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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Keto Enol Tautomerism

Is the term keto enol tautomerism familiar to you? If not, give attention to the ensuing discussion. Organic chemistry would be of no value if not for multiple bonds and functional groups.  Such groups impart the reactivity needed to sustain life. One attachment is carbonyl (–C=O). Tautomerism Given the right environment, atoms of a carbonyl group tautomerize. That is, they can exchange a proton (hydrogen). In so doing a π-bond is formed or disappears. The carbonyl or keto form assumes the alkene-alcohol or enol form, or vice versa. For acetone, the two forms are, CH3–(C=O)–CH3 keto form CH2=C(OH)–CH3 enol form See the image above. [sc name="MidArticleAdsense"] Both Aldehydes and Ketones Participate A ketone has its carbonyl carbon atom linked to two other carbon atoms. An aldehyde has its carbonyl carbon…
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