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...