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Give the decreasing order of acidic character of the following : (a) (1) Benzene (2) CH_3OH (3) H_2 O (4) CH_3 SH (b) (1) CH_3CH_2COOH (2) HOCH_2CH_2COOH (3) C1 CH_2 CH_2 COOH (4) O_2 NCH_2CH_2COOH ( c) (d) (1) [Fe(H_2O)_6]^(2 +) (2) [Fe(H_2O)_6]^(3 +) (3) [A1(H_2O)_6]^(3 +) (4) [Cr(H_2 O)_6]^(3 +) ( e) (1) HCHO (2) CH_3 CHO PhCHO (4) CH_(3)-overset(O)overset(||)(C)-CH_(3) (5) R-overset(O)overset(||)C-CI (6) (CH_3CO)_2 (7) (RCOOR) (8) RCONH_2. |
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Answer» Solution :(a) `4 gt 2 gt 3 gt 1 [(4)` THIOLS `gt (2)` Methanol `gt (3) H_2 O gt (1)` BENZENE `(sp^2)`] (i) Thiols are stronger acid than alcohols (ii) Methanol is a stronger acid than `H_2O (pK_a of CH_3OH = 15.5, pK_a` of `H_2O = 15.7)` (see Table 4.3) (b) `4 gt 3 gt 2 gt 1` (`- I` effect of `-NO_2 gt -C1 gt -OH`). In `(I) + I` effect of `Et`. ( c) `4 gt 1 gt 2 gt 3 [(4) - I` effect of five `F` atoms `gt (1) - I` effect of `O` atom `o` position `gt (2) - I` effect of `O` atom at `m` position `(gt(3) LP overline e` donation from `N` atom by resonance] (d) `3 gt 2 gt 4 gt 1` The acidic character of the complexes is explained on the BASIS of charge density (ionic charge/ionic size). According to Fajan's rule, small cation and high charge mean more covalent and more ecidic character. Charge on the central metal : `(1) Fe^(2 +), (2) Fe^(3 +) , (3) A1^(3 +), (4) Cr^(3 +)` Ionic radius : `A1^(3 +) lt Fe^(3 +) lt Cr^(3 +) lt Fe^(2 +)` `(3)` is the strongest acid due to high charge density (high `3 +` charge and small ionic radius) Charge density and acidity ORDER : `3 gt 2 gt 4 gt 1` ( e) `1 gt 2 gt 3 gt 4 gt 5 gt 6 gt 7 gt 8` [see Section 4.10.1(f)]. |
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