Question

In hyponitrous acid, the number of hydroxyl group present are:
(A). $1$
(B). $2$
(C). $3$
(D). $4$

Answer 1

Hint: There are two possible structure of acid, Tans and cis Trans-Hyponitrous acid forms with crystals that are explosive when dry. In solution, it is a weak acid $\left( {P{K_{a1}} = 7.21,P{K_{a2}} = 11.54} \right)$ and decomposed to laughing gas and water with a half lifetime of $16$ days at $25^\circ C$ at pH $1 - 3$ .

Complete answer:
Hyponitrous acid is chemical compound with formula ${H_2}{N_2}{O_2}$ or $HON = NOH$ . it is an isomer tautomer of nitramide, ${H_2}N - N{O_2}$

The number of hydroxyl groups present are two. The preparation of hyponitrous acid ( trans isomer ) can be done with silver hyponitrite and anhydrous $HCl$ in ether.
$A{g_2}{N_2}{O_2} + 2HCl \to {H_2}{N_2}{O_2} + 2AgCl$
Ferrous hydroxide will also reduce nitrates to hyponitrites.
Pure ferrous sulphate is precipitated with milk of lime, and to the cooled mixture is added $1$ mol of $NaN{O_3}$ for each $10$ moles of $FeS{O_4}$ . The hyponitrite is precipitated with silver nitrate.
A solution of $NaN{O_2}$ is added to an answer of $N{H_2}OH - {H_2}S{O_4}$ . The mixture is heated quickly to $60^\circ C$ and $AgN{O_3}$ is added at once. This method does not give good yields, but it proves that acid may be a dioxime. It proceeds as follows-
$HO - N{H_2} + O = N - OH \to HO - N = N - OH + {H_2}O$

Therefore, option B is correct.

Additional Information:
Salt of Hyponitrous acid is soluble in water and hydrolyse and gives an alkaline solution . Whereas normal salts of other bases are less soluble .
In the dry state it decomposes at $100^\circ C$ , giving $AgN{O_3}$ and it explodes at $150^\circ C$ .It is not decomposed by alkalies. It reacts with alkyl iodides, giving alkyl hypritrites.

Note:
It was shown by Divers in $1871$ , the sodium nitrate reduced with sodium amalgam gives a salt $NaNO$ (empirical composition) , the acid being an isomer of nitramide.
Since the discovery of hyponitrous acid by Dr. Divers , no new oxyacid of nitrogen has been described .