Question

Is Fructose a Ketopentose?

Answer 1

Hint: First we know ketopentose is a pentose having a single ketone group at the 2-position. The smallest carbohydrates are called monosaccharides. As the name implies, these are single sugar molecules.
A five-carbon(pentose) monosaccharide which in its linear form contains either an aldehyde group at position $ 1 $ (aldopentose) or a ketone group at position $ 2 $ (ketopentose).

Complete answer:
An aldose is a monosaccharide that contains an aldehyde group ( $ - CHO $ ) at position $ 1 $ whereas a ketose is one that contains a ketone ( $ C = O $ ) at position $ 2 $ . Thus, an aldoheptose is a heptose with an aldehyde group. A ketoheptose is a heptose with a ketone functional group located in position $ 2 $ .
Since, the structure of fructose can be expressed as a six-carbon linear chain with hydroxyl and carbonyl groups. Its molecular formula is $ {C_6}{H_{12}}{O_6} $ . Hence fructose is classified as a monosaccharide, the most important ketose sugar, a hexose, and is a reducing sugar. Thus, fructose sugar is a ketohexose. It contains six carbon atoms and a keto group.

Additional Information:
Ribose is a single-ring pentose sugar. Fructose forms both pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and the furanose form predominates in many fructose derivatives.

Note:
Fructose is also called levulose. It is present as a free compound in ripe fruits, plant tissues, and in honey. When fructose is bound to glucose, it forms sucrose (sugar cane). Compounds with $ OH $ groups on the carbon next to a ketone carbonyl group are very readily oxidized, almost like aldehydes. Hence fructose is a reducing sugar.