Orgo Question

[ahs4n]

How do you change the Haworth projection or fisher projection in terms of hydroxyl groups being (up/down or left/right or alpha/beta) into the chair conformation?

I'm asking this because I understand haworth to fisher but I can't understand when to do the axial or equatorial positions in the chair positions for a glucose molecule given the haworth or fisher.

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[ahs4n]

56 views and no one can tell me when looking at the fisher or haworth projection of glucose, how to tell whether to put the OH group equatorial or axial?

 

[muchomaas]

I'm not sure, but here's what I think.

Each sugar has its own most stable chair form, so there isn't a set rule based on whether or not an OH is up or down in the Haworth. For example, glucose is maybe so common because all of the OH groups can be equatorial.

 

[aevea]

How do you change the Haworth projection or fisher projection in terms of hydroxyl groups being (up/down or left/right or alpha/beta) into the chair conformation?

I'm asking this because I understand haworth to fisher but I can't understand when to do the axial or equatorial positions in the chair positions for a glucose molecule given the haworth or fisher.

For beta-D-glucose, all the -OH groups are equatorial. For alpha-D-glucose all are equatorial except on C₁. As far as up/down goes, a good (dumb) mnemonic is "it's beta to be on top". The beta-D-glucose anomeric -OH will generally be "up" in both Haworth and chair projections.

I don't think you'll have to deal with converting any other sugars, but if you do, start by drawing a chair frame and then putting in the bulky -CH₂OH group. It's almost certainly going to be equatorial in the most stable chair. Then, groups that are on the right in the Fischer or down in the Haworth are also down in the chair formation. Just alternate between equatorial and axial in the normal way for chair projections.

 

[0Complications]

How do you change the Haworth projection or fisher projection in terms of hydroxyl groups being (up/down or left/right or alpha/beta) into the chair conformation?

I'm asking this because I understand haworth to fisher but I can't understand when to do the axial or equatorial positions in the chair positions for a glucose molecule given the haworth or fisher.

This question used to get me as well.

It does depend on the sugar you are referring to, but if you are referring to simply how to determine alpha vs beta at the anomeric carbon, then you do this by looking at the CH2OH group, if it's up (means the OH that did the attacking came from the right side of the fischer projection) and your OH group on the anomeric carbon is up, then it's beta, but if the CH2OH is down (OH doing the attacking comes from the left side of the fischer projection) and the OH group on the anomeric carbon is up then it's alpha and vice versa. I don't think you'll need to get that involved in it for the MCAT, so typically up is beta and down is alpha.


The other thing, is that during the ring closure the OH's on the right side typically end up on the botton of the ring when going from the Fischer to the Haworth projection.

As far as Axial vs Equitorial, it depends on A) the sugar you are referring to and B) what the question states/asks. If you want to know the most stable conformation it will be with the most substituents on the equitorial plane, however that works out for your molecule. Just count it out. If you want the least stable then you would go for the most substituents on the axial plane.

So, if you notice your typical glucose counting from carbon 1 would be (either Beta or alpha) then C2 down, C3 up, C4 down. This maintains equitorial substituents. It would be very much less stable if you were to invert say C3's OH and make it point down, because then it would become axial and more steric strain would be introduced to the molecule.

Hope that makes sense.