Explain the following :
(a) Highly branched acids such as are less acidic than unbranched acids.
(b) `K_(a_1)` for maleic acid is greater than that for fumaric acid or `pK_(a_1)` for maleic acid is less than that for fumaric acid.
( c) `K_(a_2)` for fumaric acid is greater than that for maleic acid or `pK_(a_2)` for fumaric acid is less than that for maleic acid.
(d) Phthalic acid is a stronger acid than either isophthalic or terephthalic acids but phthalate monoanion is weaker than isophthalate and terephthalate monoanions.
( e) `o-`Nitro benzoic acid `(pK_a = 2.21)` is stronger acid than `3,5-`dinitrobenzoic acid `(pK_a = 2.80)` but weaker than `2,4-`dinitro benzene acid in water.

Explanation

(a) The `(-COO^(Ө))` of the branched acid is shielded from the solvent molecules and can not be stabilised as effectively as an anion of an unhindered acid.
(b) Both fumaric acid and maleic acid have positive ionisable hydrogen. Meleate ion is stabilished by intramolecular `H-`bonding, so maleic acid is greater than `K_(a_1)` of fumaric acid or `pK_(a_1)` of maleic acid is less than `pK_(a_1)` of fumaric acid as shown below :

( c) The second ionisable `H` atom of maleate monoanion is involved in `H-`bonding and hence more energy needed to remove this `H-`bonded hydrogen. The maleate monoanion is, therefore, a weaker acid. Therefore, `K_(a_2)` of fumaric acid is greater than maleic acid or `pK_(a_2)` of fumaric acid is less than maleic acid.
(d) (i) Same explanation as in `(b)` and `( c)` above.
The phthalate monoanion is stabilished by intramolecular `H-`bonding. while the anions of isophthalic and terephthalic acids do not form `H-`bonding since the meta group in isoohthalic acid and the para group in terephthalic acid are far point as in the case of fumaric acid. Hence phthalic acid is the strongest of the three acids because `H-`bonding stabilises its conjugate base.

(ii) The second ionisable `H` atom of `(II)` is involved in `H-`bonding and is difficult to ionise. Thus, the same `H-`bonding makes the monoanion the weakest acid.

( e) The decreasing acid strength of the following :
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