Difference Between Primary and Secondary Kinetic Isotope Effect

Thekey differencebetween primary and secondary kinetic isotope effect is thatprimary isotope effect describes the isotopic substitution at the broken bond whereas secondary isotope effect describes the isotopic substitution at the bond adjacent to the broken bond.

动力学同位素效应或KIE是指化学反应反应速率的变化isotope. Here, an atom in a reactant is replaced by its isotope so the reaction rate would be different from the initial rate. Then we can determine a value for KIE by dividingrate constantfor the reaction involving the light isotopically substituted reactant from the rate constant for the reaction involving the heavy isotopically substituted reactant. Therefore, KIE larger than 1 is considered as normal kinetic isotopic effect while KIE less than 1 is considered as an inverse kinetic isotopic effect.

CONTENTS

1.Overview and Key Difference
2.What is Primary Kinetic Isotope Effect
3.What is Secondary Kinetic Isotope Effect
4.Side by Side Comparison – Primary vs Secondary Kinetic Isotope Effect in Tabular Form
5.概括

What is Primary Kinetic Isotope Effect?

Primary kinetic isotope effect is the change of reaction rate due to isotopic substitution at the site of bond breaking. Here, this substitution is in the bond-breaking stage at the rate-determining step of the reaction. Therefore, this type of isotopic effect is indicative of bond breaking or bond-forming to the isotope at the rate-limiting step.

Fornucleophilic substitutionreactions, the primary kinetic isotope effect is applied for leaving groups, nucleophiles and alpha-carbon at which the substitution occurs. This type of kinetic effect is less sensitive than an ideal KIE. This is due to the contribution of non-vibrational factors.

什么是次级动力学同位素效应？

Secondary kinetic isotope effect is the change of the rate of a reaction due to isotopic substitutions at a site other than the bond-breaking site. In other words, it indicates that no bond to the isotopically labelled atom is broken or formed. Like primary kinetic effect, this also takes place in the rate-determining step. There are three types of secondary kinetic effects named as alpha, beta and gamma effect.

Unlike primary KIE, secondary KIE tends to be much smaller. However, this type of KIE is still very useful in elucidating reaction mechanisms because per Deuterium atoms, the secondary KIE is considerably large. Apart from that, the magnitude of secondary kinetic isotopic effects is determined by vibrational factors.

主要动力学同位素效应之间有什么区别？

动力学同位素效应或KIE是指化学反应替代同位素后的反应速率的变化。主要的和次级动力学同位素效应之间的主要区别在于，主要同位素效应描述了断裂键的同位素取代，而次级同位素效应描述了与断裂键相邻键的同位素取代。此外，与初级KIE不同，次要Kie往往要小得多。

此外，次级动力学同位素效应的大小由振动因子确定，而主要动力学同位素效应由于非振动因子而对敏感性较小。

Below infographic summarizes the difference between primary and secondary kinetic isotope effect.

概括– Primary vs Secondary Kinetic Isotope Effect

动力学同位素效应或KIE是指化学反应替代同位素后的反应速率的变化。主要的和次级动力学同位素效应之间的主要区别在于，主要同位素效应描述了断裂键的同位素取代，而次级同位素效应描述了与断裂键相邻键的同位素取代。

Reference:

1.“动力学同位素效应”。维基百科,维基Foundation, 8 Nov. 2019,Available here.
2. “Kinetic Isotope Effects.” Chemistry LibreTexts, Libretexts, 5 June 2019,Available here.

Image Courtesy:

1. Aykutaydin的“ SN1 VS SN2” - 自己的作品(CC BY-SA 3.0)viaCommons Wikimedia