Answer: position of the carbonyl carbon
Explanation:
In the linear structure of sugars having an aldehyde group, are called aldoses, and sugars having a keto group are called ketoses.
Like adehydes H-C=O the carbonyl carbon is the first/last in the chain it is bounded to Hydrogen, double bounded to oxygen and a single bond to the chain. An aldose linear sugar like D-glucose have the carbonyl carbon as it first carbon.
But ketones R-COR the carbonyl carbon is in between the chain, it is bounded to two carbons on both sides. Like D-fructose the keto carbon is the second carbon.
In a open ring structure the 1st carbon, adehyde group is condensed with the hydroxy group (OH) of the 5th carbon forming a ring. The first carbon is now bound to the chain, Hydrogen, OH and oxygen which forms the ring.
In the keto open ring structure of D-fructose, the second carbon(keto group) is condensed with the OH of the 5th carbon
