mixture of naphthalene (750 mg, 5.86 mmol), trifluoroacetic anhydride (102 g, 0.49 mol), and aluminium tris-trifluoroacetate (3.61 g, 9.9 mmol) in a 250-mL two-necked round-bottom flask. A water condenser with a drying tube was fitted to one neck and the second neck was stoppered to allow easy access for sampling. Samples (0.5 mL) of the reaction mixture were removed at 15-min intervals for eight hours. The samples were placed in an NMR tube that had an insert capillary containing D2O to provide a clean lock. Traces of acetone were used as a reference and were in turn referenced to TMS. The
Acknowledgment We acknowledge the receipt of an Australian Research Council grant (to SS). Literature Cited 1. Streitwieser, A.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillian: New York, 1985; p 981. 2. Morrison, R. T.; Boyd, R.N. Organic Chemistry, 5th ed.; Allyn and Bacon: Boston, 1987; p 1180. 3. Roberts, J. D.; Caserio, M. C. Basic Principles of Organic Chemistry; Benjamin: New York, 1965; p 798. 4. Loudon, G. M. Organic Chemistry, 3rd ed.; Benjamin/Cummings: Redwood City, CA, 1995; p 1174. 5. March, J. Advanced Organic Chemistry—Reactions, Mechanisms, and Structure, 4th ed.; Wiley: New York, 1992; p 508. 6. Heaney, H. In Comprehensive Organic Chemistry, Vol. 1; Stoddart, J. F., Ed.; Pergamon: Oxford, 1979; p 290. 7. Fierz, H. E.; Weisenbach, P. Helv. Chim. Acta 1920, 3, 312. 8. Witt, O. Chem. Ber. 1915, 48, 743. 9. Field, L. D.; Sternhell, S.; Wilton, H. V. Tetrahedron, 1997, 53, 4051. 10. Barfield, M.; Fallick, C. J.; Hata, K.; Sternhell, S.; Westerman, P. W. J. Am. Chem. Soc. 1983, 105, 2178.
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