Qualitative Analysis of Carbohydrates

Qualitative Analysis of Carbohydrates

Preliminary Tests

• Benedict's Test:

  • Detects reducing sugars (e.g., glucose, fructose).
  • Reaction with copper(II) sulfate results in color change (blue to green, yellow, orange, or red).

• Barfoed's Test:

  • Differentiates between monosaccharides and disaccharides.
  • Red precipitate indicates presence of monosaccharides after heating.

• Molisch's Test:

  • General test for carbohydrates.
  • Formation of a purple ring with alpha-naphthol and concentrated sulfuric acid indicates carbohydrates.

Specific Tests for Reducing Sugars

• Benedict's Test:

  • As described in Preliminary Tests; confirms reducing sugars.

• Fehling's Test:

  • Similar to Benedict's, uses Fehling's A (copper(II) sulfate) and Fehling's B (alkaline tartrate).
  • Red precipitate indicates reducing sugars.

• Tollen's Test:

  • Silver nitrate solution; reducing sugars reduce Ag+ to metallic silver.
  • Formation of a silver mirror on the test tube indicates presence.

Qualitative Indicators

• Iodine Test:

  • Specific for starch; blue-black color indicates presence.

• Phenol-sulfuric Acid Test:

  • Detects all carbohydrates; color development indicates presence (yellow to brown).

Functional Group Identification

• Alcohol Groups:

  • Carbohydrates contain hydroxyl (-OH) groups; can be identified using reagents like phenylhydrazine.

• Carbonyl Groups:

  • Aldoses and ketoses can be identified through specific tests (Benedict's, Fehling's).

• Ring Structure Identification:

  • Anomers (alpha or beta forms) can be distinguished using specific reagents and NMR spectroscopy.

• Fermentation Tests:

  • Utilizes yeast or bacteria to identify fermentable sugars by the production of gas or acid.

Qualitative Analysis of Carbohydrates

Preliminary Tests

• Benedict's Test detects reducing sugars such as glucose and fructose through a color change from blue to green, yellow, orange, or red when reacting with copper(II) sulfate.
• Barfoed's Test differentiates between monosaccharides and disaccharides; heating leads to red precipitate indicating the presence of monosaccharides.
• Molisch's Test serves as a general test for carbohydrates; a purple ring forms when alpha-naphthol reacts with concentrated sulfuric acid, indicating carbohydrate presence.

Specific Tests for Reducing Sugars

• Benedict's Test is reiterated for confirming reducing sugars, mirroring preliminary test conditions.
• Fehling's Test uses two solutions (Fehling's A and B) to confirm reducing sugars, producing a red precipitate.
• Tollen's Test utilizes silver nitrate; reducing sugars catalyze the reduction of Ag+ to metallic silver, resulting in a distinctive silver mirror effect inside the test tube.

Qualitative Indicators

• Iodine Test is specific for starch; when starch is present, a blue-black color is produced.
• Phenol-sulfuric Acid Test detects all carbohydrates; color change from yellow to brown signifies carbohydrate presence.

Functional Group Identification

• Alcohol Groups in carbohydrates are characterized by hydroxyl (-OH) groups, identifiable via reagents like phenylhydrazine.
• Carbonyl Groups are associated with aldoses and ketoses, identifiable through tests like Benedict's and Fehling's.
• Ring Structure Identification enables differentiation of anomers (alpha or beta forms) via specific reagents and NMR spectroscopy.
• Fermentation Tests assess the presence of fermentable sugars by monitoring gas or acid production from yeast or bacteria.