Since carbohydrates contain both alcohol and aldehyde or ketone
functional groups, the straight-chain form is easily converted
into the chair form - hemiacetal ring structure. Due to the tetrahedral
geometry of carbons that ultimately make a 6 membered stable
ring , the -OH on carbon #5 is converted into the ether linkage
to close the ring with carbon #1. This makes a 6 member ring
- five carbons and one oxygen.
Steps in the ring closure (hemiacetal synthesis):
1. The electrons on the alcohol oxygen are used to bond the carbon
#1 to make an ether (red oxygen atom).
2. The hydrogen (green) is transferred to the carbonyl oxygen
(green) to make a new alcohol group (green).
The chair structures are always written with the orientation
depicted on the left to avoid confusion.
Hemiacetal Functional Group:
Carbon # 1 is now called the anomeric carbon and is
the center of a hemiacetal functional group. A carbon that has
both an ether oxygen and an alcohol group is a hemiacetal.
graphic of hemiacetal in a new window