CHEM 1406 Concept Review: Carbohydrates

Back to Chemistry 1406 Concept Reviews

Carbohydrates:  A class of molecules composed of carbon, oxygen, and hydrogen that store chemical energy. 
(Simple sugars have the general formula $C_n\left(H_2O\right)_n$, hence the name carbo-hydrate.)

Monosaccharides:  a simple carbohydrate which cannot be hydrolyzed into smaller carbohydrates.  Monosaccharides can be classified as either ketoses or aldoses. 

Ketose:  A monosaccharide that contains a ketone group.

Aldose:  A monosaccharide that contains an aldehyde group.

 

 

Naming Monosaccharides

While monosaccharides have different common names with no standard pattern, there is a general systematic way of naming monosaccharides based on whether it is an aldose or a ketose and the number of carbons it has.

Prefix	Number of Carbons	Type of Monosaccharide
Tri-	3	triose
Tetr-	4	tetrose
Pent-	5	pentose
Hex-	6	hexose

The monosaccharide will get an aldo- or keto- prefix depending on whether the carbonyl (C=O) is in the middle of the chain (keto-) or at the end of the chain (aldo-).

 

 

D vs. L Monosaccharides

A monosaccharide is labeled “D” or “L” based on the position of the –OH group on the last chiral carbon furthest away from the carbonyl (C=O).  Fischer projections are very useful for determining this.  See below:

Carbon #5 is the last chiral carbon for glucose.  If the –OH group is on the right side, that’s a “D” sugar. 

If the –OH is on the left side, that’s a “L” sugar.

 

 

Disaccharides and Polysaccharides

Disaccharides:  a carbohydrate composed of two monosaccharides linked together by a glycosidic bond.

Glycosidic bond: an ether bond that connects two monosaccharides.

 

Common Disaccharides

Disaccharide formed from…                             Monosaccharide Components
Sucrose	glucose and fructose
Lactose	glucose and galactose
Maltose	glucose and glucose

 

Polysaccharides:  carbohydrates composed of many monosaccharides linked together in a long chain

 

 

 

Drawing Hayworth Projections

Most monosaccharides normally exist not as straight chains but rather as cyclic structures.  These structures can be represented by Hayworth projections.  An example of a conversion from a Fischer projection to a Hayworth projection can be seen below:

To complete a conversion like this, the following steps can be used:

 

Step 1:  Turn the open-chain condensed structural formula clockwise 90˚.

Example:  

Step 2:  Rotate the 3 groups on the last chiral carbon (Carbon #5 in the example) clockwise so that the –OH is in the horizontal position. (For an “L” monosaccharide, you would rotate the 3 groups counterclockwise.)

Example:  

Step 3:  Fold the carbon chain into a hexagon with the –OH on the last chiral carbon (Carbon #5 in the example) attaching to the carbonyl carbon (Carbon #1 in the example).  Five-membered rings are called furanoses.  Six-membered rings are called pyranoses.

Example:
	D-Glucose	α-D-glucopyranose               β-D-glucopyranose