Introductory Chemistry Fourth Edition Nivaldo J. Tro

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

2. Carbohydrates
• Learning objectives:
• Use chemical tests to distinguish between
mono-, di-, and polysaccharides

3. Review of carbohydrates:

4. Carbohydrates
• Carbon, hydrogen, and oxygen
• The basic formula -
• Example:
C3H6O3
I
(CH2O)n or H - C - OH
I

6. 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

8. 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

9. 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)

10. 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

11. Glucose can be found in a ring
structure or linear structure
• In Water
Haworth projection

12. 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)

13. 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
H
OH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose

14. Cyclization of glucose produces a new carbon where the
OH can be up or down. The 2 structures are called
anomers,  & .
 (OH below the ring)
  (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

15. Carbohydrate Tests

16. 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.

17. Benedict’s Test For Reducing
Sugars
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.

18. Benedict’s Test For Reducing
Sugars
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

19. Benedict’s Test For Reducing
Sugars
All mono- and most disaccharide's will reduce
CuSO4, producing a precipitate of copper (I)
oxide on heating, so they are called reducing
sugars.
 A precipitate indicates reducing sugars
 Original Pale Blue = no reducing sugar
 Brown/Red = reducing sugar

20. Benedict’s Test For Reducing
Sugars

21. • Ketoses can also be reducing sugars
because they can isomerise to aldoses:
•
Benedict’s Test For Reducing
Sugars

22. 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.

23. 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

24. Seliwanoff’s Test for Ketoses
Distinguishes between ketohexoses (ketone)
and aldohexoses (aldehyde) sugars.

25. Seliwanoff’s Test for Ketoses
Rapid appearance of a dark red solution indicates
the presence of a ketose
Aldohexoses react slower and the appearance of
the red solution takes longer

26. • Distinguishes polysaccharides and other
carbohydrates
Iodine Test for Polysaccharides

27. • 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

28. Some examples of polysaccharides…

29. 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

30. • 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
Negative
Benedict’s test
Positive
Benedict’s test

31. 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

32. 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.