Carbohydrates

• Carbohydrates

  Polyhyrdoxy aldehyde or ketone substances<|>Compounds containing C, H, O<|>General Formula: (CH2O)n<|>Functional Groups: C=O or CHO
• Types of carbohydrates

  • Monosaccharides (Glucose and Fructose [ketoses])
  • Disaccharides (Lactose [glucose + galactose], Maltose [2 glucose] and Sucrose [glucose + fructose])
  • Oligosaccharides [3-10 sugar chains]
  • Polysaccharides (Starch, Glycogen)
• Molish's Test


  General tests for carbohydrates or sugar detection<|>Principle: Carbohydrates when treated with concentrated H2SO4 [sulfuric acid] undergo dehydration [process] to give furfural derivatives [products]. These compounds condense with a-naphthol to form reddish violet colored products.
• Molish's Reagent


  Alpha-naphtol [95% ethanol]<|>Concentrated sulfuric acid
• Molish's Test Result

  Purple Ring at the junction of 2 layers (Positive result)<|>All monosaccharides and disaccharides give positive result<|>End result becomes black or charcoal color, it is because of the sulfuric acid [there's water-acid interaction because of sulfuric acid that produces heat that causes charring of the carbohydrates]
• Benedict's Test


  Principle: They reduce the cupric ions of Benedict's reagent to red cuprous oxide. The cupric hydroxide formed during the reaction is kept in solution by metal chelators like citrate [REDUCTION]
• Benedict's Reagent


  Copper sulfate + Sodium carbonate + sodium citrate
• Benedict's Test Result

  Green-yellow to brown or orange-red or brick red ppt (Positive)<|>Confirmatory test; manual detection<|>Frequently used in testing diabetic patients<|>There are substances that cause false positive in benedict's test [ascorbic acid]
• Fehling's Test

  Principle: The tartarate from Fehling's (B) solution chelates cupric ion, releasing it slowly for reduction thus preventing the formation of black cupric oxide.
• Fehling's Solution


  Fehling's solution A: - Contains copper sulfate solution<|>Fehling's solution B: - Contains potassium hydroxide and sodium potassium tartrate. [Rochelle's Salt]
• Fehling's Test Result

  Green-yellow to brown or orange-red ppt<|>The Benedict's test is more sensitive than Fehling's test<|>The compounds can interfere with the test
• Barfoed's Test


  Principle: Barfoed's test differentiates between reducing monosaccharides and disaccharides. This is a reduction test in acidic medium.
• Barfoed's Reagent

  Copper acetate<|>Glacial acetic acid
• Barfoed's Test Result

  Red ppt<|>Reduction test in acidic medium<|>Benedict's test = [differs in pH]<|>Perform its test in alkaline
• Foulger's Test


  Principle: This is a specific test for ketohexoses. Conc. H2SO4 dehydrates ketohexoses to form furfural derivatives which condense with urea in the presence of stannous chloride to give a deep blue color.
• Foulger's Reagent

  urea + stannous chloride + sulfuric acid
• Foulger's Test Result

  A colorless solution is seen in case of glucose. Fructose gives a deep blue color solution.<|>SHORT CUT PRINCIPLE: DEHYDRATION FOLLOWED BY FORMATION OF COLORED COMPLEX
• Seliwanoff's Test


  Principle: This is also a specific test for ketohexoses. Concentrated HCl dehydrates ketohexoses to form furfural derivatives which condense with resorcinol to give red color solution.
• Seliwanoff's Reagent


  resorcinol dissolve in conc. HCl & distilled water
• Seliwanoff's Test Result

  Pink or red color
• Rapid Furfural Test

  Principle: Ketohexoses are rapidly converted to hydroxymethyl-furfural by HCl and form purple colored complex with 1% alcoholic alpha naphthol. This is a test for ketohexoses.
• Rapid Furfural Test Reagents

  Alcoholic alpha-naphtol<|>HCl
• Rapid Furfural Test Result


  Violet color within 30 sec of boiling (fructose)<|>DIFFERENTIATION TEST FOR ALDOSES AND KETOSES
• Bial's Orcinol Test (Bial's Test)


  Principle: Conc. HCl acts on pentoses to form furfurals. This condenses with orcinol to give green color compounds.
• Bial's Reagent


  Dilute solution of orcinol in 30% HCl
• Bial's Test Result


  Green color solution
• Osazone Test

  Principle: A solution of reducing sugar when heated with phenylhydrazine, characteristic yellow crystalline compounds called Osazone are formed.
• Osazone Test Reagents

  Phenyhydrazine hydrochloride<|>Glacial acetic acid<|>Sodium acetate
• Osazone Test Result

  Glucose – needle-shaped crystals arranged like a broom<|>Maltose – sunflower-shaped or petal-shaped crystals<|>Lactose – hedgehog or pincushion with pins or powder puff shaped crystals<|>Formation of crystals separate out as the sample cools off
• Disaccharides


  Two monosaccharides can combine by a glycosidic linkage to form disaccharides. The union can take place by fusion of a free aldehyde or ketone group of one with the free aldehyde or ketone group of the other. In the case of sucrose, there is no reducing group present in the resulting compound.<|>For other disaccharides, the union also takes place by fusion of the aldehyde or ketone group of one with an alcoholic group of other, so one aldehyde group remains free and exhibits reduction test, etc.
• Acid Hydrolysis Test


  Specific test to detect sucrose<|>Principle: Sucrose on hydrolysis by conc. HCl is converted into glucose and fructose (the reducing monosaccharides) which answer the Benedict's test.
• Acid Hydrolysis Test Reagents

  HCl<|>20% sodium carbonate
• Acid Hydrolysis Test Procedure

  To 5 ml of sucrose solution, add 3 drops concentrated HCl and boil for 2 minutes. This will hydrolyze cane sugar to its constituent units glucose and fructose. Now neutralize the solution with 20% sodium carbonate using red litmus paper as indicator (this is essential as Benedict's test requires alkaline medium). Boil 1 ml of the neutralized solution with 5 ml Benedict's solution and note the reduction produced.
• Acid Hydrolysis Test Result

  Acidic/acid
• Flowchart of Identification of Unknown Carbohydrate Solution


  1. Since Molish's test is the general test for carbohydrates.

  2. Kapag nagpositive sa molish test, meaning may carbohydrates na present

  3. Kapag nagnegative sa molish test, no carbohydrates present [we perform benedict's test]

  4. Kapag sa benedict's test ay negative, non-reducing disaccharide.

  5. But, if positive sa benedict's test, monosaccharides are present or reducing disaccharides are present.

  6. Barfoed's test – to know if it is a reducing disaccharide [negative] or monosaccharides [positive]

  7. We have two types of monosaccharides: fructose and glucose

  8. Seliwanoff's Test – test for monosaccharides

  9. Positive [fructose]

  10. Negative [glucose]

  11. We have two types of reducing disaccharide: lactose and maltose. So, we use the osazone test

  12. Sunflower or petal shape [maltose]

  13. Powder puff [lactose]