Which form of glucose is obtained by crystallization from hot and saturated aqueous solution at 371K?

β-form

The correct answer is option 3. β-form.

Glucose, a simple sugar, exists in two main forms: alpha \((\alpha )\) and beta \((\beta )\). These forms are not entirely different molecules, but rather variations of the same molecule with a slight difference in structure. This difference arises from the way a hydroxyl group \((OH)\) is positioned on a specific carbon atom \((C_1)\) in the glucose molecule.

Here's a breakdown of why the β-form crystallizes from a hot, saturated aqueous solution at 371K:

Stability:

Although both \(\alpha \) and \(\beta \) forms of glucose can exist, the β-form is slightly more stable due to reduced steric hindrance (repulsion between nearby atoms). In the \(\beta \)-form, the \(OH\) group on \(C_1\) is positioned away from the bulky ring structure of the glucose molecule, minimizing repulsion. In the α-form, the \(OH\) group is closer to the ring, leading to some steric hindrance.

Temperature and Solubility:

While both forms can dissolve in water, the \(\beta \)-form is generally slightly more soluble than the α-form at higher temperatures. This means that at \(371K\) (around \(98^oC\)), a hot solution can hold more \(\beta \)-glucose molecules before becoming saturated (unable to dissolve more).

Crystallization:

During crystallization from a saturated solution, the most stable and most soluble form under those conditions will preferentially come out of the solution and form crystals.

Putting it together:

At \(371 K\), the increased temperature favors the slightly more soluble \(\beta \)-form.

The \(\beta \)-form's overall lower steric hindrance makes it slightly more stable than the \(\alpha \)-form, especially in a hot environment.

Therefore, when a hot, saturated glucose solution cools down and crystals begin to form, the \(\beta \)-form is more likely to come out of the solution and form crystals due to its better solubility and slightly higher stability at that temperature.