lec04

Lecture 4: Mutarotation, Optical Activity, and Physical Properties of Sugars

A. Isomerism

  • Isomerism: The presence of more than one compound with the same molecular formula.

    • Example: Monosaccharides (hexoses) have the same formula (C6H12O6) but differ in physical and chemical properties.

Types of Isomerism

  1. Functional isomers:

    • Example: D-glucose (aldehyde) and D-fructose (ketone) differ in the position of the carbonyl group.

  2. Stereoisomerism:

    • Occurs in compounds with asymmetric carbon atoms, differing in spatial arrangement.

    • Types of stereoisomerism:

      • Geometrical (cis-trans): Not noticed among carbohydrates, involves spatial arrangement across a double bond.

      • Optical isomers: Differ in arrangement around an asymmetric carbon atom. The number of optical isomers = 2^n (n = number of asymmetric carbons).

        • Example: Glucose has four asymmetric carbons, leading to 16 optical isomers.

Specific Types of Stereoisomers

  • Epimers:

    • Monosaccharides that differ in configuration around a single carbon atom, excluding the carbonyl carbon.

    • Example: Mannose and glucose (Carbon 2), Galactose and glucose (Carbon 4).

  • Enantiomers:

    • Non-superimposable mirror images, differing in their effect on polarized light.

    • Example: D-glucose (rotates light right) and L-glucose (rotates light left).

  • Diastereomers:

    • Stereoisomers that are not mirror images of one another.

    • Example: D-glucose, D-mannose, D-galactose are all diastereomers.

B. Optical Activity

  • Ordinary Light:

    • Vibrates in all directions perpendicular to travel direction.

  • Plane Polarized Light:

    • Light after passing through a Nicol prism, vibrating in one direction.

  • Interaction with Sugar Solutions:

    • Optically active sugar solutions rotate plane-polarized light.

    • Dextrorotatory: Rotate light right (+).

    • Levorotatory: Rotate light left (-).

Measurement of Optical Activity

  • Polarimeter: Device for measuring optical activity.

  • Specific Rotation (α):

    • Specific rotation calculated with the formula:

    [ Observed rotation (α) = \frac{Rotation}{Length of tube (dm) \times Concentration} ]

Specific Rotation Values of Sugars

  • D-glucose (dextrose): +52.2

  • D-fructose (levulose): -92.0

  • D-galactose: +80.5

  • D-mannose: +14.6

  • L-arabinose: +104.5

  • Sucrose: +66.5

C. Mutarotation

  • Definition: Change in optical rotation when an aqueous sugar solution stands.

  • Condition for Mutarotation:

    • Sugars with free aldehyde or keto groups exhibit mutarotation.

    • Some sugars exist in two crystalline forms.

    • Example: D-glucose yields different forms based on crystallization conditions.

Observations of Mutarotation

  • Fresh aqueous solution of α-D-glucose: Specific rotation of +113°

  • After standing, it changes to +52.2°.

  • Fresh solution of β-D-glucose: Specific rotation of +19° changes to +52.2° on standing.

  • Mechanism:

    • The hemiacetal ring of glucose opens and reforms, producing a mixture of α- and β-D-glucose with a specific rotation of +52.2°.