Compound `(A)` on reaction with iodine in the solvent diglyme gives a hydride `(B)` and hydrogen gas. The product `(B)` is instantly hydrolysed by water or aqueous alkali forming compound `( C)` and liberating hydrogen gas. The compound `( C)` in aqueous solution behaves as a week mono basic acid. But in presence of certain organic polyhydroxy compound behaves as a strong monobasic acid. The hydride `(B)` in air catches fire spontaneously forming oxide which gives coloured beads with transition metal compounds.
Aqueous solution of product `(C)` can be titrated against sodium hydroxide using phenolphthalein indicator only in presence of:

Options

A. `cis-1,2 diol`
B. trans-`1,2` diol
C. borax
D. `Na_(2)HPO_(4)`

Answer

A

Explanation

If certain organic polyhydroxy compounds such as glycerol, manitol or sugars are added to the titration mixture, then `B(OH)_(3)` behaves as a strong monobasic acid and it can be now titrated with `NaOH` and the end point is detected using phenolphthalein as indicator `(pH=8.3-10.0)`.
The added compound must be a cis-diol, to enhance the acid properties. The cis-diol forms very stable complex with the `[B(OH)_(4)]^(-)` thus removing it from solution.The reaction is reversible and thus removal of one of the products shifts the equilibrium in the forward direction and thus all the `B(OH)_(3)` reacts with `NaOH` in effect it acts as a strong acid in the presence of the cis-diol.
`B(OH)_(3)+NaOH to Na[B(OH)_(4)]+NaBO_(2)+2H_(2)O`

Reactions involved
`(A)2Na[BH_(4)](A)+I_(2)underset("solution")overset("in diglyme")to B_(2)H_(6) , (B)+H_(2)+2NaI`
`B_(2)H_(6)+6H_(2)Oto2H_(3)BO_(3)( C)+3H_(2)`
`B(OH)_(3)+2H_(2)O hArrH_(3)O^(+) + [B(OH)_(4)]^(-) , pK=9.25`

`B_(2)H_(6)(B)+3O_(2)toB_(2)O_(3)+3H_(2)O`
`CoO + B_(2)O_(3)to Co(BO_(2))_(2)`(cobalt metaborate -blue colour bead).