Give the decreasing order of acidic character of the following :
(a)
(b)
( c)
(d)
( e)
(f) .

Explanation

(a) `(II) gt (III) gt (I) gt (IV) (o-gt m- gt "standard" gt p-)`

(b) `(II) gt (IV) gt (III) gt (I) (orarr p- gt m- gt standard)`

Note : `(-R)` and `(+R)` at `o- and p-` are same and equal.
( c)
Note : `(III)` has `(-I)` effect of `(OMe)` group and `(II)` has ortho-effect, so their `pK_a` values are equal.
(d) `(II) gt (I) gt (IV) gt (III) (p-NO_2) B.A gt p-Me gt P-OMe)`.

Note : Resonance effect `(+R)` is greater than the combined effect of `(+ I)` and `H.C.(H.C)` refers to hyper-conjugation).
( e) `(II) gt (III) gt (IV) gt (I) (NO_2) gt CN gt Cl gt` Standard In `(II), (-I)` and `(-R)` effects of `(NO_2)` group at p-position. In `(III),(-I)` and `(-R)` effect of `(-C -= N)` group at p-position
`EW` power of `(-NO_2) gt (-CN) gt (-Cl) gt` Standard.

Note : According to `R` and `I` effect and ortho effects, the acidic order of the above should be `II gt III gt I gt IV`. But the observed order according to their `pK_a` value is as shown above. `(I gt III gt IV gt II)`. This is an exceptional case.