1. The Carbohydrates: Sugars, Starches, and Fibers
Photosynthesis
Carbohydrates
2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2
Carb-o-hydrates
= sugars
The Carbohydrates: Sugars, Starches, and Fibers
© 2008 Thomson - Wadsworth

2. Ribose (aldo-pentose)
Carbohydrates
Carbohydrates are made of carbon, hydrogen and oxygen atoms.
Polyhydroxy-aldehydes and polyhydroxy-ketones
Fischer Projection
Ribose (aldo-pentose)
Aldo = aldehyde
Pent = 5 carbons
ose = suffix for sugar
ketohexose
keto = ketone
Hex = 6 carbons
ose = suffix for sugar

3. Classification: Carbohydrates
More general classification scheme where sugars are broken into groups
Monosaccharides, Disaccharides and Polysaccharides
Monosaccharides are single sugars (most are hexoses).
Glucose serves as the essential energy source, and is commonly known as blood sugar or dextrose.
Fructose is the sweetest, occurs naturally in honey and fruits, and is added to many foods in the form of high-fructose corn syrup.
Galactose rarely occurs naturally as a single sugar.
Disaccharides are formed by a linkage between a glucose and another monosaccharide
Sucrose is a glucose and a fructose linked together
Maltose has 2 glucose linked together
Lactose is glucose and galactose
Polysaccharides are made by joining many monosaccharides together
Starch is a polysaccharide made from glucose
Cellulose is a polysaccharide made from glucose also

4. Open Chain and Ring Structures
Monosaccharides exist in rings more than the open chain form
Happens because the aldehyde can react with a neighboring hydroxy (OH) group
Glucose
Open Chain

5. © 2008 Thomson - Wadsworth

6. Physical Properties
Carbohydrates are poly alcohols – they form hydrogen bonds in water
Therefore all monosaccharides and disaccharides are soluble in water
Some polysaccharides are soluble in water and insoluble in organic solvents
Many carbohydrates are sweet to taste
Sweetness is a function of their structure
Saccharin (artificial sweetener)
sucrose

7. Reactivity: oxidation
Oxidation: The open-chain form of glucose (< 1%) is an aldehyde and can be oxidized
Carbohydrates that undergo oxidation are called reducing sugars
All monosaccharides are reducing sugars because they either have an aldehyde group or they have an alpha hydroxy ketone and can be converted to an aldehyde

8. Reactivity: oxidation
Fehling’s Reagent
Used to test for a reducing sugar
A red precipitate signals that you have a reducing sugar
Barfoed’s Reagent
Mild oxidizing agent that can distinguish between reducing monosaccharides and reducing disaccharides
Reducing disaccharides react slowly if at all
Reducing monosaccharides react quickly
A red precipitate signals that you have a reducing sugar
Cu2O(s)
Cu2+(aq)

9. Reactivity: Dehydration of Sugars
Carbohydrates can undergo dehydration (-H2O) reactions with non-oxidizing acids
Saliwanoff’s Reagent
Used to distinguish between aldoses and ketoses
Ketoses with resorcinol in 6M HCl goes cherry red
Aldoses react much more slowly

10. Reaction with Iodine: Starch Test
Starches (polysaccharides) react with iodine to make a blue/black solution
Only large starches give a positive test with I2 not say dextrins
Amylose Amlyopectin

11. Reactivity: Hydrolysis of Sugars
Polysaccharides and Disaccharides can be broken down with an acid
Disaccharides are broken down completely
Polysaccharides (like starch) are partially broken down to make dextrins
We will test the reducing properties of hydrolyzed sucrose and starch and the ability of the hydrolyzed starch to react with iodine

12. The Experiment Fehling’s test (oxidation)
Substance
Observations
Reaction?
Glucose
Fructose
Lactose
Sucrose
Starch
Unknown
Lactose
Fehling’s test (oxidation)
Barfoed’s test (mild oxidation)
Seliwanoff’s test (dehydration)
Iodine test (starch)
Hydrolysis of sucrose and starch
Starch