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Carbohydrates

Polyhydroxy aldehydes and ketones or substances that hydrolyse to yield polyhydroxy aldehydes and ketones.

Carbohydrates can be named by IUPAC rules, but generally common names are used because they are simpler and are universally understood.

Sugar combination of Sanskrit words: su (sweet) + gar (sand) “sweet sand”The Latin word for sugar is saccharum, and the derived name saccharide is the basis of a system of carbohydrate classification.

The word carbohydrate derives historically from the fact that glucose, has the molecular formula C6H12O6 and was originally thought to be a “hydrate of carbon, C6(H2O)6.”This view was soon abandoned, but the name survived.

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Types of carbohydrates

Simple carbohydrate = simple sugars = are monosaccharides. They are all sweetComplex carbohydrates = complex sugars = are made of two or more simple sugars.

Oligosaccharides oligo = few “dextrins”

Hydrolysis = breakdown with water

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Fischer Projections of Monosaccharides

Places the most oxidized group at the top.Shows chiral carbons as the intersection of vertical and horizontal lines.

The −OH group on the chiral carbon farthest from the carbonyl group determines an L or D isomer.

• Left is assigned the letter L for the L-isomer.• Right is assigned the letter D for the D-isomer.

LatinLevo = leftDextro = right

Emil Fischer determined the structure of glucose in 1900 and won the Noble Prize in Chemistry in 1902

A chiral carbon is bonded to 4 distinct groups and posses a special property, they are not superimposable with their mirror image.

EnantiomersStereoisomers which arenon-superimposable with their mirror image.

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D-fructoseIs a ketohexose C6H12O6

Is the sweetest carbohydrate.Is found in fruit juices and honey.Converts to glucose in the body.

D-galactose Is an aldohexose C6H12O6

Not found free in nature.Obtained from lactose, a disaccharide.A similar structure to glucose except for the –OH on C4.

Isomers

D-glucose is an aldohexose

C6H12O6

Found in fruits, corn syrup, and honey. Known as dextrose, grape sugar, and blood sugar.The monosaccharide in polymers of starch, cellulose, and glycogen.

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Aldoses

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• A ketose has one fewer asymmetric center than an aldose.

• Therefore, ketoses have fewer stereoisomers than aldoses.

• Naturally occurring ketoses have the ketone group in the 2-position.

Ketoses

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1. What is the classification of the following sugar?

a. aldotriose b. aldopentose c. ketotetrose d. ketohexose

2. What are the functional groups and the number of -OH groups in an aldohexose? a. aldehyde and four -OH groups b. aldehyde and five -OH groups c. ketone and five -OH groups d. ketone and four -OH groups

Practice

3. Structure A and B are shown below. Determine whether each structure is the D or L isomer.

a. Structure A is an L-isomer and Structure B is an L-isomer. b. Structure A is a D-isomer and Structure B is a D-isomer. c. Structure A is a D-isomer and Structure B is an L-isomer. d. Structure A is an L-isomer and Structure B is a D-isomer.

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4. Which carbohydrate is this?a. D-glucose b. L-glucose

c. D-ribose d. L-ribose

5 . This carbohydrate has how many chiral carbons and has which D/L orientation?

a. Three chiral carbons and an L-orientation b. five chiral carbons and an L-orientation c. five chiral carbons and a D-orientation d. Three chiral carbons and a D-orientation

7. Which monosaccharide is also known as dextrose or blood sugar?

a. Glucose b. ribose c. galactose d. fructose

6. Which monosaccharide is also known as fruit sugar? a. ribose b. galactose c. glucose d. fructose

Practice

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Aldehydes and ketones can react with an alcohol to form a hemiacetal.

Cyclic Structures of Monosaccharides (Haworth Structures)

Cyclic hemiacetals form when the C=O group and the —OH are part of the same molecule.• For hexoses, the hydroxyl group on C-5 reacts with the aldehyde group or ketone group.• Are the prevalent form of monosaccharides with 5 or 6 carbon atoms.

English chemist Sir Norman Haworth

Leads to an equatorial-equatorialconfiguration referred to as the beta (β) isomer.A special type of stereoisomer called an anomer.

Leads to an equatorial-axial

configuration referred to as the alpha (α) isomer.

An anomer is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal carbon, also called the anomeric carbon. The anomeric carbon is the carbonyl carbon, C-1.

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D-glucose

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D-Glucose exists in three different forms:

anomer

anomer

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8. Which of the following is the -anomer ?

A B

9. Which of the following is the correct Haworth structure for D-mannose?

A B C D E

Practice

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Chemical Properties of Monosaccharides

Oxidation

[O]Aldehydes carboxylic acid

Aldoses

have an aldehyde group

:. Reducing sugars

In aq. solution ketoses slowly rearrange to aldoses :. react

Ketoses

rearrange to an aldehyde group

:. Reducing sugars

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The oxidizing agent is Cu2+

The oxidizing agent, itself is reduced: Cu2+ + e- Cu+

Oxidation by Benedict’s reagent

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• Tollens reagent reacts with aldehyde, but the base present in the solution, promotes rearrangements, so ketoses react too.

• Sugars that give a silver mirror with Tollens are called reducing sugars.

The oxidizing agent is Ag+The oxidizing agent, itself is reduced: Ag+ + e- Ag

Oxidation by Tollens Reagent

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Reduction

[H]Aldehydes 1˚ alcohols

• Produces sugar alcohols called alditols.• Such as D-glucose gives D- glucitol also called sorbitol.• Sorbitol is about 60 percent as sweet as sucrose with one-third fewer calories

and may be useful to people with diabetes. • Sorbitol has been safely used in processed foods for almost half a century. • Since it's very slow to be metabolized by the body, sorbitol does not cause

insulin levels to increase as much as sugar. It also doesn't lead to tooth decay and is used in many sugar-free cough syrups. It is a popular addition to gel toothpastes as it helps add transparency.

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10. When glucose is treated with Benedict's solution, blue Cu2+ ions are reduced and a red precipitate forms. What changes occur to the glucose molecule? The terminal alcohol group is oxidized to a carboxylic acid. The last chiral alcohol group is oxidized to a ketone. The ketone group is reduced to an alcohol. The aldehyde group is oxidized to a carboxylic acid.

11. What structural change occurs when glucose is treated with H2? The ketone group is reduced to an alcohol. The aldehyde group is reduced to an alcohol. The terminal alcohol group is oxidized to a carboxylic acid. The last chiral alcohol group is reduced to a alkane.

12. What transformation is happening in this reaction?

the reduction of D-galactose the oxidation of L-galactose the oxidation of D-galactose the reduction of L-galactose

Practice

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Disaccharides

Because cyclic sugars are hemiacetals, they can react with an alcohol to form the carbohydrate equivalent of an acetal, which is called a glycoside.

Therefore, 2 monosaccharides can react, one as a hemiacetal and the other as an alcohol to give the equivalent glycoside.

Aldehyde + alcohol hemiacetal (e, a)

Hemiacetal + alcohol acetal (e, e)

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In naming glycosidic bonds, it is necessary to name the configuration as well as the carbons involved in the bond formation.

In the case of maltose, the glycosidic bond is specified as(1→4) and is simply stated as alpha-one-four.

If the –OH group had been in the beta configuration when the glycosidic bond was formed, the bond would be in the β(1→4) configuration. The molecule formed would be named cellobiose and would have a different two-dimensional and three-dimensional shape than maltose.

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The three forms of maltose present in aqueous solution.

α β

Maltose is always an (14) glycoside, but can have and anomers

Lactose, milk sugar, is composed of galactose & glucose, with b(14) linkage from the anomeric OH of galactose. Its full name is b-D-galactopyranosyl-(1 4)-a-D-glucopyranose

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• Sucrose is known as table sugar.• It is the most abundant disaccharide found in nature.• Sucrose is found in sugar cane and sugar beets.• Both anomeric carbons of the monosaccharides in sucrose are bonded, therefore, sucrose is not a

reducing sugar. It will not react with Benedict’s reagent.

Glycosidic bond between C-1 of glucose (in the -position)And C-2 of fructose (in the β-position)

The designation here is different since two anomeric Cs are involved in the glycosidic linkage.

Sucrose

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What you need to know

Monosaccharides DisaccharideGlucose + glucose maltose + H2O (14)

Glucose + galactose lactose + H2O β(14)

Glucose + fructose sucrose + H2O ,β(12)

Aldohexoses Glucose 3Galactose 3, 4

Ketohexoses fructose 3

Aldopentose Ribosedeoxyribose

Ribose deoxyribose

fructose

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13. Which compound is a disaccharide that is not a reducing sugar? lactose maltose sucrose glucose

14. Which disaccharide is composed of two D-glucose molecules connected through an -1,4 glycosidic linkage? sucrose lactose maltose fructose

15. What kind of glycosidic linkages are in lactose? -1,4-glycosidic bond between a D-glucose molecule and a D-galactose molecule β-1,4-glycosidic bond between two D-glucose molecules -1,4-glycosidic bond between two D-glucose molecules β-1,4-glycosidic bond between a D-glucose molecule and a D-galactose molecule

Practice

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16. Melibiose is a disaccharide that is 30 times sweeter than sucrose.

a. What are the monosaccharide units in melibiose?

b. What type of glycosidic bond links the monosaccharides?

c. Identify the structure as α- or β-melibiose.

Practice

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PolysaccharidesCellulose and starch are the two most widely occurring polysaccharides.

Cellulose consists of several 1000s D-glucose units linked by (14) glycosidic linkages.

Different cellulose molecules interact to form a large aggregate structure held together by hydrogen bonds.

Nature uses cellulose as a structural material to impart strength and rigidity to plants. Leaves, grasses, and cotton are primarily cellulose.

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The linear nature of the molecules allows them to act like stiff rods, a feature that enables then to align side by side into well organized water insoluble fibers. There are not enough water molecules on the surface of the fiber to pull individual molecules apart, extensive H-bonds.

Cellulose

Cellulose• Cellulose is an insoluble fiber in our

diet because we lack the enzyme cellulase to hydrolyze the (1→4) glycosidic bond.

• Whole grains are a good source of cellulose.

• Cellulose is important in our diet because it assists with digestive movement in the small and large intestine.

• Some animals and insects can digest cellulose because they contain bacteria that produce cellulase.

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Amylose: a linear (unbranched) continuous chain. Linked by -1,4 glycosidic bonds.insoluble in cold waterseveral 100s of glucose molecules

Amylopectin: a branched-chain polysaccharide can contain up to 106 units.soluble in cold waterHas -1,4-glycosidic bonds between the glucose units and -1,6 bonds to branches.

Starch can be separated into 2 fractions:

Potatoes, corn, and cereal grains contain large amounts of starch.

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Storage polysaccharides

Starch (in plants)

Glycogen(in animals)

Amylose ~ 20% by weight of starchlinear glucose polymer several 100’s (up to ~4,000), α(14)

more branched and compact than amylopectin.α(14)glucose polymerwith α(16) linked branches

Amylopectin ~ 80% by weight of starch branched glucose polymer several 1000’s (up to ~10,000), α(14) and α(16)

Total amount in human body ~ 350g divided ~equally in liver and muscle

The small, dense particles (blue) within this electron micrograph of a liver cell are glycogen granules.

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Cellulose: glucose polymer made up of 1,4’--glycoside linkages

O

HO

HO

HO

O

HO

HO

HO

OO

HO

HO

HO

OO

OHO

HO

HO

OO

HO

HO

HO

OO

HO

HO

HO

OO

H H H H H H

O

OHO

HO

HO

O

OHO

HO

HO

O

OHO

HO

HO

O

OHO HO

O

OHO

HO

HO

O

OHO

HO

HO O

HOH H H H H H

Amylose: glucose polymer made up of 1,4’--glycoside linkages

Difference

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Different glycosidic linkages in starch and cellulose different physical properties.

Linkages in starch

amylose has an helix that promotes H-bonding to water starch is soluble in water

β Linkages in cellulose promotes extensive intramolecular H-bonding forming

linear arrays Insoluble in water

Amylose nor amylopectin is truly soluble in water, too big to form a true solution, they form dispersions.

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Use of iodine to test for starch.

Blue liq. = I2 in presence of starch

Clear liq.= solution of I2

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17. Animal starch that is stored in the liver and muscles is which polysaccharide? cellulose sucrose glycogen amylose

18. What is the monosaccharide used to build polysaccharides of amylopectin? glucose galactose Lactose amylose

19. Which disaccharide occurs as a breakdown product of amylose? Sucrose Maltose Glycogen amylopectin

Practice