[a-fortiori]

"The numbered carbons in the open-chain forms correspond to the same numbered carbons in the hemiacetal forms"

Why is this statement considered correct? I am struggling to find an explanation.

I totally understand that the linear Glucose contains an aldehyde group at one of it its terminal carbons, so numbering starts from there:



The moment linear Glucose becomes a hemiacetal however, it may be a derivative of an aldehyde, but it does not contain one. It is, as far as I can see, a different compound (due to differing bonds, functional groups) and should be numbered according to the rules. Aldehyde > Alcohol > Ether (in priority)



Is it because carbon 1 on the hemiacetal glucose  is bonded to both a hydroxyl and an ether functional group? As opposed to the other terminal carbon (6) which is only bonded to a single hydroxyl group. If that is the case I was unaware nomenclature was like a game of p oker i.e My hand of an ether and hydroxyl beats your hydroxyl.

The only other alternative explanation is the hemiacetal carbon numbers correspond to the linear numbers for the sake of simplicity, which doesn't really sit right with me...

Are either of my reasons correct?

Edit- For some reason I could not type p-o-k-e-r because it would turn into ignore me I am dishonest.... Strange.

[discodermolide]

As far as I remember the open chain forms and the ring are in an equilibrium with each other, the ring being favoured, Dan will correct me if I am wrong.
In the pyranose form the carbon you labelled 1 is at the aldehyde oxidation level, it is an acetal, and actually not different from the linear compound at least it can behave as an aldehyde.

[a-fortiori]

It seems as if I am always forgetting something important with these organic compounds. Yes, you are indeed right, it is an acetal group. I am currently looking at the priority list for functional groups; I take it acetal, as a derivative of an aldehyde, is on the same rung of priority? Or possibly just below it? I take it as read then that an acetal group has a higher priority than an alcohol group

[discodermolide]

It seems as if I am always forgetting something important with these organic compounds. Yes, you are indeed right, it is an acetal group. I am currently looking at the priority list for functional groups; I take it acetal, as a derivative of an aldehyde, is on the same rung of priority? Or possibly just below it? I take it as read then that an acetal group has a higher priority than an alcohol group

This sort of acetal anyway.

[a-fortiori]

Thanks again for all your help. It is nice to know how I am wrong; these couple of issues that you helped me with have been bugging the last few days. I know now one should not make assumptions based on one owns lack of knowledge on the subject matter.

[discodermolide]

Thanks again for all your help. It is nice to know how I am wrong; these couple of issues that you helped me with have been bugging the last few days. I know now one should not make assumptions based on one owns lack of knowledge on the subject matter.

Glad to be of assistance.

[Borek]

For some reason I could not type p-o-k-e-r because it would turn into ignore me I am dishonest.... Strange.

Because of the spam pоker is filtered out by the censoring engine.

[Dan]

I arrived late and have nothing to add here with respect to aldoses.

I will point out though, monosaccharide numbering is not necessarily decided on the basis of priority of groups. The IUPAC conventions for carbohydrates can be quite subtle.

For example, polyols, in which there is no distinguishing higher oxidation level at any carbon still has a well defined numbering convention based on the assigned parent name (gluco, manno etc.). An example would be, say, D-glucitol, in which C1 is the C that corresponds to C1 in D-glucose. This sort of thing also applies to dialdoses and aldaric acids.

Uronic acids, which have an aldehyde at one terminus and an acid at the other, offer another numbering point of interest in that C1 is the aldehyde C, not the more highly oxidised acid C; so do be aware that:

I am currently looking at the priority list for functional groups; I take it acetal, as a derivative of an aldehyde, is on the same rung of priority?

The standard priority list for functional groups does not necessarily apply for sugars, they have their own nomenclature.