INTERMEDIATES IN NUCLEOPHILIC AROMATIC-SUBSTITUTION .20. RATE-LIMITING PROTON-TRANSFER IN THE FORMATION OF MEISENHEIMER COMPLEXES BETWEEN 1,3,5-TRINITROBENZENE AND AMINES - EFFECT OF DIMETHYL-SULFOXIDE ON PROTON-TRANSFER RATES - RELATIVE LEAVING-GROUP ABILITIES OF AMINES AND ALKOXIDE IONS

The kinetics of the reversible Meisenheimer complex formation between 1,3,5-trinitrobenzene and methylamine, dimethylamine in 10% dioxane-90% water (v/v), piperidine, pyrrolidine, and n-butylamine in 30% Me2SO-70% water (v/v) has been studied by the temperature-jump method. At low pH and low amine concentration proton transfer between the zwitterionic and the anionic Meisenheimer complex is rate limiting, while at high pH and/or high amine concentration nucleophilic attack by the amine is rate determining. Our results necessitate a reinterpretation of data published in 1970 on the reactions of 1,3,5-trinitrobenzene with piperidine and pyrrolidine in 10% dioxane. Deprotonation of the zwitterionic complex by OH- is about tenfold slower in 30% Me2SO comparedto 10% dioxane, possibly due to intramolecular hydrogen bonding of the ammonio proton to the o-nitro group, or due to mtermolecular hydrogen bonding to Me2SO. A comparison of rate constants of expulsion of amines with those of expulsion of alkoxide ions shows that for a given basicity amines and alkoxide ions have comparable leaving-group abilities. This behavior is intermediate between that of tetrahedral addition compounds of the N,O-trimethylenephthalimidium ion, where amines are better leaving groups than alkoxide ions of the same pK by a factor of about 105, and that of tetrahedral addition compounds of formaldehyde, where alkoxide ions appear to be much better leaving groups than amines of the same pK. © 1979, American Chemical Society. All rights reserved.