When an aldose cyclizes, the hydroxyl group on the second to last carbon undergoes an intramolecular reaction with the carbonyl group of the aldehyde. The product resulting from aldose cyclization is a hemiacetal. The cyclization of glucose is shown below with the group characteristic of a hemiacetal shown in red.Click to see full answer. Consequently, how is glucose a Hemiacetal?The hemiacetal forms when an aldehyde reacts with an alcohol. One of the most commonly known hemiacetals is sugar molecules such as glucose. For example, an intra-molecular reaction occurs when the linear glucose molecule becomes a cyclic glucose molecule. The alcohol on carbon 5 reacts to form the hemiacetal.One may also ask, how does glucose form a ring? Forms of Glucose: A Monosaccharide Sugars can exist in one of two forms: linear or ring (cyclic). Although we often draw them as linear structures, they quickly fold up into ring-shaped molecules in water, as a result of the reaction of their keto or aldehyde groups with a hydroxyl group. Keeping this in consideration, why do sugars Cyclize? Glucose, an aldo-hexose, is a central sugar in metabolism. It and other 5 and 6C sugars can cyclize through intramolecular nucleophilic attack of one of the OH’s on the carbonyl C of the aldehyde or ketone. Such intramolecular reactions occur if stable 5 or 6 member rings can form.Is glucose a Pyranose?For an aldohexose such as glucose, the C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called pyranose because of its similarity to pyran (Figure 11.4).