Compare sucrose to lactose. There is a fundamental difference. In lactose, there is still one hemi-acetal function left in the molecule, C 1 of the glucose. An equilibrium will be established between the open chain, a and b form of the molecule. Sucrose is not in equilibrium with the open chain form of the molecule.
Acetals are stable once they have formed.
Remember what happens when aldehydes are treated with mild oxidants, they are oxidized to carboxylic acids . All monosaccharides are reducing sugars . If sucrose is hydrolyzed, broken back down into fructose and glucose, the solution will reduce Benedict's solution, due to the presence of the monosaccharides. We had said that ketones can't be oxidized. In saccharides, we have ketoses not ketones .