Carbohydrates

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Carbohydrates
Definition:
 Organic compounds composed of C, H and O with H
and O present in the same ratio as in water.
 E.g. Glucose C6H12O6.
 Exceptions:
 Deoxy sugars such as Rhamnose C6H12O5, digitoxose
C6H12O4
 Some non carbohydrates follow the definition:
 Acetic acid C2H4O2
 Formaldehyde HCHO
 Lactic acid C3H6O3
 New definition:
Optically active Polyhydroxy aldehydes or ketones,
or substances that hydrolyze to yield polyhydroxy
aldehydes or ketones.
Carbohydrates
Non-Sugars (Polysaccharides)
Sugars (Saccharides)
More than 10 sugar unites
Monosaccharides
Oligosaccharides
(3- 10 unites)
Disaccharides
Trioses e.g. Glyceraldehyde
Trisaccharides
e.g. Raffinose
Tetroses e.g. Erythose
Pentoses e.g. Arabinose
Hexoses
Reducing
Lactose
Maltose
Cellibiose
Non-Reducing
Sucrose
Tetrasaccharides
Pentasaccharides
Aldohexeoses
Glucose
Ketohexeoses
Fructose
Non-Sugars (Polysaccharides)
More than 10 sugar unites
Homopolysaccharides
Holosaccharides
Hexosanes
Glucosans
Starch
Glycogen
Cellulose
Dextrins
Heteropolysaccharides
Pentosans
Xylans
Fructosans
Inulin
Gums
Mucilages
Pectins
Agar
Heparin
Alginate
Physical Characters
 Condition:
Sugars are white, crystalline in shape and with
sharp melting points, while polysaccharides are
white amorphous solids.
 Taste:
Sugars have a sweet taste. Polysaccharides are
tasteless.
 Solubility:
Monosaccharides are soluble in cold water and
hot alcohol. Polysaccharides are partially soluble
in hot water.
 Optical activity:

A compound is optically active when, in solution, it is capable to rotate
the plane of polarized light either to right (dextrorotatory, + or d) or to
the left (levorotatory, - or l).
Nickel prism
Plane polarized light
Normal light

The optical activity of a compound is measured by determination of its
specific optical rotation ([] Dt) using a polarimeter, and applying the
following equation:
[]D25 =  /LC
Where:  = extension of rotation,
L = length of tube (light path) in decimeter,
C = concentration g /ml,
25 = operating temperature (t) in 0C,
D = line spectrum of sodium light (589 nm).
Sugar isomers:
 Hexoses like glucose have 4 asymmetric (chairal) carbons.
 Number of isomers can be calculated from the formula:
Number of isomers = 2n
= 24 = 16
CHO
* CH OH
*
HO CH
* CH OH
* CH OH
CH2OH
D and L in sugars:
 A monosaccharide in which the OH group attached to the carbon
atom next to the CH2OH (farthest asymmetric carbon atom from the
carbonyl group) is always to the right is designated as a “D-sugar”
and that with the same OH to the left as “L -sugar”.
 D and L designations are like (R) and (S) designations in that they are not
necessarily related to the optical rotations of the sugars to which they are
applied. Thus, one may encounter sugars that are D (+) or D (-) and
others that are L (-) or L (+).
CHO
H
CHO
OH
CH2OH
D-Glyceraldehyde
HO
H
CH2OH
L-Glyceraldehyde
- and - anomers of glucose:
 When sugars under go cyclization C-1 became a new chairal
carbon and two isomers exist. They are called “ Anomers”.
 In the -anomer the OH group is directed downside and in the
-anomer is directed to the upper side.
 These two forms have different specific rotation, in solution an
equlibrium exsit between the two forms (mutarotation
phenomenon).
H OH
H OH
H O
HO
HO
H
H
1 H
OH
OH
HO
HO
-glucopyranoside
Chair forms
6
H O
H
H
H
1 OH
OH
H
-glucopyranoside
CH 2OH
CH 2OH
O
H
OH
H
H
H
1
OH
OH
H
OH
-glucopyranoside
H
OH
O
OH
H
1
H
OH
H
OH
-glucopyranoside
Haworth formulations
Mutarotation
When a sugar is dissolved in water, the specific rotation of
the solution gradually changes until it reaches to a
constant value due to equilibrium between the  and 
forms.
e.g. freshly prepared solution of  -glucose has a specific
rotation +112o. When this solution is allowed to stand the
rotation falls till reach + 52.7o.
The equilibrium reached is:
36%  –D- glucose []D = + 18.7 0
64%  –D- glucose []D = + 112 0
The sum is + 52.7o
Terms used to describe isomerism:
 Glucose and Galactose are different from each other in the
stereochemistry of carbon 4. They are described as “Epimers”.
 Glucose and Mannose are different from each other in the
stereochemistry of carbon 2. They are described as “Epimers”.
Epimers:
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
CHO
CH OH
HO CH
HO CH
CH OH
CH2OH
D-Galactose
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
CHO
HO CH
HO CH
CH OH
CH OH
CH2OH
D- Mannose
 Galactose and Mannose are different from each other in
the stereochemistry of carbons 2 and 4. They are
described as “Diastereoisomers””Diastereomers”.
Diastereomers:
CHO
CH OH
HO CH
HO CH
CH OH
CH2OH
CHO
HO CH
HO CH
CH OH
CH OH
CH2OH
D-Galactose
D- Mannose
 Glucose and Fructose have the same molecular
formula C6H12O6. They have different structures with
different function groups. They are described as
“Structural Isomers”.
Structural isomers:
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
CH2OH
CO
HO CH
CH OH
CH OH
CH2OH
D-Glucose
D-Fructose
Chemical Reaction Related to Colour
Tests for Carbohydrates
 Effect of conc. acids:
Treatment with conc. mineral acid (HCl or H2SO4) leads to dehydration of
sugars and formation of the corresponding furfural.
Pentoses
O
CHO
Hexoses
HOH2C
O
CHO
Furfural
Volatile
5-Hydroxymethyl
furfural
less volatile
 Reaction of furfural with amines resulted in Shiff’s bases with
different colours used as colour tests such as:
 Molisch’s test:
 Any carbohydrate + Alcoholic -naphthol then add conc. H2SO4 on
the wall of the test tube
Violet ring between the two layers.
 Resorcinol test (for keto-hexoses):
 Sugar solution + few crystals of Resorcinol + Equall volume of conc.
HCl and warm on water bath
Rose Red Colour.
 Furfural test (Differentiate between Pentoses and Hexoses):
 Pentose + Coc. Acid and heat, expose the vapours to Aniline
acetate paper
Red colour
 Hexoses give negative result.
Effect of Alkalis
 Strong alkalis:
 Polymerization.
 Weak alkalis:
 Isomerization.
H
C=O
CH OH
HO
CH
CH OH
CH OH
CH2OH
CHOH
C OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
CH2OH
C=O
HO CH
CH OH
CH OH
CH2OH
D-Fructose
CHO
HO CH
HO CH
CH OH
CH OH
CH2OH
D- Mannose
Oxidation
 Mild Oxidation:
 These are oxidizing agents like Bromine water that convert the CHO
group to COOH to produce “onic acids”.
 Colour tests based on this reaction:
 Fehling’s reduction test:
Sugar solutions + Fehling’s A (CuSO4) + Fehling’s B (NaOH, NaK tartarate
rochell salt), heat on water bath
Red Precipitate
RCHO + Cu++
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
RCOOH + Cu2O
I2 or Fehling's
COOH
CH OH
HO CH
CH OH
CH OH
CH2OH
Gluconic acid
Barfoed’s test:
Sugar solution + Barfoed’s reagent (Cu Acetate/Acetic acid),
heat for 3 minutes on boiling water bath
Red ppt with
monosaccharides only.
Acidic medium decreases the oxidation power of Cu++.
Strong Oxidation
 These are oxidizing agents like HNO3 that convert the CHO
and CH2OH group to COOH to produce “aric acids”.
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
HNO3
COOH
CH OH
HO CH
CH OH
CH OH
COOH
Saccharic acid
 Glalactaric acid (Mucic acid) test:
 Oxidation of galactose resulted in the formation of
Galactaric acid. It is a meso compound insoluble in water
and have zero optical rotation.
CHO
CH OH
HO CH
HO CH
CH OH
CH2OH
D-Galactose
HNO3
COOH
CH OH
HO CH
HO CH
CH OH
COOH
Galactaric acid
Enzymatic oxidation
 Takes place in plants and resulted in the oxidation of the
primary alcohol group only producing “uronic acids”.
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
CHO
CH OH
HO CH
CH OH
CH OH
COOH
Glucuronic acid
COOH
O
H H
OH
OHOH
H
H H
OH
Reduction
 This resulted in the reduction of the CHO to CH2OH
producing “Sugar Alcohols”. Sodium borohydride
or H2/Pt are examples of reducing agents.
 Glucose
 Galactose
 Mannose
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
Sorbitol
Dulcitol
Mannitol
CH2OH
CH OH
HO CH
CH OH
CH OH
CH2OH
Sorbitol
Pentoses
Examples:
 -D-Ribose: found in all plant and animal cells as the carbohydrate part of
nucleic acids e.g. ribonucleic acid (RNA).
 -D-Xylose (or wood sugar): prepared from corncobs, bran, straw (or any woody
material) by boiling with acids, fermenting out any glucose present with yeast,
and crystallizing the D-xylose from the evaporated solution.
 -L-Arabinose (or pectin sugar): found in gums, pectic substances,
accompanying hemicelluloses and forms the sugar part of several glycosides.
CHO
CHO
CHO
CH OH
CH OH
CH OH
HO CH
CH OH
HO CH
CH OH
CH OH
CH2OH
CH2OH
CH2OH
L-Arabinose
D-Ribose
HO CH
D-Xylose
Hexoses
1- -D-Glucose
(dextrose, grape sugar, blood sugar or cornmon sugar)
 Occurrence:
Widely distributed in nature. Present in Grape and blood.
 Preparation:
 D-Glucose is commercially prepared from starch by:
 Autoclaving (at 150 0C) an aqueous starch suspension (15-20%) with dilute
acid (0.03 N hydrochloric acid) for 30 minutes (complete hydrolysis).
CHO
CH OH
HO CH
CH OH
CH OH
CH2OH
D-Glucose
CH2OH
O OH
H H
OH H H
OH
H
OH
D-Glucopyranose
 Uses:
 As source of energy either by mouth or IV injection.
 IV solutions to restore blood volume.
 Shocks following insulin administration.
 As osmotic diuretic.
 Sweetening agent for Pharmaceutical preparations, ice-cream
and candy.
 Liquid glucose:
 Preparation:
 It is prepared by partial acid hydrolysis of starch using dilute
hydrochloric acid and heating for 20 minutes at about 30
pounds pressure.
 Composition:
 It consists of a mixture of glucose, dextrin, maltose and water.
 Uses:
 Used as sweetening agent, as substitute for sucrose and as an
excipient in massing pills.
2- Fructose
(Levulose, Fruit Sugar)
 Preparation:
 Acid hydrolysis of Inulin.
 Hydrolysis of Sucrose.
Sucrose
Acid or Enzyme
Fructose + Glucose
Ca(OH) 2
Ca fructosate
ppt
Ca glucosate
Solution
CH2OH
C O
HO CH
CH OH
CH OH
CH2OH
D-(-)-Fructose
 Uses:
 Infant food.
 Diabetic food.
 Diet control.
HOH2C
O
OH
HO CH2OH
OH
OH
-D-(-)-Fructofuranose
Some Monosaccharide derivatives in
Pharmacy and Medicine
 Gluconic acid and its salts:
Preparation:
 Gluconic acid is prepared from glucose by mild oxidation using either
dilute HNO3 or Br2/Na2CO3 or Electrically or by fermentation using
Acetobacter aceti.
Uses:
 Ca gluconate is used (by i.v. or orally) for treatment calcium deficiency.
 Ferrous gluconate, (orally or by i.v.) is used in iron deficiency.
 These salts are characterized by being more easily
adsorbed than other Ca or Fe salts.
 Glucuronic Acid:
 Naturally present in Gums and Mucilage's. It can be prepared by
Enzymatic oxidation of glucose.
 Uses:
 Treatment of certain arthritic condition as it is a component of
cartilages, joint capsules and fluids, nerve sheath and tendons.
 Aurothioglucose:
 Treatment of rheumatic arthritis by IM injection.
CH2OH
O
H H
H
OH
OH
H
H S
Au
OH
Aurothioglucose
 Auranofin:
 It is the alkyl Phosphine Gold complex with Acetylated thioglucose.
 Treatment of rheumatic arthritis orally.
CH2OR
O S
H H
OR
 Sorbitol and Mannitol:
 Preparation:
OR
H
H H
Au P(C2H5)3
R= COCH3
OR
Auranofin
 Sorbitol is prepared by reduction of glucose and mannitol by reduction of mannose.
 Uses:
 Sorbitol is used as a mild laxative, in preparation of spans and tweens,
sweetening agent, osmotic diuretic and in some food and cosmetics
industries.
 Mannitol is used as an osmotic diuretic, laxative, , vasodilator (mannitol
hexanitrate), antineoplastic (mannitol- (mannitol hexanitrate). nitrogen
mustard) and in laboratory diagnosis of kidney function.
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