The most common sugar of all, glucose, is a hexose with n = 6,
and with the molecular formula C6H12O6.
Glucose also equilibrates between a closed-ring form and an open
chain in aqueous solution. The ring form is shown on the facing
page. Glucose has the chair structure of the cyclohexane ring seen
in Chapter 19, with all of its -OH groups extending out from the
perimeter of the ring (the "equatorial" positions) where
they are farthest from one another, and hence most stable.
Each of two other common hexoses, mannose and galactose, has a
single -OH group moved from the equatorial position to an axial
position, extending up or down from the average plane of the puckered
six membered ring.
The same atoms that are present in these hexoses also can combine
into a five-membered ring, still with the molecular formula C6H12O6.
This is fructose (fruit sugar), which has a ring structure similar
to ribose (right) although it is a hexose and not a pentose.