Kinetics, isotope effects of the reaction of 1-(4-nitrophenyl)-1-nitroalkanes with DBU in tetrahydrofuran and chlorobenzene solvents

Abstract

WLODZIMIERZ GALEZOWSKI and ARNOLD JARCZEWSKI. Can. J. Chem. 68, 2242 (1990).The kinetics of the reaction of (R = Me, Et, i-Pr; NPNE, NPNP, MNPNP respectively; L is H or D) with 1,s-diazabicyclo[5.4.O]undec-7-ene (DBU) base in tetrahydrofuran (THF) and chlorobenzene (CB) solvents are reported. The products of these proton transfer reactions are ion pairs absorbing at A,,,, = 460-480 nm. The equilibrium constants in THF were K :~'~ = 2060,560, 3.7 and in CB K ? ,~'~ = 500, 220,4.7 mol-I dm3 for NPNE, NPNP, MNPNP respectively. The thermodynamic parameters of the reactions are also quoted. The substrate reacts with DBU in both THF and CB solvents in a normal second-order proton transfer reaction. In the case of deuteron transfer, isotopic D/H exchange is much faster than internal return. The reactions show low values of enthalpy of activation AH' = 14.3, 18. I , 24.2 and 13.0, 15.1, 18.6 kJ mol-' for NPNE, NPNP, and MNPNP inTHF and CB respectively, and large negative entropies of activation -AS' = 141, 139, 146; 140, 146, 160 J mol-I deg-I for the same sequence of substrates and solvents. The kinetic isotope effects are large, (kH/kD)ZOaC = 12.2, 13.0, 10.1; 12.9, 12.0, 10.2 for the above sequence of substrates and solvents, and show no difference with changes in either steric hindrance of the C-acids or polarity of the solvents.Key words: proton transfer, kinetic isotope effect.WLODZIMIERZ GALEZOWSKI et ARNOLD JARCZEWSKI. Can. J. Chem. 68, 2242 (1990).On a examint la cinttique des reactions du t-

Study of the dissociation of the products of some proton transfer reactions in acetonitrile solvent

“…They predicted that in high-polarity solvents the proton transfer was coupled with the motion of the 'Author to whom correspondence may be addressed at: Departmolecules of the solvent, which reduced the effective deu- vour large KIE values by the "uncoupled mechanism" (14, stants for the ionogenic proton transfer reactions were too 15). Consequently for comparison, kinetic experiments were small in low-polarity solvents, constraining kinetic meaperformed in both low-and high-polarity aprotic solvents (10, surements to high-polarity solvents, mostly acetonitrile (8)(9)(10)(11)(12)(13)(14)16). However, very frequently, the equilibrium con-10).…”

Equilibrium and kinetic study of the proton transfer reactions between nitroalkanes and strong organic bases — phosphazenes in tetrahydrofuran solvent

“…This effect could be caused by weaker interaction between ions in ion-pairs formed by phosphazene cation in comparison with ionpairs formed by DBUH + in THF. This weaker hydrogen bonding between ion-pairs involving phosphazenium cations is supported by the fact that absorption bands for the phosphazene reaction products (see λ max values collected in Table 2) are shifted by ~25 nm to the red compared to the bands of the DBUH + salts in the same THF solvent [13].…”

“…The equilibrium constant, K, for the proton transfer reaction between NPNE and DBU was found to be 2060 M -1 (Table 8), while for the phosphazenes studied the equilibrium constants were too large to be [13,14]. Moreover, the large difference in entropy of activation can be seen between values obtained for the reaction of phosphazene with NPNE and MNPNE.…”

“…Previously prepared samples of carbon acids: 1-nitro-1-(4-nitrophenyl)ethane (NPNE), 2-methyl-1-nitro-1-(4-nitrophenyl)propane (MNPNP) and deuterated analogues have been used experimentally [13].…”

“…These results show that the steric factor and ordering effects in forming the transition state play an important role, confirmed by negative and relatively large entropies of activation: ∆S ‡ = -149.7, -176.5, -178.7, -227.8 J mol -1 K -1 . The reactions between nitroalkanes with different steric hindrance and classic nitrogenous bases such as amidines or guanidines in acetonitrile and tetrahydrofuran have been disscussed previously [12][13][14][15].…”