Why Some Sugars Have a Cyclic and a Chain Structure

Chemistry, Education
In your current course of organic chemistry, you're studying sugars. You notice in a text or see on the web a particular sugar, you've searched for by name. What is its true structure? You see it drawn as a chain structure with pendant groups. As you read about it, you see reference to another structure... a cyclic structure! What gives? Sugars: Example Fructose Some chemicals undergo change with the most minimal modification of environment. One example is keto-enol tautomerism. Fructose and certain other sugars experience something similar. It reacts reversibly, to form two cyclic hemiketals. Fructose Hemiketals A hemiketal forms by combination of an alcohol group with a ketone group. Fructose supplies both reactive groups, internally. For the generic reaction for hemiketal formation, see the accompanying illustration. Note the presence…
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The Quintessential Aromatic Hydrocarbon Benzene (Pt.2)

[caption id="attachment_6069" align="alignright" width="440"] Benzene De-localized Pi-system - CCA Share Alike 3.0 Unported by Vladsinger[/caption] In Part One of The Quintessential Aromatic Hydrocarbon Benzene, we saw that the so-called 1,3,5-cyclohexatriene was really not completely described by the structure that that name implies. It is equivalent to 2,4,6-cyclohexatriene (if you flip the 1,3,5- structure over, and you get the 2,4,6- structure), and includes a variety of other contributing factors. Although we did not list them, they may include ionic contributors.1 In general, these are discounted. Visualizing Benzene's Structure The conclusion is that the compound actually has six equivalent bonds between adjacent carbon atoms, forming six internal angles angle each equal to 60°. The bonds are each, then, “single-and-one-half” bonds between each carbon pair. The molecule is flat, even as a collection…
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