The effects of vinylic and allylic fluorine substitution in iso‐butylene

Abstract

Molecular orbital calculations using the 3‐21G basis set have been performed for iso‐butylene (IB; 2‐methyl‐1‐propene), difluoro‐iso‐butylene (DFIB; 1,1‐difluoro‐2‐methyl‐1‐propene), hexafluoro‐iso‐butylene (HFIB; 3,3,3‐trifluoro‐2‐(trifluoromethyl)‐1‐propene), and perfluoro‐iso‐butylene (PFIB; 1,1,3,3,3‐pentafluoro‐2‐(trifluoromethyl)‐1‐propene). The effects of fluorine substitution were studied by comparison of several calculated quantities of the fluorinated compounds with those of IB. Through an analysis of the computed electron density distributions, it is suggested that a vinylic fluorine acts as a π acceptor, by electron transfer into the CF bond, and a π repeller, by polarization of the adjacent π electrons. An allylic fluorine acts as a π attractor through electrostatic effects, although in HFIB a minor contribution from hyperconjugation was evident. Finally, electrostatic potentials for the molecules were calculated. These show that fluorine substitution has large effects on the electrostatic potential associated with the π electrons. These effects change the sign of the calculated electrostatic potential in the plane containing the π bond to such an extent that PFIB is quite susceptible to nucleophilic attack.