In Glucose, for example, both 5th and 6th Carbon atoms have -OH groups. But always, only the -OH group on (either 4th or) 5th Carbon atom attacks the Aldehyde group (1st C atom) to generate the ring structure. Why can not the -OH group on the 6th Carbon atom attack the Aldehyde group? It should become easier to make a ring when the Carbon chain in the ring becomes longer!
Hi Sn
Considering glucose for example, I think the following could be part of the reason.
1. Stereochemical arrangements of the subsequents in space positions the C-5 OH group closer to the carbonyl of the aldehyde.
2. Preference towards adopting a stable chair conformation void of 1,3-diaxial interaction.
3. Ring stability. 5 and 6 membered rings can adopt a conformation with minimal ring strain compared to 7.
Its higher entropic cost to make a 7 memebered ring as opposed to 6 memebered ring.
You might also want to look up Baldwin’s Rules for Ring Closure which tell you favorable and unfavorable ring-forming reactions.
Both Kavnen and Greg are right. The septanose forms of the hexoses are 3-8 kcal/mole less stable than the pyranose or furanose structures.
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