sugar

[chiddler]

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why is glucose preferred over mannose?

oh! it's because glucose can have one more equitorial hydroxyl group, right?

 

[milski]

Wild guess: OH-H attraction would make it slightly more energetically favorable?

 

[MrNeuro]

why is glucose preferred over mannose?

oh! it's because glucose can have one more equitorial hydroxyl group, right?

if i remember correctly

Killiani Fischer tells you about the orientation of the OH group on C2 if you get glucose as a major product that means that your staring material is going to be D-arabinose due to interactions of the hydroxyl groups not being favorable. however if mannose is your major product the starting material would most likely be the C-2 epimer of D-arabinose.

 

[chiddler]

why is one product preferred over the other?

 

[ridethecliche]

Sterics?

Glucose is less hindered in that position.

 

[lkthlttr]

Yes, essentially glucose is slightly favored because it adopts a chair conformation with more equatorial hydroxyl groups than the mannose.

 

[MedPR]

Yes, essentially glucose is slightly favored because it adopts a chair conformation with more equatorial hydroxyl groups than the mannose.

Wouldn't axial be preferred because of hydrogen bonding between the neighboring OH's?

 

[lkthlttr]

Wouldn't axial be preferred because of hydrogen bonding between the neighboring OH's?

Steric clash between axial groups in a chair conformation has a greater contribution to the energy than any hydrogen bonding between hydroxyls.

 

[chiddler]

Steric clash between axial groups in a chair conformation has a greater contribution to the energy than any hydrogen bonding between hydroxyls.

yes. i just did a passage that confirms this, coincidentally.

alpha of some sugar allows hydrogen bonding
beta is equitorial but not hydrogen bonding

alpha is 36%
beta is 64%

 

[lkthlttr]

yes. i just did a passage that confirms this, coincidentally.

alpha of some sugar allows hydrogen bonding
beta is equitorial but not hydrogen bonding

alpha is 36%
beta is 64%

Well alpha/beta is complicated by anomeric effect as well, but for the most part you can just remember that the conformation with the most equatorial groups is typically the most energetically favored.

 

[chiddler]

Well alpha/beta is complicated by anomeric effect as well, but for the most part you can just remember that the conformation with the most equatorial groups is typically the most energetically favored.

what do you mean complicated by anomeric effect?

 

[lkthlttr]

Well, I think it might be outside of the scope of the MCAT, but the anomeric effect essentially describes an exception to the rule that equatorial substituents are favored. For most sugars the alpha position (OH of the hemiacetal in an axial position) is favored compared to the beta (OH of the hemiacetal in the equatorial position). There are molecular orbital explanations for why this is the case, but they're a little complicated so this is why I am just saying that it's probably best to try to remember that equatorial is favored.

 

[chiddler]

Well, I think it might be outside of the scope of the MCAT, but the anomeric effect essentially describes an exception to the rule that equatorial substituents are favored. For most sugars the alpha position (OH of the hemiacetal in an axial position) is favored compared to the beta (OH of the hemiacetal in the equatorial position). There are molecular orbital explanations for why this is the case, but they're a little complicated so this is why I am just saying that it's probably best to try to remember that equatorial is favored.

oh.

thanks for your help!

 

[MedPR]

Maybe I was thinking of gauche vs anti instead of equatorial vs axial?