Chemistry of Carbohydrates

• Monosaccharides are classified based on functional groups.

Chemistry of Carbohydrates

• Carbohydrates consist of polyhydroxyl aldehydes or ketones.
• Carbohydrates consist of compounds that yield polyhydroxyl aldehydes or ketones through hydrolysis.
• Carbohydrates are classified based on the number of monosaccharide units they contain.
• Monosaccharides are the building blocks of carbohydrates and cannot be broken down into simpler carbohydrate forms.
• The general formula for carbohydrates is (CH2O)n.
• Disaccharides yield two monosaccharide units upon hydrolysis.
  • Examples of disaccharides: sucrose, maltose, and lactose.
  • The general formula for disaccharides is C12H22O11.
• Oligosaccharides are carbohydrates that hydrolyze into 3 to 10 monosaccharide units.
• Polysaccharides are carbohydrates that yield more than 10 monosaccharide units.
  • The general formula for polysaccharides is (C6H10O5)n.
• The number of carbon atoms in carbohydrates varies:
  • Triose: 3 carbons
  • Tetrose: 4 carbons
  • Pentose: 5 carbons
  • Hexose: 6 carbons
  • Heptose: 7 carbons
• Starch is an example of homopolysaccharide.
• Heteropolysaccharides consist of repeating units of different types of monosaccharides.
• Triose is an aldose called glyceraldehyde and a ketose called dihydroxyacetone.
• Tetrose is an aldose called erythrose and a ketose called erythrulose.
• Pentose is an aldose called ribose, xylose, or arabinose and a ketose called ribulose.
• Hexose is an aldose called glucose, mannose, or galactose and a ketose called fructose.
• Heptose is an aldose called sedoheptulose.
  • Sedoheptulose also can be a ketose.
• Homopolysaccharides are made up of repeating units of the same type of monosaccharide.
  • Examples: starch, glycogen, and cellulose.
• Heteropolysaccharides consist of repeating units of different monosaccharides.

Stereoisomers

• Stereoisomers are compounds with the same elemental composition and connectivity of atoms but differ in spatial arrangements.
• Stereoisomerism occurs due to the presence of a chiral center (asymmetric carbon).
• Chiral carbons are attached to four different functional groups.
• The number of stereoisomers a compound has equals 2n, where n is the number of chiral centers (Van't Hoff's rule).
• Enantiomers are stereoisomers that are non-superimposable mirror images.
• Diastereomers are stereoisomers that are not mirror images of each other.

Classification of Monosaccharides

• Functional Group: Aldehyde or Ketone
• Number of Carbons
• Orientation: D or L configuration
• Stereoisomers of monosaccharides are classified as either D or L sugars.
• The classification of D or L depends on the orientation of the H and OH groups around the penultimate carbon.
• Optical activity refers to the ability of a substance to rotate plane-polarized light when passed through a solution.
• Stereoisomers or compounds with chiral carbons exhibit optical activity.
• Practically, D-glucose can rotate plane-polarized light.
• The body mainly metabolizes D-sugars.
• Enzymes that act on monosaccharides have a preference for specific sugar configurations.
• L-sugars can be used as sweeteners.
• Fructoses are ketoaldohexoses, which exhibit the opposite optical activity compared to their aldo counterparts.
  • D-fructose rotates light to the left.
  • L-fructose rotates plane-polarized light to the right.
• Enantiomers have equal but opposite optical activity.
  • D-glucose has a +52.7 rotation.
  • L-glucose has a -52.7 rotation.

Racemic Mixtures

• A racemic mixture is an equimolar mixture of two enantiomers.
• These enantiomers are non-superimposable in a solution.
• The optical activity of a racemic mixture is 0 degrees.
  • The extent to which the D-compound is rotated is the same as the L-compound, effectively canceling each other out.
• Resolution separates two enantiomers from a mixture through crystallization.
• Diastereomers differ in the orientation of groups around only one carbon.
  • These are called epimers.
• Glucose has four epimers.
• Glucose has 14 diastereomers.
• Glucose has 1 enantiomer.
• Open-chain structures are shown through Fischer Projections.

Ring Structure

• The fifth carbon interacts with the first carbon through oxygen.
• This forms a covalent bond
• Glucose forms a pyranose ring structure.
• Also known as a Haworth projection.
• The first carbon ring becomes a chiral carbon.
• Pyranose glucose exists in two forms.
• Achieved by interchanging the H and OH groups.
• The pyranose forms are either alpha or beta.
• For α glucose, the OH is down.
• Where as for β glucose, is up.
• Anomers are stereoisomers that vary only in the orientation around the carbon with functional groups.
  • They are known as anomeric carbon structures.
• Mutarotation occurs when a solution of sugar changes in optical activity.
• Sugar undergoes alteration if the solution has existed over a long period of time.
• Mutarotation happens due to spontaneous anomeric conversion.

Enediol Formation

• This is also called Tautomerization or Lobry de Bruyn-van Ekenstein transformation.
• Tautomerism involves the transformation of two isomers between two forms.
  • Usually between a keto and an enol form.
• Enediol is the interconversion of isomers like Keto and enol.
• Both isomers exist in equilibrium due to hydrogen atom movement.
• Monosaccharides undergo tautomerization in alkaline solutions to form enediols.
• Isomers of glucose include fructose and mannose.
• This interconversion of monosaccharides throughout a typical enediol structure, is called Lobry de Bruyn-Van Ekestein transformation.

Osazone Formation

• Crystalline derivatives of sugars.
• These are formed when sugar is placed in a phenylhydrazine and acetic acid solution.
• Osazones have distinct shapes, melting points, and precipitation times.
• Used to identify sugars.
• Glucosazone crystals are needle-shaped.
• They look like bundles of hay.
• Maltosazone crystals are petal-shaped.
• They look like sunflowers.
• Lactosazone crystals look like cotton balls or pin cushions.

Furfural Formation

• Also known as dehydration.
• Monosaccharides react with concentrated mineral acids.
• Sulphuric acids dehydrate by losing H20.
• Forms furfurals.
• Hexoses go into hydroxyl methyl furfural.
• Furfurals react with alcohols.
• Basis for monosaccharide tests.
• Furfurals react with α-naphthol, resulting in a red-violet ring in Molisch’s test.
  • non-specific for all sugars
• Resorcinol yields a cherry-red color in Seliwanofts test.
  • Specific for fructose.

Oxidation (Formation of Sugar Acids)

• Monosaccharides undergo oxidation with hypobromous acid.
  • Forms carboxylic acids.
• Hexose oxidation on carbon 1 forms aldonic acid.
• Hexose oxidation on carbon 6 forms uronic acid.
• Oxidation on carbons 1 and 6 forms dicarboxylic acids.
  • Saccharic acids are created through strong oxidation.
• Sugar acids undergo oxidation and sugar alcohol reduction.

Reduction (Formation of Sugar Alcohols)

• Monosaccharides undergo reduction using sodremamalgan.
• Creates corresponding alcohols.
• Glucose forms sorbitol.
• Galactose forms galactitol or dulcitol.
• Mannose forms mannitol.

Formation of Iodo Compounds

• Monosaccharides react with hydrogen iodide (HI).
• Oxygen is lost in the carbon chain and an iodo compound is created.
• Example: glucose with conc. HI becomes iodohexane.
• Glucose with conc. HI yields Iodohexane.
• Oxygen is replaced with iodine.