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El ácido es clave para la actividad antioxidante de los óxidos de nitrógeno

Evan A Haidasz1, Derek Meng1, Riccardo Amorati2

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Los dialquilnitróxidos persistentes como TEMPO son clave en los estabilizadores de luz de aminas obstaculizados. Este estudio revela un mecanismo alternativo en el que el TEMPO es regenerado por radicales alquilo, lo que explica su actividad antioxidante.

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Área de la Ciencia:

  • Química orgánica
  • Ciencia de los Polímeros
  • Ciencias de los materiales

Sus antecedentes:

  • Los dialquilnitróxidos persistentes, como el 2,2,6,6-tetrametilpiperidina-1-oxilo (TEMPO), son componentes cruciales en los estabilizadores de luz de aminas con impedimento (HALS).
  • Los aditivos HALS impiden la degradación fotooxidativa de varios productos de consumo e industriales.
  • El mecanismo establecido para HALS implica un ciclo catalítico con nitróxidos, radicales alquilo y radicales peroxilo.

Objetivo del estudio:

  • Proporcionar pruebas de un mecanismo de reacción alternativo de los dialquilnitróxidos persistentes en la degradación (foto) oxidativa.
  • Investigar el papel de la catálisis ácida en la reacción entre los nitróxidos y los radicales peroxilo.
  • Elucidar la vía de regeneración de TEMPO a partir de su ion oxoamonio por radicales alquilo.

Principales métodos:

  • Estudios cinéticos de la reacción entre TEMPO y los radicales peroxilo en presencia de ácidos.
  • Determinación de la constante de velocidad de la reacción entre el ion oxoamonio de TEMPO y los radicales alquilo.
  • Investigación del mecanismo de reacción en las autoxidaciones de hidrocarburos a temperaturas elevadas.

Principales resultados:

  • TEMPO reacciona con los radicales peroxilo a velocidades controladas por difusión en presencia de ácidos.
  • La regeneración de TEMPO a partir de su ion oxoammonio por los radicales alquilo se produce con una constante de velocidad de aproximadamente 1-3 × 10 M 1 s 1 .
  • Esta vía de regeneración compite efectivamente con el O2 para los radicales alquilo, y su velocidad está influenciada por la fuerza del ácido.

Conclusiones:

  • Se propone un mecanismo alternativo catalizado por ácido para la actividad del nitróxido en HALS, que implica la formación de iones oxoamonio y su posterior reducción por radicales alquilo.
  • Este mecanismo explica la actividad antioxidante catalítica de captura de radicales de TEMPO observada en las autoxidaciones de hidrocarburos.
  • Los hallazgos contribuyen a comprender la química fundamental que subyace a la eficacia de los antioxidantes basados en HALS y TEMPO.