[a-fortiori]

I am looking at the full structure of sucrose right now and I don't see any functional groups apart from hydroxyls. Every atom is saturated with bonds i.e there are no double bonds present, so that rules out any aldehydes, ketones or esters. Based on that fact, the numbering of carbons for both hexose subunits (previously cyclical glucose and fructose) will start with the terminal carbon in closest proximity to a functional group. In the case of the glucose-unit, through hydrolysis it has lost one of its terminal hydroxyl groups, so numbering should begin at the other terminus. As for the fructose-unit, both terminal ends have a hydroxyl group; simply for the sake of naming the glycosidic bond that joins both hexose subunits the lowest number should be used (in this case both ends are equal i.e one hydroxyl group does not overtake the other in importance).

Here is a picture that I have made to explain what is quite difficult for me to put into words:



This leads me to believe that the true name of the glycosidic bond is 6-2, but then again I have not ever seen it written this way in textbooks, websites etc. I have seen 1-2, occasionally 1-5, but never 6-2. One of my lecturers is under the impression that it is a 1-4 glycosidic bond, which I believe is incorrect.

If 6-2 is incorrect, could somebody explain why?

Just to put this in to some context, I am a Biology student and have recently done an exam where the question was "What is the name of the bond that joins glucose and fructose?"

I put down 1-5 glycosidic. Would that be acceptable enough for biologists? (I usually see the numbers of the carbons in the reverse of what I have in the picture above)

P.S I currently see 4 possible answers, in descending order of correctness as I see it:
6-2
1-2
6-5
1-5
But certainly not 1-4...

[discodermolide]

The bond connecting the two rings is an ether, -C-O-C- linkage. In this case it is also a gylcosidic bond, 1-2.
Have a look here:
http://en.wikipedia.org/wiki/Sucrose

[a-fortiori]

Ahhh, now I see it, sorry for being so block-headed. The ether overtakes the hydroxyl group in importance. Which also explains why with maltose it is a 1-4 rather than a 1-3. I take it also that, in sucrose, the reason it is a 1-2 instead of a 1-5 is simply because the 2 is a lower number?

Biologists are a little fast and loose with such things, so given this new information:

Absolutely correct in the eyes of chemistry = 1-2 glycosidic
Iffy, but possibly passable in the field of biology (certainly not in chemistry!) = 1-5
More iffy, less likely to be passable (again, not in chemistry) = 6-2, 6-5
Completely wrong = 1-4

Would anyone agree?

Edit: typing error, I meant ether...

[discodermolide]

Ahhh, now I see it, sorry for being so block-headed. The ester overtakes the hydroxyl group in importance. Which also explains why with maltose it is a 1-4 rather than a 1-3. I take it also that, in sucrose, the reason it is a 1-2 instead of a 1-5 is simply because the 2 is a lower number?

Biologists are a little fast and loose with such things, so given this new information:

Absolutely correct in the eyes of chemistry = 1-2 glycosidic
Iffy, but possibly passable in the field of biology (certainly not in chemistry!) = 1-5
More iffy, less likely to be passable (again, not in chemistry) = 6-2, 6-5
Completely wrong = 1-4

Would anyone agree?

Well as the only correct answer is 1-2 for biology and chemistry all the others are incorrect.

[a-fortiori]

OK, but then again, it could be argued that my interpretation of 1-5 was closer than my own lecturer's opinion that it was 1-4. This is the picture they sent me when I questioned them about it:



Again, this is my Molecular and Cellular Biology Lecturer sending me this, just before an exam that featured this question....

[discodermolide]

OK, but then again, it could be argued that my interpretation of 1-5 was closer than my own lecturer's opinion that it was 1-4. This is the picture they sent me when I questioned them about it:



Again, this is my Molecular and Cellular Biology Lecturer sending me this, just before an exam that featured this question....

Point them in the direction of the Wiki page or Chemspider:
http://www.chemspider.com/Chemical-Structure.5768.html?rid=f1a99535-2a0b-4aeb-84ec-129b29824850
But here you need to look for the relevant item we are discussing.

[a-fortiori]

I am certainly now not under the false illusion that my answer was in any way correct, thanks to your reply. I was just making the comment that this incorrect view of chemical nomenclature was taught to 130 students, and if I hadn't questioned it, would probably be taught to next years students as well. I don't think it is too much to ask that a Lecturer teaching said material actually has an understanding of it themselves...

[discodermolide]

I am certainly now not under the false illusion that my answer was in any way correct, thanks to your reply. I was just making the comment that this incorrect view of chemical nomenclature was taught to 130 students, and if I hadn't questioned it, would probably be taught to next years students as well. I don't think it is too much to ask that a Lecturer teaching said material actually has an understanding of it themselves...

This is certainly correct, at least the Uni lectures should get it right.

[Dan]

It is a 1,2-glycosidic linkage. Carbon 1 of glucose is linked to carbon 2 of fructose.

For correct numbering of the monosaccharides (including glucose and fructose), see IUPAC carbohydrate nomenclature.