Mechanistic Assessment
of S<sub>N</sub>Ar Displacement
of Halides from 1‑Halo-2,4-dinitrobenzenes by Selected Primary
and Secondary Amines: Brønsted and Mayr Analyses
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Abstract
Pseudo-first-order rate constants (<i>k</i><sub>obsd</sub>) have been measured spectrophotometrically for nucleophilic
substitution
reactions of 1-X-2,4-dinitrobenzenes (<b>1a</b>–<b>d</b>, X = F, Cl, Br, I) with various primary and secondary amines
in MeCN and H<sub>2</sub>O at 25.0 ± 0.1 °C. The plots of <i>k</i><sub>obsd</sub> vs [amine] curve upward for reactions of <b>1a</b> (X = F) with secondary amines in MeCN. In contrast, the
corresponding plots for the other reactions of <b>1b</b>–<b>d</b> with primary and secondary amines in MeCN and H<sub>2</sub>O are linear. The Brønsted-type plots for reactions of <b>1a</b>–<b>d</b> with a series of secondary amines
are linear with β<sub>nuc</sub> = 1.00 for the reaction of <b>1a</b> and 0.52 ± 0.01 for those of <b>1b</b>–<b>d</b>. Factors governing reaction mechanisms (e.g., solvent, halogen
atoms, H-bonding interactions, amine types) have been discussed. Kinetic
data were also analyzed in terms of the Mayr nucleophilicity parameter
for the amines with each aromatic substrate. Provisional Mayr electrophilicity
parameter (<i>E</i>) values for 1-X-2,4-dinitrobenzenes
have been determined: <i>E</i> = −14.1 for X = F, <i>E</i> = −17.6 for X = Cl and Br, and <i>E</i> = −18.3 for X = I. These values are consistent with the range
and order of <i>E</i> values for heteroaromatic superelectrophiles
and normal 6-π aromatic electrophiles
