Discusses about monosaccharides definition, classification, structure and reactions of glucose, galactose, and fructose. Qualitative tests for carbohydrates with reaction scheme. Terminologies in carbohydrates such as epimeris, anomers and mutarotation.
Carbohydrates - Monosaccharides and its qualitative tests - Part 1
1. DR. S.M.Y. MOHAMED MUKTHAR ALI
DEPARTMENT OF CHEMISTRY
SADAKATHULLAH APPA COLLEGE, TIRUNELVELI
EMAIL ID: SMYMUKTHAR@GMAIL.COM
CARBOHYDRATES PART 1
2. • Defined as polyhydroxyaldehydes or ketones or compounds which produce
them on hydrolysis.
• General formula Cm(H2O)n. Polyhydrates of carbons.
• Functions:
• Most abundant dietary source of energy (4 Cal/g).
• Precursors for many organic compounds i.e. fats, amino aicds.
• Participate in the structure of cell membrane and cellular functions.
• They are the structural compounds as cellulose of plants, exoskeleton of some insects
and cell wall of microorganisms.
• Serve as a storage of energy (as glycogen) for immediate energy demand of the body.
CARBOHYDRATES DEFINITION
3. • Carbohydrates are referred as saccharides.
• They are classified based on the number of sugar units.
• Mainly classified into monosaccharides, oligosaccharides and
polysaccharides.
Carbohydrates Classification
4. Monosaccharides:
• They are the simplest group of sugars or carbohydrates. They cannot be further hydrolysed.
• General formula Cn(H2O)n.
• They are further classified based on functional groups and number of carbon atoms present.
• Based on functional group classification:
Aldoses – contain aldehyde function groups e.g. glyceraldehyde, glucose.
Ketoses – contain keto function groups e.g. dihydroxy acetone, fructose.
• Based on number of carbon atoms:
trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C) e.g. glucose is
aldohexose while fructose is ketohexose.
C
O
H
C
O
Carbohydrates Classification
5. Oligosaccharides:
• They contain 2 to 10 units of monosaccharide molecules.
• Oligosaccharides on hydrolysis produces monosaccharides.
• Oligosaccharides can be further subdivided into disaccharides, trisaccharides etc. based on
the number of monosaccharides.
Polysaccharides:
• They contain many number of monosaccharide unit of molecules.
• They have very high molecular weight (upto million).
• On hydrolysis, produces monosaccharides and oligosaccharides.
• Polysaccharides are of two types – homopolysaccharides and heteropolysaccharides.
Carbohydrates Classification
6. • Molecular formula of glucose is C6H12O6.
• It is an aldohexose i.e. 6C system with aldehyde functional group. It is a reducing sugar.
• It has four secondary hydroxyl groups and one primary hydroxyl group.
• It has two enantiomers i.e. L-Glucose and D-Glucose. Enantiomers are mirror image compounds of a
molecule.
• Glucose can exist in cyclic and acyclic forms.
Structure of Monosaccharide - Glucose
Reference compounds
7. • In cyclic form, it can exist in two different forms namely -D-Glucopyranose and -D-Glucofuranose.
• Pyranose is 6 membered ring structure while furanose is 5 membered ring structure. The name
pyranose is derived from pyran, while furanose is derived from furan.
• The structure is depicted using Haworth projection formulae.
• Of these two cyclic forms, pyranose based structure is most stable.
Structure of Monosaccharide - Glucose
8. • There are two different cyclic forms of Glucose.
• The and cyclic forms of D-glucose are known as anomers.
• They differ from each other configuration only around C1 is known as anomeric carbon.
• In -anomer, the –OH group on the anomeric carbon is placed on the opposite side of the –CH2OH
group.
• In -anomer, the –OH group on the anomeric carbon is placed on the same side of the –CH2OH group.
Anomers of Glucose
-D-Glucopyranose -D-Glucopyranose
9. Tautomerization or Enolization:
• The process of shifting a hydrogen from one carbon atom to another to produce enediols is known as
tautomerization.
• Anomeric carbon of glucose undergo tautomerization in alkaline solutions.
• This reaction is known as Lobry de Bruyn-von Ekenstein transformation.
Reactions of Monosaccharide - Glucose
10. Reducing property of glucose:
• Sugars are classified as reducing or non-reducing sugars. Glucose is a reducing sugar.
• Reducing property depends on the free aldehyde or free keto group of anomeric carbon.
• Reducing sugars can be differentiated using Benedict’s test, Fehling’s test and Barfoed test.
• CuSO4 is reduced to Cu2O (red ppt) by the reducing sugar.
Reactions of Monosaccharide - Glucose
11. Oxidation:
• Depends on the types of oxidizing agent either aldehyde group or terminal CH2OH group or both can
be oxidized.
• Aldehyde group oxidation lead to the formation of gluconic acid.
• Terminal CH2OH oxidation lead to the formation of glucuronic acid.
Reactions of Monosaccharide - Glucose
12. Reduction:
• In the presence of Na-amalgam, D-glucose is reduced to D-sorbitol.
• During the process, aldehyde group is reduced to alcohol.
Reactions of Monosaccharide - Glucose
13. Dehydration:
• On treatment with conc. H2SO4, glucose undergo dehydration with an elimination of 3 water molecules
to produce hydroxymethyl furfural.
• This hydroxymethyl furfural on reaction with -napthol produces coloured compounds in Molisch test.
Reactions of Monosaccharide - Glucose
15. Ester formation:
• Alcoholic groups of glucose can be esterified by enzymatic or non-enzymatic reactions.
• Esterification of glucose with phosphoric acid is a common reaction in metabolism.
Reactions of Monosaccharide - Glucose
16. • Molecular formula of galactcose is C6H12O6. Found in milk as disaccharide, lactose.
• It is an aldohexose i.e. 6C system with aldehyde functional group. It is a reducing sugar.
• It has four secondary hydroxyl groups and one primary hydroxyl group.
• It has two enantiomers i.e. L-Galactose and D-Galactcose. Enantiomers are mirror image compounds
of a molecule.
• Glucose and galactose are epimers.
• Galactose can exist in cyclic and acyclic forms also.
Structure of Monosaccharide - Galactose
Epimers?
17. Structure of Monosaccharide - Galactose
• In cyclic form, it can exist in two different forms namely -D-Galactopyranose and -D-
Galactofuranose.
• Pyranose is 6 membered ring structure while furanose is 5 membered ring structure.
• The structure is depicted using Haworth projection formulae.
• Of these two cyclic forms, pyranose based structure is most stable.
22. • Molecular formula of frutcose is C6H12O6. Found in honey and fruit juices.
• It is an ketoohexose i.e. 6C system with keto functional group. It is a reducing sugar.
• It has three secondary hydroxyl groups and two primary hydroxyl group.
• It has two enantiomers i.e. L-fructose and D-fructcose.
• Fructose can exist in cyclic and acyclic forms also.
Structure of Monosaccharide - Fructose
23. Structure of Monosaccharide - Fructose
• In cyclic form, it can exist in five membered ring structure such as -D-fructofuranose and -D-
fructofuranose.
• The structure is depicted using Haworth projection formulae.
26. Reactions of Monosaccharide - Fructose
• Fructose undergoes dehydration to form hydroxymethyl furfural similar to glucose and galactose.
• It also undergoes ester formation and fructosazone formation
27. Mutarotation - Glucose
• Mutarotation is defined as the change in the specific optical rotation representing the interconversion of and
forms of the sugars to an equilibrium mixture
• The and form of fresh glucose show different optical rotations.
• Specific optical rotation of freshly prepared -D-glucose in water is +112.2° while the -D-glucose +18.7°.
• Due to mutarotation, the value gradually reaches an equilibrium value of +52.7°.
• Mutarotation occurs faster in alkali solution.
In D-Glucose, Equilibrium mixture
contains, 63% of form, 36% of form
and 1% of open form.
28. Epimerisation
• If two monosaccharides differ from each other in their configuration around a single specific carbon (other than
anomeric carbon) is referred to as epimers to each other.
• The interconversion of epimers is known as epimerisation.
• Epimerase enzyme catalyses the epimerisation in biological systems.
30. Conversion of D-Arabinose to D-Glucose
• During the conversion of D-Arabinose to D-Glucose, one carbon chain length is increased.
• The conversion is known as Kiliani-Fischer synthesis.
• It involves three steps namely, 1. Nucleophilic attack by CN- ion, 2. Hydrogenation and 3. Hydrolysis.
31. Conversion of D-Glucose to D-Arabinose
• During the conversion of D-Glucose to D-Arabinose, one carbon chain length is decreased.
• The conversion is known as Ruff Degradation.
• It involves two steps namely, 1. Oxidation with bromine water, 2. Loss of CO2.
32. Interconversion of D-Glucose and D-Fructose
• Interconversion of D-Glucose and D-Fructose involves enediol formation followed by rearrangement.
• Both the steps are reversible.
• The steps involve proton abstraction and rearrangement.
33. QUALITATIVE TEST FOR MONOSACCHARIDES
https://www.biologydiscussion.com/carbohydrates/test/qualitative-and-quantitative-tests-for-
34. Molisch Test:
• To the aqueous solution of carbohydrates, conc. H2SO4 is added to get the
dehydrated 5-hydroxymethyl furfural.
• Alcoholic alpha -naphthol with 5-hydroxymethyl furfural forms the furfural
derivatives. This compound forms a reddish-violet coloured ring at the junction of
the two liquids. Molisch’s reagent is 5% solution of alpha naphthol in alcohol.
• This confirms the presence of carbohydrates.
QUALITATIVE TEST FOR MONOSACCHARIDES
35. Seliwanoff’s Test:
• It distinguishes aldoses and ketoses.
• Seliwanoff reagent is 0.5% resorcinol in con. HCl.
• To the sugar solution, 2 ml of Seliwanoff reagent is added and the mixture is
heated.
• Strong colour change indicates ketoses while slower colour change indicates
aldoses.
QUALITATIVE TEST FOR MONOSACCHARIDES
36. • Free aldehydic or free ketonic groups can be used for reducing the various metallic ions.
Fehling’s Test:
• It contains two freshly prepared stock solutions A and B.
• Solution A – 6.93 g of CuSO4.5H2O in 100 ml. Solution B – 20 g of KOH and 34.6 g of sodium potassium tartarate in 100 ml.
• To the sugar solution, 5 ml of Fehling’s solution are added and the mixture is heated.
• Appearance of yellow or red precipitate confirms the fructose, glucose, galactose.
QUALITATIVE TESTS FOR MONOSACCHARIDES – REDUCTION
TESTS
37. Benedict’s Test:
• It is a modified Fehling’s test.
• Benedict solution is prepared by dissolving 173 g of sodium citrate and 100 g of sodium carbonate in 100 ml. To this solution,
CuSO4 (17.3 g in 100 ml) solution is added.
• To the sugar solution, Benedict solution is added and heated in water bath.
• Appearance of red precipitate confirms the fructose, glucose, galactose.
QUALITATIVE TESTS FOR MONOSACCHARIDES – REDUCTION
TESTS
38. Barfoed’s Test:
• It is also copper reduction test but in the presence of acid medium.
• Barfoed reagent is CuSO4.5H2O in acetic acid medium.
• To the sugar solution, Barfoed solution is added and heated in water bath.
• Appearance of red precipitate confirms the fructose, glucose, galactose.
QUALITATIVE TESTS FOR MONOSACCHARIDES – REDUCTION
TESTS