If you need a refresher on the mechanism, hover here or click this link. Like hydrate formation, hemiacetal formation is an equilibrium and the equilibrium tends to favor the starting aldehyde. Similarly, aldehydes can react with alcohols to form hemiacetals. Refresher on mechanism – hover here to bring up a picture of the mechanism or click this link. Hydrates form readily in solution, but they tend not to be easy to isolate the equilibrium tends to favor the starting aldehyde. ( relevant article: Acetals, Hemiacetals, Hydrates). You may recall that aldehydes (and ketones, but we’ll focus on aldehydes here) can reversibly react with water to form “ hydrates“. Hydrates, Hemiacetals, and Cyclic Hemiacetals And not just one cyclic form, but several!īefore diving into these, it’s worth a quick refresher on the two main functional groups in glucose (and other sugars) which make this possible: hydroxyl groups and aldehydes (or ketones, in the case of keto-sugars like fructose). That’s because glucose, like a snake that bites its own tail, or a belt – can adopt a cyclic form as well. The fact that I had to specify “open-chain form” might tip you off that something is amiss. In its open chain form, when drawn as a Fischer projection, D-glucose looks like this: If, by “glucose”, you mean the enantiomer we commonly encounter as blood sugar, then you’re referring to D-glucose.

The first question to ask, which we covered in our recent post on D- and L- sugars, is: “Which enantiomer are you talking about?”. Conclusion: Pyranose vs Furanose vs Open-Chain Forms of GlucoseĪ simple question! but one with many “right” answers.Glucose Has Several Structures, All In Equlibrium With Each Other?.Ring-Chain Tautomerism In Glocose, II: The “Furanose” Form.Ring-chain Tautomerism In Glucose: The “Pyranose” Form.Hydrates, Hemiacetals, and Cyclic Hemiacetals.This gives rise to two diastereomers which for historical reasons are called anomers and designated “ alpha” or “ beta” depending on the orientation of the -OH group. In the process of forming the hemiacetal a new chiral center is created.Equilibrium tends to favor the six-membered cyclic form, called “ pyranose” since it resembles pyran, although some of the five-membered cyclic form (“ furanose“) is also present along with the open-chain form. In the case of glucose, a 5- or 6- membered ring can form, depending on which hydroxyl group adds to the aldehyde.Sugars such as glucose exist in equilibrium between their open-chain form and various cyclic forms where an OH group and an aldehyde combine to form a cyclic hemiacetal.Pyranoses, Furanoses, Straight-Chain Glucose, And Ring-Chain Tautomerism

Anomeric carbon in fructose