Incase of CF3+, here back bonding occurs from the electrons present in Flourine outermost shell to the positive carbon atom hence it tries to reduce the positive charge to gain stability. In case of C6H5CH2+ here it is benzene ring, hence here the electron cloud present in benzene ring is in resonance with the cation atom and hence gain stability but not as compared to the stability due to backbonding. In case of CH2Cl+ here NO backbonding is possible due to large size of chlorine atom hence due to electronegative behaviour of Cl it increases the positive charge decreasing the stability. On the other hand in last case C6H5+,this is most unstable situation in benzene ring because this is not possible to have a benzene ring with a positive charge.
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