Kinetic study of the proton transfer reaction between 1-nitro-1-(4-nitrophenyl)alkanes and P1-t-Bu phosphazene base in THF solvent
✍ Scribed by Żaneta Wisłocka; Iwona Nowak; Arnold Jarczewski
- Publisher
- Elsevier Science
- Year
- 2006
- Tongue
- English
- Weight
- 154 KB
- Volume
- 788
- Category
- Article
- ISSN
- 0022-2860
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✦ Synopsis
The rates of proton transfer reactions between C-acids of the series of nitroalkanes with increasing bulk of RZH, Me, Et, i-Pr substituents as 4-nitro-phenylnitromethane (1), 1-(4-nitrophenyl)-1-nitroethane (2), 1-(4-nitrophenyl)-1-nitropropane (3), 2-methyl-1-(4-nitrophenyl)-1-nitropropane (4) and the P 1 -t-Bu phosphazene have been measured in THF at pseudo-first-order conditions. The product of the proton transfer reaction with P 1 -t-Bu is associated into ion pair. The equilibrium constants decrease along with growing bulk of alkyl substituent in the reacting C-acid: O100 000, 1170, 590, and 11.8 for 1, 2, 3, 4, respectively. The second order rate constants (k 2H ) for this very strong base and C-acids are rapidly declining: 9357, 2.31, 0.66, 0.09 dm 3 mol K1 s K1 for 1, 2, 3, 4, respectively, and could not be accounted for the small values of the enthalpies of activation DH s H Z 6:1; 18:0; 20:7 and 11:1 kJ mol K1 . The appropriate values of the entropies of activation were all negative and relatively large DS s H ZK149:7;K176:5;K178:7;K227:8 J mol K1 deg K1 indicating an importance of steric and ordering effects in forming the transition state. The primary deuterium kinetic isotope effects are large showing tendency of reverse relation towards steric hindrance of the reacting C-acids, k H /k D Z 15.8, 13.6, 13.2 for 1, 2, and 3, respectively. The explanation of the unusual slow proton transfer abstraction caused by this extremely strong base is undertaken.