A highly unstable contributor to the resonance structure when trifluoromethyl benzene undergoes electrophilic substitution is

First, draw the trifluoromethyl benzene molecule. Trifluoromethyl benzene consists of a benzene ring with a trifluoromethyl group (-CF3) attached to one of its carbons. Due to the electronegativity of fluorine atoms, the trifluoromethyl group is an electron-withdrawing group, pulling electron density away from the benzene ring through the inductive effect. Resonance structures help visualize the distribution of electron density across the benzene ring's pi bonds, especially during electrophilic substitution. Identify all possible resonance structures by moving electrons around the ring.

The most unstable resonance structure for the reaction will have the most overall positive charge on the ring atoms, especially on carbon, which will make it least stable. This is because carbon, being electronegative, favors electron-rich environments, and positive charge destabilizes it. In case of electrophilic substitution on trifluoromethyl benzene, an intermediate carbocation is formed as the electrophile forms a bond with one of the carbons on the benzene ring. The destabilizing effect of the electron-withdrawing trifluoromethyl group will be most prominent when the positive charge is adjacent to the -CF3 group. Draw various resonance structures with electrophilic substitution at different positions and look for the one where the positive charge is maximally destabilizing due to the trifluoromethyl group's inductive effect.

Upon examining the different resonance structures, you will find that the most unstable contributor occurs when the positive charge is ortho or para to the trifluoromethyl group, i.e., next to or opposite to the -CF3 group on the benzene ring. In these resonance structures, the carbon with positive charge directly neighbors the electron-withdrawing trifluoromethyl group, leading to significant destabilization of the molecule due to the greater electron deficiency. This makes the electrophilic substitution reaction less likely to proceed through these positions and thus results in a highly unstable resonance contributor.