Nucleophilic displacement reactions kinetic isotope effects

An S-12C bond breaks a little faster than an S-13C bond in a nucleophilic displacement reaction. This primary kinetic isotope effect (KIE)3a 3b/31a-d is usually discussed first for breakage of a C-H bond. 

The chlorine kinetic isotope effect in nucleophilic displacement at saturated carbon in para-substituted benzyl chlorides, with thiolate and analogous oxygen nucleophiles, has been examined . The reactions proceed via a concerted transition state. 

---