carbs powerpoint

Test for Carbohydrates: Mono-, Di-, and Polysaccharides

Carbohydrates

• Learning objectives:

• Use chemical tests to distinguish between mono-, di-, and polysaccharides

Review of carbohydrates:

Carbohydrates

• Carbon, hydrogen, and oxygen

• The basic formula -

• Example:

C3H6O3

I (CH2O)n or H - C - OH

I

Classification of carbohydrates

• Monosaccharides • Trioses, tetroses, pentoses, hexoses

• Oligosaccharides• Di, tri, tetra, penta, up to 9 or 10 • Most important are the disaccharides (sucrose)

• Polysaccharides or glycans

Monosaccharides

Aldoses (e.g., glucose) have an aldehyde group at one end.

Ketoses (e.g., fructose) have a keto group, usually at C2.

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

Most of the names of carbohydrates end in -ose

• Glucose-What plants make • Maltose- used in making beer

(disaccharide)• Fructose – found in fruit (monosaccharide)• Sucrose- Table sugar (disaccharide)• Lactose – In milk (disaccharide)

D vs L Designation

D & L designations are based on the configuration about the single C with four different groups around it in glyceraldehyde.

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

CHO

C

CH2OH

HO H

CHO

C

CH2OH

H OH

L-glyceraldehydeD-glyceraldehyde

L-glyceraldehydeD-glyceraldehyde

Glucose can be found in a ring structure or linear structure

• In Water

Haworth projection

Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.

Cyclic glucose forms as the C1 aldehyde & C5 OH react, to form a 6-member ring.

H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

23

4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose (linear form)

Fructose forms

a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.

CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

HOH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

Cyclization of glucose produces a new carbon where the OH can be up or down. The 2 structures are called anomers, a & b.

a (OH below the ring) b (OH above the ring).

H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

23

4

5

6

1 1

6

5

4

3 2

Carbohydrate TestsCarbohydrate Tests

Benedict’s Test For Reducing Sugars

Distinguish reducing from non-reducing sugars.

Reducing sugar …

Sugars that contain aldehyde groups that are oxidized to carboxylic acids

In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde.


Benedict’s reagent is a mild oxidant with CuSO4, Cu (II) sulfate, as one of the reagents.

Copper (II) sulphate, Sodium carbonate and Sodium citrate.


oxidized reduced

In the presence of a reducing sugar, the blue solution of Cu (II) or Cu+2, is changed to a brick red/brown precipitate of Copper (I) or Cu+1 oxide,Cu2O.

Formation of a brick red/brown precipitate is indicative of the presence of reducing sugars.

precipitate


All mono- and most disaccharide's will reduce CuSO4, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars.

Þ A precipitate indicates reducing sugars

Þ Original Pale Blue = no reducing sugar

Þ Brown/Red = reducing sugar

• Ketoses can also be reducing sugars because they can isomerise to aldoses:

•


Why sucrose is non-reducing sugar…sucrose has a anomeric carbon which is not free since the carbon links glucose and fructose and fructose does not have free -OH group to undergo reducing reaction and to open the ring so sucrose is non-reducing.

At the end of the lab…• You should…

• Distinguish between reducing and non-reducing sugar

• Understand the theory for Benedict’s test

• Know which of the tested carbohydrates are positive or negative for Benedict’s solution and why

Seliwanoff’s Test for Ketoses

Distinguishes between ketohexoses (ketone) and aldohexoses (aldehyde) sugars.

Seliwanoff’s Test for KetosesRapid appearance of a dark red solution indicates the presence of a ketose

Aldohexoses react slower and the appearance of the red solution takes longer

• Distinguishes polysaccharides and other carbohydrates

Iodine Test for Polysaccharides

• Polysaccharides will react with iodine to form a blue, red, violet, or purple product which is a positive indicator of the presence of a polysaccharide.

• Monosaccharides and disaccharides are too small to trap iodine molecules and do not form any dark color with iodine

Iodine Test for Polysaccharides

Some examples of polysaccharides…

Disaccharides and polysaccharides can be hydrolyzed in acidic solution into their component monosaccharides, and then submitted to chemical tests like Benedict's test.

Hydrolysis Test for Di- and Polysaccharides

• Starch and sucrose do not give positive test with Benedict’s. After hydrolysis, sucrose produces fructose and glucose that give a positive Benedict’s test.

• Sucrose + H2O -> Glucose + Fructose

Hydrolysis Test for Di- and Polysaccharides

NegativeBenedict’s test

PositiveBenedict’s test

This test distinguishes carbohydrates that undergo fermentation and those that do not.

Yeast is used as a source of an enzyme, zymase. The products are CO2 and ethanol.

C6H12O6 + yeast C2H5OH + CO2

Formation of bubbles of carbon dioxide is used to confirm the fermentation process

Fermentation Test

Benedict’s Test For Non-Reducing Sugars

Non-Reducing sugars do not reduce copper sulphate. However, if it is first hydrolysed to its constituent monosaccharides, it will then give a positive Benedict’s Test.

First test a sample for reducing sugars, to see if there are any present before hydrolysis.

Then using a separate sample,

Boil the test solution with dilute hydrochloric acid for a few minutes to hydrolyse the glycosidic bond.

Neutralise, by adding small amounts of solid sodium hydrogen carbonate until it stops fizzing.

Perform the Benedict’s test

A positive result indicates the presence of simple non-reducing sugar.

carbs powerpoint

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