Lecture on Enomers

Introduction

• Discussion focused on enomers, a type of isomer in carbohydrates.
• Use of D-glucose as a primary example.

Carbohydrate Structures

D-Glucose

• A six-carbon carbohydrate.
• Numbering starts from the aldehyde group.

D-Mannose

• Similar stereochemistry to D-glucose at several carbons.
• Different configuration at the second carbon (C2 epimer).

D-Galactose

• Differs from D-glucose at the fourth carbon (C4 epimer).

Epimers

• Definition: Carbohydrates differing in configuration at only one carbon.
• Epimers exist as distinct compounds.

Enomers

Definition and Formation

• Enomers are cyclic forms of epimers.
• Formed when D-glucose dissolves in water, undergoing nucleophilic attack to form a cyclic structure.

Configuration

• Alpha D-glucose: OH group below the plane at the first carbon.
• Beta D-glucose: OH group above the plane at the first carbon.

Representation

• Haworth Projection:
  • Pyranoses: Six-membered ring systems with oxygen.
  • Alpha configuration: Group below the plane.
  • Beta configuration: Group above the plane.

Stability and Reactivity

• Stability:
  • Beta D-glucose is more stable (64% in solution) than alpha D-glucose (36%).
• Reactivity:
  • Alpha D-glucose is more reactive due to less stability.

Optical Properties and Mutarotation

• Optical Rotation:
  • Alpha D-glucose: +112.2 degrees in pure form, decreases to +52.7 degrees in solution.
  • Beta D-glucose: +18.7 degrees in pure form, increases to +52.7 degrees in solution.
• Mutarotation:
  • Change in optical rotation over time due to interconversion between alpha and beta forms in water.

Enomeric Effect

• Reflects differences in chemical properties and stability due to configuration at the enomeric carbon.
• Beta D-glucose forms more due to higher stability.

Summary

• Enomers are cyclic epimers, differing at one carbon in cyclic form.
• They have distinct stability and reactivity, leading to phenomena such as mutarotation.