How do you number carbons in ketoses?
In aldoses, the carbon in the aldehyde group is the first carbon and the rest are numbered accordingly by going clockwise from there. But how do you number the carbons in ketoses?
In aldoses, the carbon in the aldehyde group is the first carbon and the rest are numbered accordingly by going clockwise from there. But how do you number the carbons in ketoses?
1 Answer
More or less the same as Aldoses..
Explanation:
The biochemical class of compounds known as Sugars can roughly be divided in two categories: Aldoses and Ketoses.
They typically contain between 3 and 7 Carbon atoms:
- 3:
#rarr# Trioses - 4:
#rarr# Tetroses - 5:
#rarr# Pentoses - 6:
#rarr# Hexoses - 7:
#rarr# Heptoses
7 seems to be the maximum: there are no (known) sugars containing 8 or more Carbon atoms, as they are too unstable.
If we start with the simplest ones we find there are 3 Trioses:
- Dihydroxyacetone;
- D-Glyceraldehyde;
- L-Glyceraldehyde.
Have a look at the glyceraldehydes :
D-Glyceraldehyde and L-Glyceraldehyde are identical, except that they are mirror-images of each other, a bit like a pair of gloves: one left-handed, the other right-handed. The glyceraldehydes are the starting point for all Aldoses, so we'll forget about them for the rest.
Next, the difference between D-Glyceraldehyde and Dihydroxyacetone :
Next, we number the Carbon atoms from top to bottom:
If we go from Triose to Hexose, we find that at each step an extra H-C-OH-group has crept in:
Observe that in each of the ketoses shown, the one-but-last H-C-OH - group "points" in the same direction: This is the group that determines D- or L-configuration. All the ketoses shown are D-configs. Starting with
NOTE: The last but one C-centre doesn't change this identity, only the D/L-configuration!!!!!!!!!!!!!!!
Footnote : I didn't include the Heptoses: theoretically there could be 8 of them, but there aren't that many around, and they don't seem to be very important in biochemical respect anyway.....
