Exp 5.2: Carbohydrates (Quali Tests for Polysaccharides)

Question 1

general tests for polysaccharides

Answer 1

molisch’s test
iodine test

Question 2

molisch’s test:
purpose

Answer 2

detect presence of carbohydrates

Question 3

molisch’s test:
reagents

Answer 3

H2SO4
EtOH (ethanol)
a-naphthol

Question 4

molisch’s test:
positive result

Answer 4

Purple ring at interphase

Question 5

molisch’s test:
principle

Answer 5

(1) Strong acid (H2SO4) to dehydrate monosaccharide to produce furfural or hydroxymethylfurfural
(2) Intermediate furfural derivatives will condense with a-naphthol

Question 6

iodine test:
purpose

Answer 6

detect presence of polysaccharides

Question 7

iodine test:
reagents

Answer 7

KI

Question 8

iodine test:
positive result

Answer 8

starch-iodo complex: blue-black solution
glycogen-iodo complex: red or pink solution

Question 9

iodine test:
principle

Answer 9

iodine slides into starch coil to give blue-black color

Question 10

qualitative tests for polysaccharides

Answer 10

benedict’s test
barfoed’s test
seliwanoff’s test
bial’s orcinol test
mucic acid test
phenylhydrazone/ osazone test

Question 11

benedict’s test:
purpose

Answer 11

Detect reducing sugars

Question 12

benedict’s test:
reagents

Answer 12

CuSO4
Na2CO3
Na3C6H5O7

Question 13

benedict’s test:
positive result

Answer 13

Brick-red ppt for all reducing saccharides (all monosaccharides)

Question 14

benedict’s test:
principle

Answer 14

(1) Cu2+ oxidizing agent (oxidation)
(2) Cupric ions reduced to cuprous to form Cu2O precipitate
(3) Sodium citrate (basic) used to keep Cu ions in solution

Question 15

barfoed’s test:
purpose

Answer 15

Distinguishes reducing monosaccharides and disaccharides

Question 16

barfoed’s test:
reagents

Answer 16

Cu(CH3COO)2
acetic acid

Question 17

barfoed’s test:
positive result

Answer 17

Brick-red ppt for all reducing monosaccharides

Question 18

barfoed’s test:
principle

Answer 18

(1) Cu2+ oxidizing agent (oxidation)
(2) Cupric ions reduced to cuprous to form Cu2O precipitate

Question 19

seliwanoff’s test:
purpose

Answer 19

Distinguish aldoses from ketoses (ketohexoses)

Question 20

seliwanoff’s test:
reagents

Answer 20

HCl
Resorcinol

Question 21

seliwanoff’s test:
positive result

Answer 21

Cherry-red solution for ketohexoses

Question 22

seliwanoff’s test:
principle

Answer 22

(1) Strong acid to dehydrate monosaccharide to produce furfural/hydromethylfurfural
(2) Will then condense with resorcinol

Question 23

bial’s orcinol test:
purpose

Answer 23

Specific for pentoses

Question 24

bial’s orcinol test:
reagents

Answer 24

HCl
FeCl3
Orcinol

Question 25

bial’s orcinol test:
positive result

Answer 25

Blue-green solution for aldopentoses

Question 26

bial’s orcinol test:
principle

Answer 26

(1) Strong acid to dehydrate monosaccharide to produce furfural/hydromethylfurfural
(2) Will then condense with orcinol

Question 27

mucic acid test:
purpose

Answer 27

Specific for galactose and lactose

Question 28

mucic acid test:
reagents

Answer 28

Concentrated HNO3

Question 29

mucic acid test:
positive result

Answer 29

Broken glass-like or Rhombic crystals

Question 30

mucic acid test:
principle

Answer 30

HNO3 – strong oxidizing agent
(1) Formyl functional group will get oxidized to carboxylic acid
(2) Primary alcohol group will get oxidized to carboxylic acid

Question 31

phenylhydrazone/ osazone test:
purpose

Answer 31

For all sugars with free carbonyl group at C1/C2 (all aldoses and ketoses)

Question 32

phenylhydrazone/ osazone test:
reagents

Answer 32

Phenylhydrazine

Question 33

phenylhydrazone/ osazone test:
positive result

Answer 33

Yellow-orange crystals (osazones)
D-glucosazone (for glucose)
No “sucrosazone”

Question 34

phenylhydrazone/ osazone test:
principle

Answer 34

Phenylhydrazine – strong oxidizing agent
(1) Formyl functional group will get oxidized to carboxylic acid
(2) Secondary alcohol will get oxidized to ketone
(3) Product : phenylhydrazone
(4) Phenylhydrazone + phenylhydrazine = osazone