We have to get the product of the addition reaction between butanal and water.
The product of the addition reaction between butanal and water is the gem di-ol which is unstable and it produces the reactant i.e. butanal only.
The reaction is shown in the figure.
From the figure we can see that in the addition reaction there the –C atom of the –CHO (aldehyde group) possess two –OH (hydroxyl group) as a product (gem-diol) of addition reaction, which is highly unstable and water molecule are regenerate to form the reactant i.e. butanal again.
