Hexose

D-Glucose.

D-Fructose.

Two important hexoses, in the Fischer projection.

In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms.^[1][2] The chemical formula for all hexoses is C₆H₁₂O₆, and their molecular weight is 180.156 g/mol.^[3]

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.^[4] The open-chain form of a hexose, which usually is favored in solutions, has the general structure H–(CHOH)_n−1–C(=O)–(CHOH)_6−n–H, where n is 1, 2, 3, 4, 5. Namely, five of the carbons have one hydroxyl functional group (–OH) each, connected by a single bond, and one has an oxo group (=O), forming a carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by seven hydrogen atoms. The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl.

Hexoses are extremely important in biochemistry, both as isolated molecules (such as glucose and fructose) and as building blocks of other compounds such as starch, cellulose, and glycosides. Hexoses can form dihexose (like sucrose) by a condensation reaction that makes 1,6-glycosidic bond.

When the carbonyl is in position 1, forming an formyl group (–CH=O), the sugar is called an aldohexose, a special case of aldose. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a ketone, and is called a ketohexose, a special case of ketose; specifically, an n-ketohexose.^[1][2] However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize;^[5] so the term "ketohexose" usually means 2-ketohexose.

In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, stereoisomers that differ in the spatial position of the hydroxyl groups. These species occur in pairs of optical isomers. Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "D-" or "L-", depending on whether the hydroxyl in position 5, in the Fischer projection of the molecule, is to the right or to the left of the axis, respectively. These labels are independent of the optical activity of the isomers. In general, only one of the two enantiomers occurs naturally (for example, D-glucose) and can be metabolized by animals or fermented by yeasts.

The term "hexose" sometimes is assumed to include deoxyhexoses, such as fucose and rhamnose: compounds with general formula C
₆H
₁₂O
_6-y that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.