Monosaccharides
Physical properties of Carbohydrates
Condition
Taste
Solubility
Optical activity of sugars
As glucose & fructose & most disaccharide ps as sucrose are white crystalline solids & melt sharply
Higher polysaccharides, eg. Starch & cellulose are amorphous solids & do not melt sharply
Low molecular weight sugars are sweet
Different degrees of sweetness
Reference degree of sweetness is that of sucrose =1
Soluble in cold water & hot alcohol
Soluble in water & insoluble in alcohol
Soluble with difficulty in cold water,
easily soluble in hot water, insoluble in alcohol
Monosaccharides
Mucilage, pectins, starch & glycogen
Gums
Insoluble in cold & hot water,
soluble in dil. alkalis
Hemicellulose
Insoluble in above solvents
Cellulose
Rotate plane of polarized light to right
Rotate plane of polarized light to left
Dextrarotatory
(+) symbol
d.
Levorotatory
(-) symbol
l.
Monosaccharides & water soluble oligosaccharides are optically active
Measured with polarimeter
Simplest carbohydrate
Reactions of monosaccharides
Cannot be further hydrolyzed
Condition: White crystalline solids
Functional group: single aldehyde/ ketone
Optically active
Found in D-configuration
Naturally occurring contain 3-7 Carbon atoms
Similar to carbonyl compounds
Similar to alcohols
Specific for carbohydrates
Glycoside formation
Oxidation
Reduction
Osazone formation
Action of mineral acids
Ether formation
Esther formation
Effects of alkalis
Reaction with oxidizing cations
Carbohydrates acetals are called glycosides, eg. Glucosides
Mild oxidation of Aldose
Strong oxidation of Aldose
Controlled oxidation of Aldose
Aldonic acid
Aldaric acid
Alduronic acid
Oxidation of D-glucose at C6
Controlled Oxidation
Oxidation of D-glucose at C1
br2, H2O
Oxidation at C1 & C6
HNO3
D-Gluconic Acid
D- glucoronic Acid
D- Glucaric acid/ Saccharic acid
Body uses glucoronic acid to detoxify alcohols& phenols. Converted in liver to glycosides of glucoronic acid —> urine
eg. IV anesthetic propofol -> water - soluble glucoronide-> excreted
Aldoses reduced to Alditols
Ketoses are also reduced
Glucose —NaBH4<- >Glucitol
Mannose —NaBH4<-> Mannitol
Galactose—NaBH4<-> Dulcitol
Fructofuranose—-> Fructose<—>NaBH4
Methyl glucoside —Dimethyl Sulphate/NaOH— > Pentamethyl glucoside
Ethers are used for determination of ring size
B or a Pentamethyl D- glucose —H3O-> B-tetramethyl D-glucose—HNO3-> C5-C6 cleavage + C4-C5 cleavage
Methyl glucoside —Pyridine/ CH3COOH-> Pentamethyl glucoside
Glucose—Phenyl Hydrazine—> Glucazone
All monosaccharides & reducing disaccharides form osazones
Osazones are beautiful crystals
Osazones are different in physical properties (shapes, and m.p.)
Give reason: why glucose, fructose, Mannose give the same osazone.
Osazone involves C1 & C2 so C3, C4, C5 will have identical configuration
Hot conc. Mineral acids (eg. HCl or H2SO4) cause the elimination of water (dehydration) from the monosaccharides
Pentoses
Methyl pentoses
Hexoses
D- Arabinose — conc. HCl or H2SO4/ -3 H2O—> Furfural
Rhamnose — Conc. HCl or H2SO4/ -3 H2O—> Methyl furfural
Glucose— conc. HCl or H2SO4/ -3 H2O—> Hydroxymethyl furfural
Furfural and it’s derivatives form colored complexes with: Phenolic compounds (eg. alpha-napthol) (Molish’s test)
amines (eg. Aniline) (Aniline acetate paper test)
Colored complexes are used for qualitative & quantitative determination of monosaccharides
Strong alkalis: Polymerization
Weak Alkalis: Isomerization
All monosaccharides & reducing disaccharides are oxidized by metal ions in alkaline medium
Eg. Fehling’s test
Fehling A: CuSO4
Fehling B: Potassium Sodium Tartrate& NaOH
Cu2+ (CuO)(Blue) ——-> Cu+(Cu2O) (Red)
Glucose —-> Gluconic acid
Barfoed’s test: Cu Acetate/ Acetic Acid gives positive with monosaccharides only, as acidic medium decreases the oxidation power of Cu2+