The General Fluorine Effect

For example, in the top example the formation of CF3CCI3 is energetically preferred by ca 5.6 kcal moh (calculation on the B3LYP/ 6-31G //B3LYP/6-31C level of theory) [24]. 

The reason for this energetic preference for fluorine accumulation at the same sp center is selfstabilizing by formation of possible ionic resonance structures, as already discussed in Section 1.4.2. 

---

One clear tendency is apparent from the equilibrium product distribution of Letvis acid-induced transfluorination and halogen scrambling if fluorine can migrate it vill al vays tend to concentrate at one carbon atom. Most preferred reaction products are trifluoromethyl derivatives, followed by geminal difluoromethy] derivatives. This thermodynamic product control is often referred to as the general fluorine effect (Scheme 2.19). 

Scheme 2.19 The general" fluorine effect in Lewis acid-induced halogen scrambling.

---