MCAT Organic Chemistry > Carbohydrates
Carbohydrates are molecules that contain carbon, hydrogen, and oxygen in the form of polyhydroxylated aldehydes or ketones.
Carbohydrates have the general formula Cn(H2O)n.
Monosaccharide – a single carbohydrate unit, commonly known as simple sugars.
Disaccharide – molecule made of two carbohydrate units
Oligosaccharides – short carbohydrate chains
Polysaccharides – long carbohydrate chains
To name monosaccharides, give the first give a numerical name relating to the number of carbons in the carbohydrate then followed by the suffix -ose. For example, triose contains three carbons, tri-, and –ose is added to the end denoting a carbohydrate.
There are two major types of carbohydrates, aldoses and ketoses which are basically modified forms of aldehydes and ketones.
- Aldose – polyhydroxylated aldehyde, also known as glyceraldehyde.
- Ketose – the simplest ketone sugar, dihydroxyacetone.
Every ketose that will be encountered on the MCAT will have the ketone group on carbon 2 of the carbohydrate.
Every carbon on a monosaccharide other than the functional group will have a hydroxyl group attached the each carbon.
Carbohydrates have their own unique distinction for figuring out the absolute configuration. Like many other molecules in organic chemistry, carbohydrates have unique stereochemistry that thus allows them to have unique physical properties in the body.
D and L designations represent the enantiomeric configurations of a sugar.
D sugars will have the hydroxide of their highest numbered chiral carbon on the right.
L sugars will have the hydroxide of their highest numbered chiral carbons on the left.
There are three types of stereoisomers for carbohydrates that you must be familiar with.
- Sugars that are the same but are in different optical families are enantiomers.
- All non-identical, meaning sugars whose structures are not mirror images of each other, sugars within the same family, with the rule that both sugars must be ketoses/aldoses, and have the same number of carbons, are diastereomers.
- Epimers are diastereomers that differ at only one chiral center.
All monosaccharides that are not glyceraldehydes are designated D or L configurations based on their relationship to glyceraldehyde.
When carbon 5 of a fructose molecule attacks the carbonyl carbon on the chain, furanose is formed. These are formed from 5-carbon aldoses or 6-carbon ketoses.
When carbon 5 of glucose attacks the carbonyl carbon on the chain, pyranose is formed. These are formed from 6-carbon aldoses or 7-carbon ketoses.
These conformations of cyclohexose are rated from most stable to least stable. Hexose rings will always try to rotate as to achieve the most stable conformation possible.
- Chair - every group is staggered
- Twist Boat - not completely eclipsed
- Boat - everything is eclipised
Chair > Twist Boat > Boat
Torsional strain occurs when there is strain due to the presence of eclipsing groups on a single bond. This causes instability in the molecule.
Axial refers to when groups are positioned on the same axis as each other and this type of stereochemistry is less stable.
Equatorial refers to when groups are positioned on opposite axis and this is most stable.
Conversion of Fischer projection to Haworth projection
A Haworth projection is a flat depiction of cyclic molecules. For sugars, the oxygen atom is always at the back, right corner of the projection and the hemiacetal carbon is at the far right.
A Fischer projection is the depiction of a 3-D chiral structure using a 2-D drawing. The vertical lines depict bonds tghat project into the plane of the paper, and horizontal ines depict bonds that project out of the plane of the paper.
In order to convert a Fischer projection into a Haworth projection, any substituent on the right of the Fischer projection will point down on the Haworth projection, and any substituent on the left of the Fischer Projection will point up on the Haworth projection.
Epimers and Anomers
Epimers are stereoisomers of sugars that differ only in configuration at the second carbon of the chain or ring. Anomers are diastereomers that differ in the configuration about the anomeric carbon, typically carbon-1 in hexoses for example.
Anomers can be either alpha (α) or beta (β), depending on the orientation of the hydroxyl or alkoxy substituent on carbon-1.