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Benzene, touene, xylene (o,m,p) and mesitylene dissolve in HBF_(4) to fromsalts. Expalin the order of basicity: Mesitylene gt m- "Xylene" gt o- and p- Xylenen gt Toulene gt Benzene |
Answer» Solution : The reaction is reversible, the more stable the `sigam-` complex is, the more will be the equilibriumon the right , i.e., the more basicis the ARENE. More the number of `EDGH` [(Me)` group], more is the stability of the complex, and more basic is the compound.  Because of `+I` effect of `(Me)` the positive charge is partically neutralised and `(Me)` group acquires `+dela` charge. i.e., there is chargespreading whichincreases the stabilityof the `sigma-` complex and thus increases the basic CHARACTEROF toluene w.r.t benzene. Mesitylene thas three `(Me)` groups so it must be thestrongest BASE accordingly. Resonating structures of `m-o-` an d`p-` XYLENES are:   In `o-` p-` xylenes the resonace-contributing stuctes (VII) and (X) are slightly less stable than other resonaitngsturctures. So one can conclue that resonatingstructers of `m-` xylene are more stable than `o-` and `p-` xylenes which increases the basic character of `m-` xylene than `sigma-` and `p-` xylenes. SO the order of basic character is: 
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