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Give the order of acidic character of the following : (i) (a) p-Nitrobenzoic acid (b) 2,4-dinitrobenzoic acid ( c) 2,4,5-Trimethylbenzoic acid (d) 2,3,4-Trinitrobenzoic acid (ii) (a) Ph_3 CH (b) Ph_2Ch_2 ( c) PhCH_3 (iii) 2,6-Di-tert-butyl phenol is a much weaker acid than phenol. (iv) (a) Phenol (b) Cyclohexanol ( c) p-Bromophemol (d) p-Methoxy phenol (v) (vi) (a) Benzoic acid (b) p-tert-Butylbenzoic acid ( c) p-Nitrobenzoic acid. |
Answer» Solution :(i)  So the order is: `b gt d gt a gt c`(ii) `a gt b gt c` a. `Ph_(3) CH + H_(2)O rarr underset(["RESONANCE stabilised by three Ph groups"])(Ph-underset(Ph)underset(|)overset(ddotTheta)C-PH+H_(2)overset(o+)O)` b. `Ph_(2)CH_(2)+H_(2)Orarrunderset(["Resonance stabilised by two Ph groups"])(Ph-overset(ddotTheta)underset(H)underset(|)(C)-Ph+ H_(3)O^(o+))` c. `underset(["Resonance stabilised by ONE Ph groups"])(PhCH_(3)+H_(2)OrarrPh-overset(Theta)CH_(2)+H_(3)O^(o+))` (iii)  Due to steric inhibition of resonance of bulky ortho-t-buty1 groups, it prevents the solvation of corresponding phenoxide ion, making it less stable than phenoxide ion. Hence, the OBSERVED effect. (iv) `c gt a gt d gt b` [(c )(-I effect of Br at o-) gt (a) Standard (phenol) gt -I and `+ R` effects of `(-OMe)` group at `p- gt (b)` CYCLIC alcohol (resonance does not take place)] (v) `a gt b` The carbanion derived from `(a)` is better stabilished by the aromatic rings which have to be coplanar than in the carbanion from `(b)` in which the aromatic rings prefer to be in different planes for steric reasons. Complanarity allows maximum `p-`ORBITAL overlap. (vi) `c gt a gt b` [( c) `- I` and `-R` effect of `(-NO_2) gt (a)` Standard `gt ( c) + I` effect of t-buty1 group.] |
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