The largest isotope effects involve hydrogen transfer, where a bond to hydrogen is broken in the rate-limiting step. Substitution of deuterium for hydrogen then slows the rate by anywhere from 1.5- to 7-fold, depending on the structure of the transition state. This is a classic primary isotope effect, one in which a bond to the heavy isotope breaks in the transition state. Secondary isotope effects are those in which a bond to the heavy isotope is not broken but is altered in the transition state. The differences between primary and secondary isotope effects can be illustrated for a specific reaction, the oxidation of ethanol to acetaldehyde in eq. 2-19.
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