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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 deg C and largely naphthalene-2-sulfonic acid at 160 deg 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 (alpha-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 beta-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 alpha-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 (I) 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...