Rank the following compounds in order of increasing amount of hydrate present at equilibrium.

When aldehyde is dissolved in water, a hydrate is formed. The aldehyde and its hydrate are in a state of equilibrium. The addition of water to most ketones, however, results in an unfavourable balance. A stable hydrate can be formed by compounds having strong electron-withdrawing groups linked to the carbonyl carbon. The hydrate is stabilised by the electron-withdrawing group. As a result, at equilibrium, aldehyde will have more hydrate than ketone. At equilibrium, an aldehyde with an electron-withdrawing group will have a higher hydrate.

The reaction for hydration of given compounds are shown below.

It is know that aldehyde with an electron withdrawing group will have a higher amount of hydrate. Here, compound (III) has an electron withdrawing group $(-\mathrm{Br})$at the alpha position. Therefore, compound (III) has a higher amount of hydrate present at equilibrium.

Now among compounds (I), (II), and (IV), compound (I) is aldehyde and (II) and (IV) are ketone. Therefore, the compound will have a higher amount of hydrate.

Now among (II) and (IV), compound (IV) has an electron withdrawing group $(-\mathrm{Br})$ at the alph position. Therefore, compound (IV) has a higher amount of hydrate present at the equilibrium.

Therefore, the compounds in the order of increasing amount of hydrate present at the equilibrium is shown below.