Explain the following :
(a) In gaseous phase, the acidic characters of the following are :
(i) `CH_3 CH_2 COOH (I) gt CH_3 COOH (II)`
(ii) `ClCH_2COOH (III) gt FCH_2 COOH (IV)`
(iii) `Me_3 C-OH (V) gt MeCH_2 CH_2 CH_2 OH (VI)`.
(b) Account for the following differences in the acidic characters of the following :
(i) `(I) Me_3 C-CH_2 -COOH gt (II) Me_3 SICH_2 COOH`
(ii) `(III) H_2 C = CHCH_2 COOH gt (IV) CH_3 CH_2 COOH`
(iii) `(V) HC -= C - CH_2COOH (pK_a 3.22) gt (VI) H_2 C = CH CH_2 COOH (pK_a = 4.35)`
(iv) `CH_(3)-underset(O)underset(||)(C)-CH_(2)-COOH gt`
(v) `(IX) N -= C - CH_2 COOH gt (X) CH_3 COCH_2 -COOH`
(vi) `(XI) PhCh_2 COOH gt (XII) CH_2 = CHCH_2 COOH`.

Explanation

a. The acidic strengths are opposite to what is expected from inductive effects, which suggest that in gaseous phase, electronic phase, there is no role of base and solvents. So in gaseous phase, a large anion is more stable.
i. (I) anion is large than (II) anion, so (I) is a stronger acid than (II).
ii. (III) anion (size of `CI gt F)` is large than (IV) anion, so (III) is a stronger acid than (IV).
iii. (V) anion (size of t-butyl `gt n`-butyl) is large than (VI) anion, so (V) is a stronger acid than (VI).
b. i. Silicon is more electropositive than C and therefore, has an acid weakening `bar(e)` donating inductive effect. consequenctly, anion of (II) is less stable than its acid. Hence, (II) is a weaker acid than (I).
ii. `sp^(2)`hybridised `C` atom of `(C=C)` attracts the bonded `bar(e)`'s more than `sp^(3)`-hybridised `C` atom and thus `(-C=C-)` has acid strengthening `EWG`. so (II) is a stonger acid than (IV).
iii. sp-hybridised `C` atom of `(C-=C)` attracts the bonded `bar(e)`'s more than `sp^(2)-` or `sp^(3)-`hybridised `C` atom and thus `(-C-=C-)` has acid strengthening `EWG`. so (V) is a stronger acid than (VI).
iv. `(C=O)` is polarised as `(overset(+delta)(C)=overset(-delta)(O))` and because of `(+delta)` charge on `C`, the `(C=O)` group has acid strengthening `EWG`. so (VII) is more acidic than (VIII).
v. Both the `(C-=N)` and `(C=O)` groups have acid strengthening `EWG`s but sp-hybridised `C` atom of `(C-=N)` attracts the bonded `bar(e)`'s more than `sp^(2)`-hybridised `C` atom of `(C=O)` so (IX) is more acidic than (X).
vi. Both have `sp^(2)-`hybridised `C` atoms, but in `(Ph-)` they are much more resonance-stabilised and thus `(Ph-`) has more acid strengthening `EWG` than `(-C=C-)`. Hence (XI) is a stronger acid than (XII).