COMPETITIVE CONDENSATION AND PROTON-TRANSFER REACTIONS - TEMPERATURE AND PRESSURE EFFECTS AND THE DETAILED MECHANISM

At high pressures (P ≿1 Torr) carbonium ions (R+) and amines (A) undergo proton-transfer reactions yielding AH+in competition with the formation of condensation products RA+. For example, i-C4H9+reacts with NH3to give NH4+in competition with the formation of i-C4H9NH3+; i-C3H7+reacts with C2H5NH2to give C2H5NH3+in competition with l-C3H7C2H5NH2+, etc. The product distribution ratio /RA+/RAH+increases linearly with third body pressure at low pressures, but levels off to small finite values at high pressures. The product distribution ratio does not vary significantly with temperature between 200 and 500 K in any of five reactions whose temperature dependence was measured. Displacement-exchange reactions such as i-C3H7+-i-C2H5NH2→-i-C3H7NH3++ C2H6do not take place even when those reactions would be more exothermic than the proton-transfer reactions. The pressure effects and the absence of exothermic displacement reactions suggest that the major channel for proton transfer does not proceed through the excited condensation-product-like σ-bonded complexes (RA+*)tight. Rather, the data is consistent with a two-stage mechanism (see Scheme II) in which (RA+*)tightand AH+are formed from a common precursor. It is proposed that the collision between R+and A forms first a loose, electrostatically bonded complex (R+-A)*loosein which R+and A undergo multiple internal collisions, some of which may produce AH+or RA+*depending on the geometry of the intracomplex collision. It is further proposed that a similar two-stage mechanism can account for some unexpected pressure and temperature effects in other ion-molecule reactions. © 1979, American Chemical Society. All rights reserved.