Animated Solution for Chemistry - Organic Chemistry: Maltose on treatment with dilute HCl gives

To truly understand what happens when maltose undergoes hydrolysis, we must first appreciate its molecular architecture. Maltose, commonly known as malt sugar, is a classic example of a disaccharide. As the name suggests, it is constructed from two simpler building blocks, or monosaccharides.

Imagine two identical $\alpha$-D-glucose molecules floating in a solution. Through a condensation reaction (where a molecule of water is removed), these two rings link up. The specific connection point is crucial: the hydroxyl group on the anomeric carbon ($C_1$) of the first glucose molecule reacts with the hydroxyl group on the fourth carbon ($C_4$) of the second glucose molecule.

This creates an oxygen bridge known as an $\alpha (1\to 4)$ glycosidic linkage. The "$\alpha$" designation tells us that the stereochemistry at the anomeric carbon of the first ring is in the alpha position (pointing downwards in a standard Haworth projection).

Now, let's introduce dilute hydrochloric acid ($\text{HCl}$) into the mix. The acid acts as a powerful catalyst for a process called hydrolysis. The word hydrolysis comes from Greek, meaning "water" (hydro) and "to break" (lysis).

The acidic environment provides protons ($H^{+}$) that protonate the oxygen atom in the glycosidic linkage, making it highly susceptible to attack by a water molecule. When the water molecule attacks, the glycosidic bond shatters. The water molecule splits, donating a hydroxyl ($-OH$) group to one glucose unit and a hydrogen ($-H$) to the other, effectively reversing the condensation reaction that originally formed the maltose.

Because maltose is a homodisaccharide—meaning it is built from two identical monomer units—breaking it apart yields two identical molecules.

Unlike sucrose, which breaks down into glucose and fructose, or lactose, which yields glucose and galactose, maltose is beautifully simple. It gives us pure D-glucose.

As an interesting side note, because the second glucose unit in the maltose molecule retains a free anomeric carbon (at $C_1$), the ring can spontaneously open in an aqueous solution to reveal a reactive aldehyde group. This structural feature is what classifies maltose as a reducing sugar, capable of reducing reagents like Tollens' or Fehling's solutions.

However, for the scope of our question, the primary takeaway is the identity of the monomers. The hydrolysis of maltose yields exclusively D-glucose, making option (d) the undeniably correct answer.